Fault finding

42 faults · 9 systems · 7 where sources disagree

SU carburettor troubleshooting: describe your symptoms and get a checklist built from these records (AI-assisted).

Results

1. A patch of oil under the car behind the sump, an oil film down the bellhousing and oil marks on the garage floor after every run; in bad cases oil reaches the clutch and the clutch starts to slip.

   LubricationCharacteristicConfidence: High

   Likely cause · Most likely first

   1. Normal weep from the crankshaft rear scroll or slinger, which is not a lip seal but a machined spiral running at close clearance and dimensionally imperfect by design.
   2. Scroll grooves packed with sludge, so the scroll no longer pumps oil back.
   3. Crankcase over-pressure from a restricted breather forcing oil past the scroll.
   4. Sump overfilled above the dipstick top mark.
   5. Worn rear main bearing, or a scroll cap left not concentric with the crank by a past rebuild.
   6. Failed gasket or sealant at the joint between scroll cap, block and sump, a separate leak path often blamed on the scroll.

   What to check · Cheapest and easiest first

   1. Check the oil level is not above the top dipstick mark; SB 168 (Jun 55) instructs that on pressed-steel sumps the level should not be above the top mark on the dipstick when the engine is hot.
   2. Inspect the breather and road-draught tube for debris, crushing and vermin nests.
   3. Clean the area, run the car and locate the actual origin, ruling out the gearbox, the sump rail and oil running back from the filter housing or the head oil feed banjo above.
   4. Check hot oil pressure at idle as a proxy for rear main wear.
   5. Only then have the sump dropped to inspect the slinger and its grooves.

   Applies to: All XK140; the scroll arrangement is common to the 3.4 across XK120, XK140 and XK150.

   Source: jag-lovers XK forum, XK140 Rear oil seal (Roger Payne, Rob Reilly, Holland-rick, Ed Nantes), jag-lovers XK forum, Crankshaft Scroll Oil Seal Mods - Yea or Nay? (Rob Reilly, John Quilter, peterjb), jag-lovers XK forum, Amount of oil & rear crank seal leaks (Paul Wigton, Peder)

2. Not one leak but many appearing together at the cam covers, sump rail, rear of the engine and anything sealed with a copper washer, with a plume of oily vapour when the filler cap is lifted on a hot engine and sometimes a noticeable drop in power.

   LubricationInvestigate soonConfidence: High

   Likely cause · Most likely first

   1. Blocked, crushed or fouled crankcase breather or road-draught tube.
   2. The internal breather baffle, an early flat plate with two holes, fitted rotated so the holes sit side by side instead of one above the other, so oil flung up by the chain cannot drain back.
   3. Emulsified sludge in the breather, which Rob Reilly describes as looking like grey butter and Charles (cb1) attributes to condensation mixed with oil lacking an anti-foaming additive after short runs and long storage.
   4. A catch bottle or PCV added without adequate venting, creating back-pressure.
   5. Worn rings and bores genuinely over-filling the crankcase with blow-by, which Paul Saltwick warns should be suspected before condemning the original breather design.

   What to check · Cheapest and easiest first

   1. Pull the breather pipe and look through it for nests, sludge and a kinked or crushed section.
   2. Hold a hand or a sheet of paper over the filler aperture at idle; strong steady pressure rather than gentle pulsing points at restriction or blow-by.
   3. Check the baffle plate orientation.
   4. Correlate with the leak pattern, since simultaneous leaks at copper-washered joints implicate pressure rather than the individual gaskets.
   5. Compression and leak-down testing to separate breather restriction from worn bores.

   Applies to: All XK140; worse on cars used for short runs or laid up for long periods.

   Source: jag-lovers XK-Engine, Crank case ventilation (Paul Saltwick, Rob Reilly, Charles (cb1), Keith Bertenshaw), jag-lovers XK-Engine, crankcase breather baffle (Cliff King, Rob Reilly, Andy Blackley)

3. Oil creeping out along the cam cover joint, typically lifting at the front corner, running down the side of the head onto the manifolds with the smell of burning oil.

   LubricationMonitorConfidence: High

   Likely cause · Most likely first

   1. Cover flange distorted by previous over-tightening; Rob Reilly notes that over-tightening the 8 studs distorts the cover so it lifts at the front, which is the dominant cause and is self-inflicted.
   2. Work-hardened copper washers reused without annealing, so they no longer conform.
   3. Poor mating surfaces, wrong gasket material, or sealant misapplied.
   4. Crankcase over-pressure, with the covers simply the easiest exit.
   5. Blocked oil drain-back from the head.

   What to check · Cheapest and easiest first

   1. Look at where the joint lifts, since a front-corner lift is the distortion signature.
   2. Check whether other copper-washered joints are also weeping, and if so treat crankcase over-pressure first.
   3. Inspect the breather.
   4. With the cover off, check the flange against a straight edge.
   5. Inspect the half-moon seals and the head face.

   Applies to: All XK140; risk rises with each time the covers have been off.

   Source: jag-lovers XK-Engine, Avoiding Cam Cover Oil Leaks (Rob Reilly, Michael Sweeney (newrynuck), Keith Bertenshaw, Paul M. Novak), jag-lovers XK-Engine, crankcase breather baffle (Cliff King)

4. Oil mist and spray around the crankshaft pulley and timing cover, oil flung onto the front chassis rails and the underside of the bonnet, and belt slip or a squealing belt from oil contamination.

   LubricationMonitorConfidence: High

   Likely cause · Most likely first

   1. Failed front crankshaft seal, noting that two different arrangements exist on the XK140 - a circular oil seal at the crankshaft front end was introduced from engine G1908 (SB 161, Feb 55) along with the rotor-type oil pump.
   2. Seal bore in an early timing cover no longer round; Paul Saltwick notes the ID does not stay round in some early covers.
   3. Worn or poorly machined seal track on the crank or pulley hub, which Paul Saltwick calls critical.
   4. Crankcase over-pressure.

   What to check · Cheapest and easiest first

   1. Establish the engine number and whether the car is before or after G1908, since the parts and the failure mode differ.
   2. Clean and run the engine to confirm the oil comes from the seal rather than the cover joint, the sump front or the head oil feed above.
   3. Check the breather.
   4. Check the pulley hub seal track for a wear groove and the cover seal bore for ovality.

   Applies to: All XK140, split at engine G1908. SB 161 (Feb 55) records that from G 1908 the engine gained a rotor-type oil pump and a circular front crankshaft oil seal, with the oil pressure valve moved into the filter head; changed parts included sump C8592, cylinder block C8610, timing cover C8614, generator bracket C8619, front engine mountings C8783-4 and dipstick C8781.

   Source: XK140 Service Bulletins 1954-57, SB 161 (Feb 55), jag-lovers XK-Engine, Front oil seal on crankshaft (Paul Saltwick, pajtas, richneary)

5. Oil weeping around the filter canister and running down the block, with the canister centre bolt found loose by about a quarter turn after roughly 100 miles; tightening it buys only another hundred miles.

   LubricationInvestigate soonConfidence: High

   Likely cause · Most likely first

   1. Wrong O-ring cross-section for the housing; Rob Reilly notes the XK140 housing has a straight-cut edge that wants a square-section ring, and owners report poor results from X-section rings.
   2. Damaged or extruded O-ring, or a chewed groove.
   3. The wrong filter installation for the engine, since from engine G6233 the canister centre bolt head moved to the bottom of the canister instead of the top of the filter head (SB 175, Dec 55).
   4. Spin-on adapter conversions that do not suit the housing.
   5. On post-G1908 engines the oil pressure relief valve lives in the filter head (SB 161), so a leak here is adjacent to and easily confused with a pressure-control problem.

   What to check · Cheapest and easiest first

   1. Mark the bolt head with paint and re-check after a run to confirm it is genuinely backing off.
   2. Identify the engine number against G6233 and confirm the canister, head and seal match.
   3. Inspect the O-ring cross-section and the groove.
   4. Clean down and confirm the oil is not actually arriving from the head oil feed banjo higher up.

   Applies to: All XK140, with a hard parts split at engine G6233 (SB 175, Dec 55: Tecalemit FA 2708 / C11524, canister 61874, filter head 41034); earlier engines use the previous top-bolt type.

   Source: XK140 Service Bulletins 1954-57, SB 175 (Dec 55) and SB 161 (Feb 55), jag-lovers XK-Engine, XK 140 Oil Filter Housing Leak (Mike Loper (TUP), Rob Reilly)

6. Oil appearing high up on the left side of the engine near the dipstick, running down and then dripping from somewhere much lower so the source is mistaken; in the worst case a sudden loss of oil pressure and a noisy top end.

   LubricationInvestigate soonConfidence: High

   Likely cause · Most likely first

   1. Cracking of the rigid oil feed pipe at the banjos; SNG Barratt state oil reaches the head via a rigid oil feed pipe from the block, which can crack at the banjos over time.
   2. The banjo bolt snapped at its cross-drilling from over-tightening; Rob Reilly notes the bolt does not bottom out in the hole and Erica Moss calls breakage a sign of applying too much torque.
   3. Wrong or reused sealing washers.
   4. Blockage in the pipe or banjo restricting oil to the camshafts.

   What to check · Cheapest and easiest first

   1. Clean the area and trace the leak upward rather than assuming the lowest drip is the source.
   2. Inspect the pipe along its length and at both banjos for a hairline crack, and the banjo bolts for damage.
   3. Watch oil pressure and listen to the top end, since starvation at the cams presents as top-end noise with pressure still showing at the gauge.

   Applies to: All XK140; risk is cumulative with each disturbance of the joint.

   Source: SNG Barratt, Classic XK Engines, jag-lovers XK forum, Oil line from block to cylinder head (Rob Reilly, Erica Moss, Mike_S, Lee140FHC)

7. The gauge sits lower than the owner expects, especially hot and at idle; or it reads normally but dives briefly to near zero under braking, hard cornering or on a steep descent.

   LubricationMonitorConfidence: High

   Likely cause · Most likely first

   1. Worn oil pressure relief valve, which Bill (1955 XK140) calls the most common cause of consistent low oil pressure, with the seat wearing off-centre; on post-G1908 engines this valve is in the filter head (SB 161).
   2. Thin modern oil; Leo's back-to-back readings show synthetic 5W/30 about 10 psi lower than 20W/50 across temperatures.
   3. Gauge error, since these are mechanical Bourdon-tube instruments.
   4. Bearing clearances at crank, rod or cam journals, Wray Schelin noting pressure depends on close tolerances on all the bearing surfaces.
   5. Worn oil pump with rotor clearances or a worn square drive, or a restricted pickup.
   6. Air drawn into the pickup, or a failed O-ring at the pump interface.
   7. For a transient drop only, oil surge from a level too low or from a non-original pump whose pickup sits higher in the sump than the original, so it is uncovered on deceleration.
   8. A missing plug from the timing chain sprayer hole letting pressure escape, found by Skip Smith and absent from the parts manuals.

   What to check · Cheapest and easiest first

   1. Check the oil level and note the grade in the engine, and establish whether the problem began when the oil was changed.
   2. Note the reading properly - cold against hot, idle against 2,000-3,000 rpm, with oil temperature known - and compare it against the published figures before doing anything.
   3. Distinguish steady low pressure from transient dips, which have different causes.
   4. Verify the gauge against a second gauge teed into the main oil passage.
   5. Inspect the relief valve and its seat.
   6. Inspect the pump for rotor clearances, scoring and a worn drive, and check the pickup and its seal.
   7. Check bearing clearances last, as the most expensive answer and the least often correct.

   Applies to: All XK140, with the relief valve location differing before and after G1908. Normal is quoted by Larry Martz as 40 psi at 2,000 rpm warm (70 C), with 15-20 psi typical at idle falling to about 10 psi in extreme heat; Keith Bertenshaw gives 40 psi at 3,000 rpm losing roughly 10 psi per 1,000 rpm down towards idle, and judged 10 psi at hot idle with 35 psi at 2,500 rpm at 80 C to be close to spec. Relief valve setting is around 60 psi and a healthy pump on test should show 60-80 psi. SB 167 (Mar 55) introduced multigrade recommendations (Shell X-100 10W/30, BP Special Energol Visco-Static, both fully detergent); SB 185 (Apr 56) recommended Esso Extra 20W/30 and 40/50 tropical; SB 207 (Feb 57) added Duckhams.

   Source: jag-lovers XK library, Oil pressure & how to install bearings (Larry Martz, Bill (1955 XK140), Wray Schelin, Skip Smith), jag-lovers XK library, Oil Pressure (Jim Flack, George Badger, Bruce Cunningham), jag-lovers XK forum, XK140 oil pressure (Keith Bertenshaw, Bill Schorse, Nick (53 XK120), Leo, Doug, Mshucard)

8. A distinct metallic clatter or knock from the head, harsher and less rhythmic than normal tappet noise, often following a known overheating episode, with valve clearance on one or more valves found to have changed.

   EngineStop drivingConfidence: High

   Likely cause · Most likely first

   1. Steel tappet guides working loose in the aluminium cylinder head; JCNA call loose tappet guides the most common repair XK heads require, with overheating expanding the alloy head so the steel sleeves back out and contact the cam lobes.
   2. A guide already broken; Randy Wilson reported finding a broken metal sleeve behind a tapping noise.
   3. Thermal cycling over a long life, with or without a single severe overheat.
   4. Head warped in the cam area, which JCNA note can remain even after the block face is resurfaced, affecting clearance particularly on cylinder one.

   What to check · Cheapest and easiest first

   1. Establish whether the car has a history of overheating, which is the trigger.
   2. Listen, since guide clatter is coarser and less regular than solid-lifter tappet noise; use a rod against the cover with your ear to the other end.
   3. Check valve clearances for one or two that have changed while the rest are consistent.
   4. Inspect the exhaust-side cam cover area, feeling under the camshaft between the first two tappet locations for retainers as Lawrence Buja's FAQ describes.
   5. Drain and cut open the oil filter and look for metal particles.

   Applies to: All XK straight-six heads in principle (2.4, 3.4, 3.8, 4.2), but incidence is not uniform; on an XK140 treat it as real but overheat-driven rather than routine.

   Source: JCNA, XK Cylinder Head Repair (no author named on the page), jag-lovers, XK Engine Tappet Retainer Stakedown FAQ (Lawrence Buja, with Randy Wilson, Silas Elash, John McDonagh, Jeffrey Gram, Tony Gardner), SNG Barratt, Classic XK Engines

9. A rhythmic tic, tic, tic from the cam covers, present when hot and unchanged for years, alarming to an owner used to modern engines.

   EngineCharacteristicConfidence: High

   Likely cause · Most likely first

   1. Nothing wrong: the XK 3.4 uses solid tappets with a designed running clearance, and MGuar notes there is a designed gap of .010 between camshaft and tappet so a tappet sound is expected.
   2. Clearances at or slightly beyond the top of the range from normal wear.
   3. Tappets of mixed grades from past work; SB 158 (Nov 54) records that from then only one grade of tappet was supplied, which matters for cars whose history includes earlier parts.

   What to check · Cheapest and easiest first

   1. Characterise the sound, since regular light ticking in time with the cams is expected while a knock like a hammer knocking on a piece of hard wood points at detonation or damage.
   2. Confirm the noise has not changed, since a new or louder noise should be treated as a loose tappet guide or a timing chain until proven otherwise.
   3. Check clearances against the manual figure; SB 156 (Nov 54) records standard valve clearances of 004/006, but work to the manual rather than to that note.
   4. Inspect the oil filter for metal only if the character has changed.

   Applies to: All XK140. Paul Saltwick's warning is worth passing on: you could dismantle the head, find nothing, rebuild it and still have the noise.

   Source: jag-lovers XK-Engine, About engine head noises, tappings, etc (MGuar, Paul Saltwick, il_conte, Car-Nut), XK140 Service Bulletins 1954-57, SB 156 and SB 158 (Nov 54)

10. A ringing, jingling rattle at idle that fades above roughly 1,200-1,500 rpm and does not clear when warm; sometimes a whirring heard strongly at the front of the car, and sometimes a noise arriving a second or two after a cold start as oil pressure comes up.

    EngineInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Slack lower timing chain with a worn or failed tensioner, the primary suspect for a front-end jingle at idle.
    2. Tensioner type mismatch or a worn early tensioner; Joel (ex_jag1) notes the spring-steel type will be noisier than the hydraulic one and can make noise from the chain rubbing on the tensioner.
    3. Tensioner slipper dropped out of position, which Dunners_Dave had.
    4. A displaced rubber buffer on the upper chain guide, which Erica Moss describes as a nice little jingle jangle at idle that diminishes when revved.
    5. Noise arriving with oil pressure after a cold start, which points at the tensioner's hydraulic action rather than simple slack.
    6. Not the chain at all - see the checks.

    What to check · Cheapest and easiest first

    1. Rule out the cheap impostors first: one documented case of supposed timing chain noise was a faulty fan belt, and an exhaust manifold blow and normal tappet noise are the other two.
    2. Note exactly when it occurs - at idle only, on cold start as pressure rises, or on the overrun - since these separate the causes.
    3. Establish the engine number against G4431 so you know which tensioner the car should have.
    4. Turn the crankshaft back and forth with a finger on the exhaust cam through the oil filler aperture; if the cam follows the crank, tension is adequate.
    5. Inspect the upper chain guides for a displaced buffer.
    6. With the front covers accessible, check lower chain lateral movement.

    Applies to: Split at engine G4431. SB 172 (Sep 55) reads verbatim: 'Lower type changed. Commencing engine no. G4431, 4411 to 4420.' The spring blade tensioner was replaced with a spring-loaded Renold chain tensioner (C10332), with new block number C8610/1, and the 2 types of tensioner were not interchangeable. SB 192 (Jul 56) records a spares kit becoming available for servicing the Renold type.

    Source: XK140 Service Bulletins 1954-57, SB 172 (Sep 55) and SB 192 (Jul 56), jag-lovers XK-Engine, Timing Chain Noise? (Terry Hilton, Joel (ex_jag1), newrynuck), jag-lovers XK-Engine, Possible lower timing chain problem (Erica Moss, Dunners_Dave, Mike Eck, Ron Smith)

11. Persistent high running temperature and disappointing performance, surviving a recored or replaced radiator, a new pump and a new thermostat, so the owner has spent money on cooling and nothing has changed.

    EngineInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Camshaft timing out by one or more teeth; Robert H. W. Cathey documents intake cam misalignment by two to three teeth causing persistent overheating that survived radiator recoring and a custom aluminium radiator.
    2. Ignition timing retarded, including a failed vacuum advance diaphragm; Arno Wahl notes defective diaphragms retard timing and cut efficiency, one unit having a small hole rusted through the side of the housing.
    3. A cracked head passage or a head gasket leak pressurising the system.
    4. Slack upper chain letting cam timing drift from its set position.

    What to check · Cheapest and easiest first

    1. Check ignition timing and test the vacuum advance unit for a holed or perforated diaphragm, which is cheap, quick and commonly the answer.
    2. Check the mixture.
    3. Check upper chain slack before assuming cam timing is where someone set it.
    4. Have cam timing verified properly on both cams, which Cathey notes requires two gauges reading both cams simultaneously.
    5. Pressure-test the cooling system with the engine running to detect a head gasket leak or head crack.

    Applies to: All XK140, most likely on any engine that has had the head off, chains disturbed or an engine change. SB 174 (Dec 55) recorded modified cylinder heads with reduced tapped-hole depth (C6733/1 standard, C7707/1 C-type, C7896/1 D-type), with the instruction that longer studs must not be fitted, a 3/16 inch difference; mismatched head and stud parts are a hazard on a car with an unknown rebuild history.

    Source: jag-lovers XK library, Overheating (Robert H. W. Cathey, Arno Wahl, Skip Smith), JCNA forum, XK140 FHC overheating: water pump? (George, SE98-32482CJ), XK140 Service Bulletins 1954-57, SB 174 (Dec 55)

12. Measurable oil consumption with little or no smoke in normal running, but a distinct cloud of blue smoke on accelerating away after a long closed-throttle descent or a period of engine braking, and after idling at a light following one.

    EngineMonitorConfidence: High

    Likely cause · Most likely first

    1. Worn valve guides and valve stem seals, where high manifold vacuum on a closed throttle draws oil down the guides to burn on the next throttle opening; Gary Otto notes heavy smoking when using the engine as a brake points at guides, and ex_jag adds guides can pass a warm tolerance check yet still pass oil under high vacuum.
    2. Wrong or incompatible valve stem seals; ex_jag warns that without matched valve hardware the seals will not work properly and can actually pump oil down the guide.
    3. Worn bores and rings, which give smoke under load and on overrun alike plus crankcase pressure.
    4. Oil grade; SB 204 (Jan 57) records Jaguar's own observation that greater oil consumption resulted from use of multigrade oils.
    5. Sump overfilled.
    6. External leaks counted as consumption.

    What to check · Cheapest and easiest first

    1. Measure actual consumption over a known distance and separately account for what is on the floor, since many oil burners are oil leakers.
    2. Note the oil grade and whether consumption changed with an oil change.
    3. Check the level is not above the dipstick top mark (SB 168).
    4. Characterise the smoke: descent-then-pull-away points at guides, while smoke under load and on overrun with crankcase pressure points at rings and bores.
    5. Check the breather, bearing in mind Paul Saltwick's caution that excess oil in the breather tube implicates ring seal before breather design.
    6. Compression and leak-down testing to settle guides against bores.
    7. Inspect the guides and valves.

    Applies to: All XK140, with a documented change at engine G6678S: SB 185 (Apr 56) records modified exhaust valves and guides fitted from G6678S, the valve part number unchanged at C7708 and guides C8312, with the valve stem reduced in diameter under the valve head to form a scraping edge inside the guide as an oil-control measure. The new valve became common to standard and C-type heads for all future production, and earlier engines do not have this feature unless previously updated.

    Source: jag-lovers XK-Engine, oil consumption diagnosis (Gary Otto, George Morrell, ex_jag), XK140 Service Bulletins 1954-57, SB 185 (Apr 56), SB 204 (Jan 57), SB 168 (Jun 55)

13. Temperature climbs steadily and a newly fitted thermostat has left the car no better or worse; or the opposite complaint, a car that never warms up properly, runs at around 55 C and spits back through the carburettors on full-throttle acceleration from about 2,000 rpm in top.

    CoolingInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. A modern thermostat without the bypass-closing feature, since the correct pattern carries an external ring or skirt that closes the bypass as the main valve opens and standard aftermarket units lack it, so coolant continues to recirculate through the bypass without passing through the radiator.
    2. A non-bellows thermostat fitted where a bellows type belongs, which Rob Reilly identifies as a known issue on the XK140 and which was the resolution in Herb22's documented case.
    3. Wrong opening temperature; SB 167 (Mar 55) records a change of thermostat from engine G2841 plus certain individual engines prior to that number, new C3731/1 (stamped 43570/5) opening at 73 C replacing earlier 43570 opening at 60 C, made to address the cold-running and carburettor spit-back complaint, and the later type was also standard on the Mk VII.
    4. Thermostat fitted the wrong way round, or a bypass blanked off entirely.
    5. An air pocket trapped at the thermostat housing, which on the XK140 sits on the inlet manifold and is therefore an air trap.
    6. The tiny bypass valve on early thermostats, described by Bob Oates and E.W. Blake as pencil-lead sized, allowing recirculation without cooling.

    What to check · Cheapest and easiest first

    1. Take the thermostat out and look at it: is it a bellows type, does it have the outer ring that closes the bypass, and does the marked opening temperature suit the engine number relative to G2841?
    2. Test it in hot water and confirm the opening temperature.
    3. Purge air from the thermostat housing, which Bob Knijnenburg suggests doing by squeezing the upper hose repeatedly.
    4. Run briefly with the thermostat removed as a diagnostic only, which Otto M. notes reveals whether the water pump is at fault.
    5. Then move on to radiator and block condition.

    Applies to: All XK140, with a documented change at engine G2841 (SB 167, Mar 55) plus certain individual earlier engines; in practice the bigger variable today is which replacement thermostat is in the car, not what left the factory.

    Source: XK140 Service Bulletins 1954-57, SB 167 (Mar 55), jag-lovers XK library, Cooling (John Spence, Cleo Bay, Mike Plechaty), jag-lovers XK library, Overheating (Bob Oates, E.W. Blake)

14. Perfectly happy on the open road, but the gauge walks up whenever the car is stationary or crawling, especially on a hot day, and recovers as soon as airflow returns; coolant is pushed out and needs topping up after every hot run.

    CoolingCharacteristicConfidence: High

    Likely cause · Most likely first

    1. The system was never designed for this; Twyford Moors state the XK120, 140 and 150 were never designed to cope with the kind of driving conditions we experience these days and that the original system lacks sufficient capacity for sustained idle operation in traffic.
    2. No header or expansion tank, which Twyford Moors identify as a primary weakness, since the car spits out coolant when heated and cannot recapture it on cooling, so the system self-depletes.
    3. Insufficient airflow at idle from the mechanical fan alone; JCNA's documented XK140 case improved to about 90 C when driving but idle cooling remained insufficient without electric fan assistance.
    4. Mixture too weak, or ignition retarded, raising heat rejection at idle.
    5. A genuinely tired radiator core or a silted block on top of the above.

    What to check · Cheapest and easiest first

    1. Establish what the car actually does at speed against at rest; if it cools down when moving, the core and the pump are probably doing their job and the problem is airflow and capacity.
    2. Calibrate expectations before spending: Nick_53 runs to approaching 92-93 as his stop on a hot day in traffic, Bob_K1 reports 90 C typical in summer even after a recore and pump rebuild, and John Spence reports 95-100 C in overdrive at 70 mph at 100 F ambient.
    3. Verify the gauge and the sender before believing the needle.
    4. Check the coolant level, the cap rating, and whether coolant is being lost at every run, since a system with no recovery tank runs itself down.
    5. Check the mixture and ignition timing.
    6. Only then assess radiator and block condition.

    Applies to: All XK140. Radiator and fan combinations varied - SB 169 (Jun 55) tabulates them: brackets C10287/C10288 with a 15 1/2 inch fan C10128, fixed head coupe only, chassis 804001-019 and 814001-034; brackets C7523/C8830 with a 16 inch fan C7516 for all models; brackets C9616/C8830 with the 16 inch C7516 for all models. C9616 is interchangeable with C7523 if the water hose is changed and the tie rods replaced by radiator steady brackets C9618 (2 off), and the bulletin warns the 2 different fans were not interchangeable either. An early FHC with the small fan is the least favourably equipped for traffic.

    Source: Twyford Moors Classic Cars Ltd, Jaguar XK120, XK140, XK150 and E-Type Cooling System Upgrades, JCNA forum, XK140 FHC overheating: water pump?, jag-lovers XK library, Cooling (John Spence)

15. Cool enough around town, but the needle rises on a sustained fast run, a long climb or at high rpm - the opposite pattern to overheating in slow traffic.

    CoolingInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Bottom hose collapsing under pump suction at high rpm; George Badger reports the two bottom hoses collapse when the revs are high and the water pump is sucking really hard, so flow falls exactly when it is needed most.
    2. Radiator core internally silted or partially blocked, so it cannot shed heat at high flow.
    3. Cam or ignition timing error, the classic cause of overheating that worsens with load.
    4. Inadequate water pump flow, noting the figure attributed to Haynes that pump delivery falls from about 38 gallons per minute at 20 C to roughly 28 at 80 C.
    5. Head gasket or head crack pressurising the system under load.
    6. A temperature bulb sitting in air rather than coolant, giving a false high reading, which Rob Reilly raises.

    What to check · Cheapest and easiest first

    1. Verify the gauge before anything else, comparing the temperature at the radiator neck with the instrument reading and confirming the bulb is actually immersed.
    2. Squeeze the bottom hoses and check whether spring reinforcement is present, since soft, aged, unreinforced hose is a candidate.
    3. Check ignition timing and the vacuum advance.
    4. Assess radiator core condition, bearing in mind Jamie's caution that V-cell cores cannot be rodded out, so once internally corroded replacement is the only route.
    5. Check cam timing.
    6. Pressure-test the cooling system with the engine running for a gasket or head leak.

    Applies to: All XK140; hose specification matters.

    Source: jag-lovers XK library, Overheating (George Badger, Jamie, Arno Wahl, and pump-flow figures attributed to Haynes), jag-lovers XK forum, New overheating problem xk140 (Rob Reilly on the bulb sitting in air)

16. Sudden total coolant loss and a recovery truck; or, found on inspection, a hose chafed through where it passes close to the crankshaft damper and a corroded steel bypass branch pipe.

    CoolingStop drivingConfidence: High

    Likely cause · Most likely first

    1. The lower hose chafing on the crankshaft damper; Rodney L Crawford (Chippewa-Rod) documents his C7546 lower hose which rubbed on the crankshaft damper and ruptured, leading to a coolant dump and a 20 mile towing job, with clearance of only about 3/8 to 1/2 inch.
    2. The C7547 steel branch pipe corroding, which is reported as prone to it.
    3. Replacement parts that do not match the originals; Crawford's replacement stainless pipe lacked the welded enlarged-diameter section of the original, leaving a quarter-inch gap incompatible with the new hose, and a replacement C8025 bracket arrived with the slant oriented the wrong way.
    4. Missing C8027 packing rubber strips, so the pipe is not held in position; Roger Payne emphasises these for maintaining pipe position and hose clearance.
    5. Hose collapse under suction.
    6. Over-pressurisation from a blockage elsewhere blowing a hose off; in one documented XK140 case the bypass hose disconnected from the inlet manifold and expelled coolant twice, the root cause being a plug of sludge in the heater matrix and a heater return elbow at the pump inlet completely blocked with crud.

    What to check · Cheapest and easiest first

    1. Look at the lower hose where it passes the crankshaft damper, checking actual clearance and looking for a rub mark or a shiny patch.
    2. Check the C8027 packing rubbers are present and the brackets are the right way round.
    3. Inspect the steel branch pipe for corrosion, especially if it is an original.
    4. If a hose has blown off rather than split, look for a blockage downstream at the heater matrix or the heater return elbow at the pump inlet rather than blaming the clip.
    5. Check hose age and internal condition, since old hose fails from the inside.

    Applies to: All XK140. Relevant part numbers: C7546 lower hose (pump to pipe, 1 1/2 inch both ends), C7545 smaller 90 degree hose (1 1/4 inch), C41091 alternative tapered hose, C7547 steel branch pipe, C8025/C8026 brackets, C8027 packing rubber strips.

    Source: jag-lovers XK forum, XK140 Cooling System - Question on Hoses and Pipe (Rodney L Crawford (Chippewa-Rod), Roger Payne, davidxk), jag-lovers XK forum, XK140 cooling system woes (PowrLok (1956 XK140 FHC), F8driver (Dick), Lee140FHC, Bob_K1, Phil Dobson, Rob Reilly)

17. Persistent overheating with a good radiator and a good pump; or coolant seeping from a disc-shaped plug in the side or rear of the block; worst case sudden coolant loss, rapid overheat and seizure.

    CoolingInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. The block water jacket silted with rust and scale; Dick Cavicke describes finding 2 quarts of rust and corrosive debris during block cleaning, and Cleo Bay and Bob Oates note core plugs conceal decades of sludge so a flush through the radiator tells you nothing about the block.
    2. Core plugs corroding from the inside out; Rob Reilly notes standard plugs rust thin from the inside, and Ed Nantes adds barrel-plated plugs are problematic because tumbling can cause a nick in the edge from where corrosion can start.
    3. Coolant left too long and losing its inhibitors, or plain water, or freezing.
    4. Plug blow-out; Gary Grant documents a rear core plug pushed out at its lower edge, allowing water to escape, causing severe overheating and engine seizure with connecting rods attempting to leave the block.
    5. Corroded cooling connections and steam holes in the head, where JCNA note welding alloy into the affected area is the only long-term repair.

    What to check · Cheapest and easiest first

    1. Look at the coolant, since rusty, brown or thin coolant means the block is probably silted whatever the radiator looks like.
    2. Inspect every core plug for distortion, staining and corrosion; Rob Reilly counts eight in the block - 3 big ones on each side, 1 at the back and a little one down by the starter motor - while a second count gives seven large plus a small rear right-hand plug for an oil passage and a large brass hex-key plug on the left side.
    3. Check for coolant staining at the bellhousing joint, which is where a rear plug shows itself.
    4. Flow-test the system rather than assuming; PowrLok used a domestic hose to find remaining obstructions.
    5. Check the heater matrix and the heater return elbow at the pump inlet, both documented blockage sites.
    6. Have the block professionally cleaned or boiled if the evidence points there, as Dick White did.

    Applies to: All XK140; risk correlates with coolant neglect, not with engine number.

    Source: jag-lovers XK library, Cooling (Rob Reilly's plug census, Dick White, John Spence, Cleo Bay, Mike Plechaty), jag-lovers XK library, Overheating (Cleo Bay, Bob Oates, Dick Cavicke), jag-lovers XK forum, Block core plugs (Rob Reilly, Ed Nantes, Gary Grant, Rsk55)

18. Overheating with clear passages and a good radiator, coolant weeping or dripping from the pump area, and sometimes a bearing rumble or play at the fan.

    CoolingInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Worn pump, which Twyford Moors list alongside old radiators and blocked waterways as a direct cause of overheating.
    2. Excessive clearance between impeller blades and the pump body, so the impeller churns rather than pumps.
    3. Pump capacity inadequate for a non-original engine; Bob Grossman states the XK140 water pump will not cope with a larger engine fitted and recommends a larger-impeller pump.
    4. Failed pump seal, or a bearing worn from over- or under-lubrication.
    5. Reduced flow simply from heat, delivery falling from roughly 38 gallons per minute at 20 C to about 28 at 80 C.

    What to check · Cheapest and easiest first

    1. Look for weeping and staining at the pump, and check for play and roughness at the fan.
    2. Establish whether the engine is the original 3.4 or a transplant, and whether the pump suits it.
    3. Check the impeller-to-body clearance, not impeller-to-timing-cover.
    4. Confirm circulation is actually occurring by removing the thermostat temporarily as a diagnostic and looking for flow.
    5. Check the fan is the correct one for the car, since SB 169 records the 15 1/2 inch and 16 inch fans are not interchangeable.

    Applies to: All XK140; acutely relevant to cars with a larger engine fitted. Fan and radiator combinations per SB 169 (Jun 55).

    Source: JCNA forum, XK140 FHC overheating: water pump? (Bob Grossman, Dick (rcmaury), Otto M. (NE08-63766), George (SE98-32482CJ), Yves-Michel Caron), Twyford Moors Classic Cars Ltd, Jaguar XK120, XK140, XK150 and E-Type Cooling System Upgrades, jag-lovers XK library, Overheating (pump flow figures attributed to Haynes)

19. Varies by failure path: coolant loss with no external leak and pressure in the system when cold, bubbles in the header or overflow, coolant seeping down the side of the block, steam or a sweet smell from the exhaust, a misfire on one cylinder, emulsified oil, or in the mildest and commonest form a persistent weep at the head-to-block joint.

    EngineStop drivingConfidence: High

    Likely cause · Most likely first

    1. A previous overheat, the dominant precursor; JCNA state the XK head rarely cracks and it usually happens from a severe overheat, with hairline fractures branching from expansion plugs between cylinders 3 and 4 and between valve seats and plug holes, and overheating also warps the head.
    2. Corroded or unflat mating faces; the jag-lovers library records oil or grease contamination on mating surfaces, warped or unflat head surfaces, and deformed or rusted stud threads as contributing factors.
    3. Gasket type and its history; SB 210 (Feb 57) records the steel type gasket C7861 being supplied, superseding the Klingerite type C2250 and the cupro-nickel gasket C3335 for XK120/140 future production, so a car may have any of these or a modern composite.
    4. Gasket bore mismatch on an overbored engine, a documented failure where a gasket intruding too far into the combustion chamber caused the nosing to split and the composite material to erode.
    5. Corrosion of the water passages and steam holes in the head.
    6. An aluminium head on an iron block, where differential expansion is inherent to the design.

    What to check · Cheapest and easiest first

    1. Check for coolant loss with no external leak, and whether the system is pressurised when cold.
    2. Look into the header or radiator neck at idle for bubbles.
    3. Check the oil for emulsion and the plugs for one that is steam-cleaned.
    4. Pressure-test the cooling system with the engine running, the test that separates gasket from head crack from unrelated overheating.
    5. Compression test, looking for two adjacent low cylinders.
    6. Establish the rebuild history: has it been overheated, has it been rebored, what gasket is in it and does its bore suit?

    Applies to: All XK140, with the gasket supply change recorded at SB 210 (Feb 57). Also note SB 174 (Dec 55) on modified heads with reduced tapped-hole depth and the warning against fitting longer studs, a mismatch here being a gasket-failure mechanism on a car with unknown history.

    Source: XK140 Service Bulletins 1954-57, SB 210 (Feb 57) and SB 174 (Dec 55), jag-lovers XK library, Head gaskets (Martin Jacobsen, Wray Schelin, Gene Burda, Cleo Bay Jr., Bob Mullins, Mike Plechaty), JCNA, XK Cylinder Head Repair

20. A rhythmic tick or puff from the manifold side, loudest cold and often quieter as things expand, soot staining at a manifold joint, black glassy flakes appearing on the engine and garage floor, a manifold that looks pitted and bare where it was once glossy black, plus heat soak in the cockpit, hot-start difficulty and paint cracking nearby.

    ExhaustInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Cracked cast iron manifold, with thermal cycling the mechanism; Bill Angel (angelw) identifies expansion and contraction as the governing factor, and cracks are often invisible until the manifold is cleaned, Ingotvein_Ks only discovering a developing crack after blasting.
    2. Failure of the original vitreous enamel finish; Peter Whyman of Zircotec states the enamel finish can't withstand the heat cycling and just doesn't last, and that even reproduction manifolds supplied with the original enamel finish suffer the same durability problem.
    3. Refinishing that fails early; Ex_jag notes porcelain is prone to flaking and spider-webbing, Tom Carson's manifolds began deteriorating almost immediately, and Ron Rader reports failures from 1000 miles to 3000 miles across multiple coating attempts.
    4. Manifold gasket blowing, or studs seized, stretched or snapped, letting a joint lift.
    5. Consequential heat problems, which Zircotec attribute to an uncontrolled manifold radiating heat - stifling cockpit temperatures, fuel evaporation problems on hot start-up and heat damage to adjacent paintwork.

    What to check · Cheapest and easiest first

    1. Listen cold, since a manifold blow is loudest on a cold engine and often quietens as the iron expands, which alone separates it from tappet and chain noise.
    2. Look for soot tracking at the manifold-to-head and manifold-to-downpipe joints.
    3. Look for flakes of black glassy material on and below the manifolds.
    4. Check whether a ticking noise being blamed on the valvegear or the timing chain is actually here, a frequent misattribution.
    5. Clean the manifold before concluding it is sound, since cracks hide under enamel and carbon.
    6. Check studs and nuts for seizure and for a joint that has lifted.
    7. If hot-start or cockpit heat is the complaint, treat the bare manifold as a contributing cause rather than looking only at the fuel system.

    Applies to: XK140 front and rear cast manifolds, part numbers in the C13822 / C13823 / C2214 family; the same manifold family and the same enamel finish ran across the six-cylinder Jaguars, which Zircotec date as used from 1948 to the mid-1980s.

    Source: Zircotec, Peter Whyman (Sales Director), press release via Propel Technology, jag-lovers, Cast iron exhaust manifold crack (Ingotvein_Ks, Bill Angel (angelw), Paul Wigton, John Walker, abowie, Marco), jag-lovers XK forum, Exhaust Manifold coatings (Ex_jag, Tom Carson, Ron Rader)

21. Switch on the ignition and the pump under the car tick-tick-ticks and keeps ticking instead of beating quickly for ten or fifteen seconds and then falling silent; sometimes it never stops at all, even with the engine running steadily.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Outlet or delivery valve disc not seating, so fuel pumped forward leaks back and the pump can never satisfy the float chambers; Burlen describe the valve disc not shutting properly when the pump is operating.
    2. Valve disc seized or held off its seat by debris - a brass disc on earlier pumps, plastic on some modern replacements.
    3. Perforated or split diaphragm, or a missing gasket between lower lid and body, so fuel passes internally.
    4. A float chamber needle valve not shutting, so the pump is simply keeping up with a leak into the carburettor.
    5. A throttle butterfly not fully closed, so the engine is drawing fuel when it should not be.
    6. An external fuel leak anywhere between pump and carburettors.

    What to check · Cheapest and easiest first

    1. Listen and time it, since a normal pump primes for 10-15 seconds and stops; Roger Herrick's fault case was beating about once every one to two seconds with the ignition on and the engine not running.
    2. Look and smell for fuel anywhere along the line, at the pump's own joints and in the carburettor overflow or vent.
    3. Check whether the ticking stops when you gently tap the pump body, which points at a debris-held valve rather than a torn diaphragm.
    4. Check the float chambers for overfilling before condemning the pump.
    5. Only then open the pump's valve cage to inspect the discs and their seats.

    Applies to: All XK140. Burlen note the seizing problem applies to both the brass discs of earlier pumps and the plastic discs of some modern replacements, especially with the higher ethanol content in modern fuels.

    Source: Burlen / SU Carburetters, What To Do If Your SU Fuel Pump Stops Working, SU Specialists, SU Electric Fuel Pumps, jag-lovers XK library, Fuel Pump Problems (Roger Herrick, David Drenzek, Bruce Cunningham)

22. Running normally, then a stumble and the engine dies as though someone turned the key off, with the normal tic, tic, tic sound missing; a thump on the pump body sometimes restores it and you drive home.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Burnt or sticking contact points in the pump, which Dave DuBois says usually causes intermittent fuel starvation and stalling.
    2. Dirt, corrosion or maladjustment of the tungsten points, in Burlen's own wording.
    3. Failure of the throw-over mechanism, the rocker that snaps the points open as the diaphragm reaches its stroke, stiff from dried oil or friction.
    4. Carbon scoring on the contact blade where bare metal points have been sparking.
    5. Loss of supply or earth to the pump - an intermittent feed wire, a corroded terminal, or a poor pump-to-chassis earth.
    6. Fractured Bakelite pedestal, which DuBois says is usually a problem only on the L and HP type pumps and usually an owner-induced failure from over-tightening the mounting screws.
    7. On cars fitted with a modern electronic SU pump, failure of the electronics rather than any of the above.

    What to check · Cheapest and easiest first

    1. When it next dies, put your ear to the pump with the ignition on; silence points here and ticking points elsewhere.
    2. Tap the pump housing sharply, and if it restarts suspect points rather than electronics.
    3. Check there is 12 V at the pump terminal and a sound earth, wiggling the harness while you watch.
    4. Inspect the points under the end cap for burning, carbon tracking and whether they are in contact at all; Burlen note that if the pump strokes when you bridge the contact plate to earth, the fault is in the points.
    5. Inspect the pedestal for cracks before refitting anything.

    Applies to: All XK140 as built, with a contact-breaker pump. Cars converted to the electronic SU pump have a different failure set: Brad (morevert) reported a new dual-polarity AZX1307 that worked, then failed dead after winter storage with 12 V present and no clicking, replaced under warranty.

    Source: Dave DuBois, SU Fuel Pumps 101 (updated Feb 2014), Burlen / SU Carburetters, Electric Fuel Pump Fault Diagnosis, Burlen / SU Carburetters, SU fuel pump guide

23. The pump ticks - perhaps faster than usual, perhaps unusually slowly - yet the car stumbles, flat-spots and runs out of fuel; or it ticks at what sounds like a healthy rate and delivers nothing at all.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Inlet valve held off its seat by foreign matter, which Burlen single out as foreign matter lodged under one of the valves, particularly under the inlet valve.
    2. Diaphragm hardened with age giving a reduced stroke; DuBois notes a stiffened diaphragm usually makes the pump run slowly or erratically, and Burlen attribute the same to stiffening of the diaphragm fabric or abnormal friction in the rocker throw-over mechanism.
    3. Clogged inlet filter in the pump, or the gauze in the tank drain plug.
    4. Tank not venting; Burlen advise checking correct tank venting by removing the filler cap, since inadequate venting causes a slow power stroke.
    5. Obstructed pipeline between tank and pump.
    6. Worn points giving a weak stroke, or low supply voltage.

    What to check · Cheapest and easiest first

    1. Slacken the filler cap and try again; if the symptom eases, the tank is not breathing.
    2. Note the character of the ticking: slow ticking suggests low voltage, worn points or a stiff diaphragm, fast ticking with no delivery suggests a leak or an unseated valve, and ticking with no delivery suggests a blocked filter, a valve not seating or a holed diaphragm.
    3. Disconnect the outlet and measure what the pump actually delivers into a jar in a timed period.
    4. Disconnect the inlet, since Burlen note that if the pump then operates freely the trouble is a restriction upstream.
    5. Clean the tank drain-plug gauze and the pump's own filter before opening the pump.

    Source: Burlen / SU Carburetters, Electric Fuel Pump Fault Diagnosis, SU Specialists, SU Electric Fuel Pumps, Dave DuBois, SU Fuel Pumps 101

24. Fine at moderate speed, but held at high rpm or sustained wide throttle on a motorway or a long climb it goes progressively flat, surges, and picks up again when you ease off.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Air leak on the suction side of the pump, which DuBois says will usually show up as fuel starvation at higher speeds - a loose inlet union, a perished flexible section or a tired O-ring.
    2. Restriction ahead of the pump, such as a silted tank gauze or a partly blocked pickup that copes with low demand and not high.
    3. Float chamber refill rate below demand, the needle valve orifice becoming the limit; Brandon Shriver reports this on an XK140 with a modified 3.8 engine, partly relieved by raising float height, and Michael Frank reports a specialist drilling valves from .070 to at least .10 for race use.
    4. Float level set low, so the reserve in the chamber is small.
    5. An in-line filter retro-fitted with too high a pressure drop; Nick (Nick_53_XK120_OTS) warns the SU system runs at only about 2.5-3 psi while many in-line filters cost 1-2 psi.
    6. A tired diaphragm or points giving adequate low-demand flow and no reserve.

    What to check · Cheapest and easiest first

    1. Slacken the filler cap to rule out tank venting first, every time.
    2. Run the pump outlet into a jar and watch for a stream of bubbles, DuBois's and Burlen's test for an air leak on the suction side, Burlen submerging the discharge pipe and treating continuous emission of bubbles as confirmation.
    3. Go over every joint from tank to pump by hand for tightness, and look at the condition of any flexible sections.
    4. Check float levels in both chambers before touching a needle valve seat, which is the contributors' own order of work.
    5. If a filter has been added, find out its pressure drop, and if that is unknown remove it and see whether the symptom goes.

    Applies to: All XK140. The needle-valve-orifice limit is reported only by owners of modified and enlarged engines (3.8-litre conversions, race use); no evidence was found that it limits a standard 3.4 in standard tune, and it should not be presented to a standard-car owner as a likely cause.

    Source: Dave DuBois, SU Fuel Pumps 101, Burlen / SU Carburetters, Electric Fuel Pump Fault Diagnosis, jag-lovers XK-Engine, Carb needle-valve size (Brandon Shriver, Michael Frank)

25. Stop for petrol or a pub lunch on a hot day, come back, and it either will not fire or fires and then coughs, spits and hunts for five to eight minutes before clearing; sometimes a smell of petrol and visible fuel weeping from the carburettor vents.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Heat soak rather than classical vapour lock; Mike Eck's mechanism is that while the car is running the carburettors are continuously provided with a supply of cool fuel, and after shutdown that flow stops and the fuel standing in the float chambers boils at under-bonnet temperature.
    2. Higher volatility of modern petrol, ethanol-blended in particular; Knight Martorell reports the problem worsened with ethanol blends, and Raindog and Roger King both report avoiding it by using non-ethanol or 97 RON fuel in hot weather.
    3. Vapour forming in the line or pump where it passes close to heat, the classical vapour lock, which Gene Burda raised and which does also occur.
    4. Reduced airflow through the engine bay; Bruce Baysinger notes removing the radiator cowling prevents proper airflow and increases running temperatures.
    5. Heat shield or carburettor insulator missing, wrong or ineffective; Bob Knijnenburg questions the thermal performance of the phenolic insulators.

    What to check · Cheapest and easiest first

    1. Establish when it happens: trouble after a stop on a hot day is heat soak, while trouble while running that worsens with time and load is more likely vapour in the line.
    2. Look into the carburettor mouths and at the vents shortly after a hot shutdown, since visible or smellable fuel confirms boiling in the chambers.
    3. Note what fuel is in the tank and whether the problem tracks the fuel grade.
    4. Check the radiator cowling and all the under-bonnet ducting is present and correct.
    5. Check the carburettor insulators and any heat shielding are fitted and intact, and look at how close the fuel line runs to the exhaust.

    Applies to: All XK140, markedly worse in hot ambient conditions and on ethanol-blended fuel. SB 167 (Mar 55), engine G2841 onward, fitted a 73 C thermostat (C3731/1, stamped 43570/5) in place of the 60 C type, addressing cold running of about 55 deg and spitting back from the carbs on full throttle - so a pre-G2841 car may have either, and a car's thermostat rating bears on its under-bonnet temperature.

    Source: jag-lovers XK, Vapor Lock Season - Cure (Mike Eck, Knight Martorell, Kenneth Wallace, Bob Knijnenburg, Allenby, Raindog, Roger King), jag-lovers XK library, XK140 running problems (Gene Burda, Bruce Baysinger), XK140 Service Bulletins 1954-57, SB 167 (Mar 55)

26. Fuel weeping at unions and float chamber joints, a rubber fuel hose soft, swollen or sticky and sometimes breaking up inside where you cannot see it, debris appearing in the float chambers from nowhere, a carburettor that floods or starves, and a pump that has gone lazy.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Elastomer attack; Burlen note some types of elastomers and polymers used for moulded and flexible rubber components may swell, shrink, begin breaking down or leak, and the FBHVC add that these materials swell and soften, weakening the elastomer structure, then on drying out can shrink and crack resulting in fuel leaks.
    2. Hose degrading from the inside out; Burlen report disintegration on the inside surface of rubber fuel hoses from E5, so the outside can look serviceable while the bore sheds material into the carburettors.
    3. Fuel pump diaphragm changed in character; Burlen are precise that thin rubber membranes in fuel pumps do not rot or split from contact with biofuel but do become either too supple or in many cases very hard so they no longer perform well.
    4. Rubber-tipped float needle valves degrading, which Burlen identify as needing upgrading and now supply with Viton tips.
    5. Main metering needles; Burlen identify original brass needles as vulnerable and now supply nickel silver (brass plus 20 percent nickel).
    6. Gum and varnish deposits and seized valve discs from fuel left standing; Burlen note ethanol content creates gunky residue which can stick things in place, a ring of residue around the outside of the disc indicating fuel buildup where it has been left to stand.
    7. Corrosion and tarnishing of metal parts; the FBHVC note ethanol has increased acidity, conductivity and inorganic chloride content and can cause corrosion and tarnishing of metal components.
    8. Failed tank sealant, which Burlen report failed most commonly of all, taking out the fuel system downstream.

    What to check · Cheapest and easiest first

    1. Flex every rubber hose and look at a cut end or into the bore, since external appearance is not evidence.
    2. Look for weeping at float chamber joints, jet glands and unions after a run rather than when cold.
    3. Look in the float chambers for rubber crumb, gum or silt.
    4. Check whether the car's soft parts are original or modern replacements, and how long the car has stood between uses.
    5. If the tank has ever been sealed internally, treat that sealant as suspect.

    Applies to: All XK140 running on E5 or E10 pump petrol, and especially cars laid up between uses. Burlen note Mazak carburettor bodies show only a light varnish or gum which has not become a problem, though they do not officially recommend Mazak for biofuel use.

    Source: Burlen Fuel Systems, ethanol article (hosted by the MG V8 Register), SU Specialists FAQ, Burlen / SU Carburetters, SU fuel pump guide

27. Owners are warned that ethanol-blended petrol attacks brass or copper carburettor floats and the solder joining their halves, giving a sinking float, flooding and fuel leaks.

    CarburationMonitorConfidence: Low

    Likely cause · Most likely first

    1. Claimed attack on the soldered seam of a brass or copper float, warned about by Guy Lachlan of Classic Oils in Hagerty UK's E10 guide.
    2. Burlen, who own SU and manufacture the floats, state the opposite: no damage has been seen with traditional brass floats and no signs of biofuel attack on the solder joining the upper and lower sections.
    3. Plastic float buoyancy affected by ethanol, which is what the FBHVC actually warn about, advising that carburettors be checked so that float levels are not adversely affected causing flooding and fuel leaks.

    What to check · Cheapest and easiest first

    1. Check float levels, and look for fuel inside the float, which is the only direct evidence of a leaking float.
    2. Establish whether the float is the traditional brass type or a plastic type such as the StayUp, since the FBHVC buoyancy warning applies to plastic.
    3. Look for flooding, a wandering fuel level or weeping at the float chamber before attributing anything to ethanol.

    Applies to: All XK140 running E5 or E10 pump petrol, to the extent the claim holds at all.

    Source: Burlen Fuel Systems, ethanol article (hosted by the MG V8 Register), Guy Lachlan, MD of Classic Oils, quoted in Hagerty UK's E10 guide, FBHVC fuels guidance (specialist Nigel Elliott)

28. Cork and fibre sealing washers in the carburettors found hardened, shrunken or weeping, and commonly blamed on ethanol in modern petrol.

    CarburationMonitorConfidence: Low

    Likely cause · Most likely first

    1. Ordinary age and service failure of the cork gland washers in the jet assembly and the fibre washer on the float pivot spindle, which are routine renewal items and routine leak points.
    2. Ethanol as the mechanism, which is very widely repeated but which the research could not source from Burlen, SU, AMAL or the FBHVC.

    What to check · Cheapest and easiest first

    1. Inspect the jet cork gland washers and the fibre washer on the float pivot spindle for hardening, shrinkage and weeping.
    2. Renew them as routine service items rather than treating the fault as ethanol damage.

    Applies to: All XK140 with twin SU H6 carburettors.

    Source: Burlen Fuel Systems, ethanol article (hosted by the MG V8 Register), FBHVC fuels guidance (specialist Nigel Elliott), Burlen / SU Carburetters, H Type Carburetter Dismantling

29. Recurring, unpredictable starvation - runs fine for weeks then stumbles - with blocked gauzes found in the tank drain plug and in the pump, and grit or fine rust-coloured dust in the float chambers, often worse shortly after filling up or after the car has stood.

    FuelInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. No filter element in the system as built, only gauzes: Rob Reilly's inventory is a brass screen filter in the gas tank drain plug, a brass or plastic screen inside the fuel pump, and brass screens in the carb float bowl banjos, all coarse strainers.
    2. Tank corrosion producing fine rust dust; Rob Reilly documents two starvation incidents from ultra-fine rust dust, one blocking the pickup in his XK120's tank drain and another accumulating in the float bowls despite those multiple brass screens.
    3. Sediment and scale disturbed by a fill-up or by cornering.
    4. Failed internal tank sealant from a past restoration.
    5. Debris from a perishing rubber inlet hose, Jim Voorhies's diagnosis.
    6. A retro-fitted filter that has done its job and blocked; Nick reports a car stalling within a mile of refuelling on a clogged 20-micron in-line filter.

    What to check · Cheapest and easiest first

    1. Pull and inspect the tank drain plug gauze, the cheapest and most informative single check in the fuel system.
    2. Inspect the gauze inside the pump and the small screens in the float chamber banjos.
    3. Look at what is in the float chambers: orange-brown dust says tank, black crumb says hose, grey silt says sediment.
    4. Correlate the symptom with refuelling and with periods of standing.
    5. If a filter has been added, note its micron rating and pressure drop and whether it sits before the pump; Roger King warns against glass-bodied filters in the engine bay on fire grounds.
    6. With the tank sender or drain out, inspect the tank interior for corrosion and sediment.

    Applies to: All XK140. The XK150 gained an AC glass-bowl paper-element filter and Bob Knijnenburg reports retro-fitting the XK150 filter to his car without pressure problems, so the XK150 arrangement is the factory precedent an XK140 owner usually reaches for. Iain Buxton's XK140 (chassis S812797DN) is a documented case: tank contamination causing petrol starvation, a pump needing rebuild, and a sender beyond repair.

    Source: jag-lovers XK, In line fuel filter (Rob Reilly, Nick_53_XK120_OTS, Bob Knijnenburg, Dave C, Roger King), jag-lovers XK, Jaguar XK140 Fuel Tank Filter - replace or clean (Brad, Nick, Bob Knijnenburg), jag-lovers XK, XK140 Petrol Tank Removal (ILOBReno (Iain Buxton), jagmike90 (Mike Kurtzweil))

30. Hunting, uneven idle that will not come down or stay put, mixture that appears weak no matter what you do, a hiss or whistle from the carburettors at idle, and in bad cases burnt exhaust valves found later.

    CarburationInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Throttle spindle and body bush wear letting unmetered air past the spindle; Charles Bishop is emphatic that this must be checked before any adjustment, because worn bores can lean out an engine and cause burned valves.
    2. Perished or missing throttle spindle seals.
    3. Air leaks at the manifold and carburettor flange joints, or at a cracked or porous insulator.
    4. A leaking auxiliary starting carburettor; Burlen note any leakage between the valve and its seating would allow the device to operate and affect the idling setting of the main carburetters.
    5. A disconnected, split or blocked vacuum line to the distributor.
    6. The two carburettors genuinely out of balance, which is the thing everyone adjusts and usually the last thing actually wrong.

    What to check · Cheapest and easiest first

    1. Try to move each throttle spindle sideways in its bearings; Burlen instruct examining the throttle spindle and its bearings in the carburetter body and checking for any excessive play.
    2. Listen at the spindle ends and flange joints at idle.
    3. Check every vacuum connection and the carburettor and manifold fasteners.
    4. Isolate the auxiliary starting carburettor before chasing the main carburettors.
    5. Only then balance and set the mixture, since adjustment applied over a mechanical air leak is wasted work.

    Applies to: All XK140 with twin SU H6 carburettors; wear is a function of mileage and of how many times the linkage has been worked on, not of chassis number.

    Source: Charles Bishop, Adjusting SU Carburetters (jag-lovers XK library; author identified as Charles Bishop, Jaguar XK #677556), Burlen / SU Carburetters, carburetter inspection (throttle spindle and bearings, play), Burlen / SU Carburetters, Auxiliary Enrichment Carburetter

31. A flat spot as you open the throttle, mixture that will not come right at both ends, heavy fuel consumption and sooty plugs with no other explanation, and a piston that does not fall back cleanly when lifted by hand.

    CarburationMonitorConfidence: High

    Likely cause · Most likely first

    1. Worn jet and needle, which SU describe as a worn needle or jet that has been elongated through years of use, and which must be treated as a matched pair.
    2. Jet off-centre or a bent needle rubbing in the jet; Bishop notes that any binding means the main jet could be off-centre or the needle bent and rubbing in it.
    3. Needle not seated correctly in the piston; Bishop makes it a hard requirement that the shoulder of the needle boss is flush with the base of the piston.
    4. Piston sticking in the suction chamber from scoring, distortion, dirt or damage.
    5. Dashpot run dry, which SU say produces flat acceleration spots due to excessive piston rise.
    6. Wrong needle for the car's specification.
    7. Worn or damaged float needle and seating, giving a wandering fuel level.

    What to check · Cheapest and easiest first

    1. Check the dashpot oil, the cheapest possible explanation for a flat spot.
    2. Lift each piston and let it drop; it should spin freely in the suction chamber without any tendency to stick, and for bores 38.0-47.6 mm SU give a fall time of 5 to 7 seconds, longer suggesting the presence of oil, foreign matter or damage.
    3. Feel for binding as the jet is moved through its range, Bishop's test for an off-centre jet or bent needle.
    4. Check the needle shoulder is flush with the piston base.
    5. Read the plugs, and use Bishop's piston-lifting test to attribute rich or weak running to the front or rear carburettor.
    6. Inspect the jet and needle for a wear step or ovality, and the float needle and seat for damage.
    7. Establish which needle the car should have and which it has.

    Applies to: All XK140, with a documented specification history. SB 168 (Jun 55), engines G3250 onward: standard carburettors with C-type heads and disc air cleaners had needles changed from SL to WO2, in connection with the spitting-back problem. SB 206 (Jan 57) on petrol economy gives standard SJ/WO2 and weak LBA/SL for C-type heads, and records plugs changed from N8B to NA8. SB 164 (Feb 55) carries tuning data.

    Source: SU Specialists FAQ (worn needle and jet as a matched pair, dashpot run dry), Charles Bishop, Adjusting SU Carburetters, Burlen / SU Carburetters, carburetter inspection (piston and suction chamber, fall time, float needle and seating)

32. Two opposite faces of the same device: either the engine runs rich, idles badly, smells and smokes and the main carburettors cannot be made to behave, or the car simply will not start from cold without a great deal of cranking while starting perfectly when warm.

    CarburationInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Auxiliary starting carburettor valve leaking past its seating, so it enriches when it should be shut; Burlen note any leakage between the valve and its seating would allow the device to operate and affect the idling setting of the main carburetters.
    2. Jet or diaphragm in the starting carburettor failed; MGCJAG's diagnosis is that leaking fuel means the jet or diaphragm needs replacing.
    3. The thermostatic Otter switch in the cylinder head coolant jacket failed closed, holding the device energised when warm; Burlen describe it as set to bring the apparatus into operation below 35 C (95 F).
    4. The Otter switch failed open, or its solenoid not energising, giving no cold enrichment at all.
    5. Gum and deposits in the jet region from long standing; Edgar Blake's case had been non-functional for twelve years.
    6. Anti-backfire valve under the manifold fouled after inactivity.
    7. Float level wrong, showing as fuel standing visible in the side chambers, which was Rgbrt_Mna's case and was resolved at the rear carburettor lever.
    8. The device disconnected or bypassed by a previous owner and never recorded.

    What to check · Cheapest and easiest first

    1. Establish whether the device is even connected, and whether it is the automatic Otter-switch version or the manual-switch version, which has a warning light.
    2. Feel whether the fault tracks engine temperature: rich when warm points at the switch or valve failing to release, while no cold start points at the switch or solenoid not energising.
    3. Check for supply at the solenoid, cold and hot, and whether the Otter switch is opening as the engine warms.
    4. Listen for the solenoid operating, and test with the throttle open, since Burlen note that if it energises at idle the valve often will not lift because manifold depression is high.
    5. Look for fuel standing in the side chambers, and check float level before dismantling anything.
    6. Isolate the device entirely and see whether the main carburettors can then be set, the clean way to separate this from throttle spindle wear.

    Applies to: All XK140 with the automatic starting carburettor. The Otter switch's mounting set screws are, per the jag-lovers library discussion, highly subject to breaking off during removal, so a diagnostic inspection here can turn into a bigger job, which the owner should know before starting.

    Source: Burlen / SU Carburetters, Auxiliary Enrichment (Thermo) Carburetter, jag-lovers XK library, Starting Carburettor (Rob Reilly, Dick Clements, Mike Morrin, Edgar Blake), jag-lovers XK-Engine, The auxiliary starting carburettor (Rob_Reilly, MGCJAG, Rgbrt_Mna)

33. Set the timing and it is wrong again shortly after, idle speed wanders, and under a timing light the mark jitters or wanders over several degrees rather than sitting still, often with general woolliness and poor high-rpm performance.

    IgnitionInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Distributor shaft and bush wear, letting the shaft move so the points gap - and therefore the timing - varies as it turns.
    2. Mechanical advance mechanism worn or seized; Bill Fair (140 FHC) identifies weakened springs, broken counterweights and rusted shafts as affecting the advance curve and potentially leaving the timing retarded, and Bruce Cunningham notes a failed mechanical advance prevents proper timing progression at higher rpm.
    3. Vacuum advance capsule perforated; Arno Wahl found the membrane in the little thing next to the distributor had given up, and Mike Plechaty notes these units fail after 40-plus years.
    4. Vacuum line blocked; Klaus Nielsen found copper vacuum pipes clogged with fifty years of sludge.
    5. Internal wiring failure; Michael P Moore found frayed wires in several locations, including the very fine wire connecting to the moving advance plate.
    6. Distributor body damaged or the clamp not holding; Moore's unit had two very deep grooves in the body from repeated forcing of the clamping ring, and a deformed clamp screw.
    7. Points gap closing up as the heel wears.

    What to check · Cheapest and easiest first

    1. Watch the timing mark under a strobe and see whether it is steady or scattered, since scatter is the signature of mechanical wear and distinguishes this from simple maladjustment.
    2. Check the clamp actually grips and the distributor cannot be turned by hand.
    3. Try to rock the distributor shaft laterally at the rotor.
    4. Check the vacuum line is connected, unblocked and not split, and test the capsule for leakage.
    5. Watch the advance come in as revs rise, and check it returns.
    6. Check the points gap has not closed.

    Applies to: All XK140, worse with age and with each past disturbance. Larry Martz cautions that MANY different distributors were used across XK models and that timing figures are model- and distributor-specific; Michael P Moore's XK140 unit is identified as DVXFA 40199E, so establish your own distributor's identity before accepting any published figure. Jaguar's published curve for an 8:1 XK140 - 10 degrees static, advance from 1200 rpm, 25 degrees mechanical, full advance 4500 rpm, 35 degrees max mechanical total, 8 degrees vacuum - is the published reference, not a universal.

    Source: jag-lovers XK, XK140 Distributor rebuild (Michael P Moore, Paul Wigton, Nick), jag-lovers XK library, Overheating (Arno Wahl, Mike Plechaty, Bill Fair, Bruce Cunningham, Klaus Nielsen), jag-lovers XK library, Ignition Timing (Mike Eck, Larry Schear, Larry Martz)

34. Intermittent misfire, worse under load, hard starting, or a clean cut with no warning, with points found blackened, pitted, or with the gap closed up.

    IgnitionInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Points burnt, pitted or closed up with heel wear.
    2. Condenser failed or failing, the classic intermittent and the classic hot-weather failure.
    3. Points affecting the coil's recovery; Nick_53_XK120_OTS notes the points greatly affect the coil's ability to recover and recharge quickly, so a coil is often wrongly blamed.
    4. Distributor cap tracking, or a cracked cap, especially when damp or hot; Roger_McWilliams raises temperature-dependent failure in caps and connections rather than in the coil.
    5. Rotor breakdown.
    6. HT leads and suppressors degraded.
    7. A loose or corroded CB or SW connection on the low tension side.

    What to check · Cheapest and easiest first

    1. Inspect the points for gap, colour and pitting, and the cap inside and out for tracking lines.
    2. Check the rotor and the cap's centre carbon.
    3. Check the low-tension connections at the coil and distributor for tightness and corrosion.
    4. Substitute a known-good condenser, which is cheaper and quicker than diagnosing one in situ.
    5. Check the spark quality at a plug lead; Rob_Reilly used a simple ignition tester light and compared a dim spark with a bright one.
    6. If it is heat-related, get the engine to the failure condition before testing anything.

    Applies to: All XK140. SB 206 (Jan 57) records plugs changed from N8B to NA8, so plug specification is a live question on a car of unknown history.

    Source: jag-lovers XK, Ignition Coil Failure (Nick_53_XK120_OTS, Roger_McWilliams, Robert_Wilkinson, Rob_Reilly, Mike_Balch, D_J2), jag-lovers XK library, XK140 running problems (extended points and condenser discussion, including Mike Morrin (B.E.E.)), XK140 Service Bulletins 1954-57, SB 206 (Jan 57)

35. Temperature climbs, especially in traffic or on a climb, with radiator, pump, thermostat and hoses all checking out or replaced and the car still running hot; sometimes pinking on a climb.

    IgnitionInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Retarded ignition timing, including timing retarded in effect because the advance mechanisms are not working; Bill Fair's point is that weak springs, broken counterweights and rusted shafts leave the timing retarded and the engine hot.
    2. Vacuum advance not functioning; Arno Wahl's overheating was cured by the vacuum capsule diaphragm and Klaus Nielsen's by clearing the blocked vacuum pipe.
    3. Weak mixture; Cleo Bay (XK120 and XK140) advises reading plug colour, with grey or tan indicating lean, and lean raising combustion temperature.
    4. Timing set from the wrong reference; Skip Smith's case had a 180-degree flywheel misalignment making correct timing unobtainable and causing overheating with a rough idle, and Rob's case was corrected by reversing ignition leads to restore the number 6 cylinder sequence.
    5. Reduced airflow with the radiator cowling removed or ducting missing.
    6. Exhaust restriction; Wray Schelin proposes a corroded internal silencer flap acting as a butterfly valve and trapping heat.
    7. Thermostat rating wrong for the engine number.

    What to check · Cheapest and easiest first

    1. Read the plugs for mixture before anything else.
    2. Check the timing with a strobe, and check the advance actually comes in with revs and with vacuum.
    3. Check the vacuum line is connected, clear and not split.
    4. Confirm the cowling and ducting are all present.
    5. Confirm the firing order and that the timing reference is genuinely TDC on number 1.
    6. Only then suspect the exhaust or the cooling system itself.

    Applies to: All XK140. SB 167 (Mar 55), engine G2841 onward: thermostat C3731/1 (stamped 43570/5) opening at 73 C replaced the earlier 43570 type opening at 60 C, introduced to address cold running of about 55 deg and spitting back from the carbs on full throttle - so an owner chasing a heat problem needs to know which thermostat his car should have and which it has.

    Source: jag-lovers XK library, Overheating (Arno Wahl, Mike Plechaty, Bill Fair, Bruce Cunningham, Klaus Nielsen, Cleo Bay, Skip Smith, Wray Schelin), jag-lovers XK library, XK140 running problems (Bruce Baysinger), JCNA forum, XK140 FHC overheating: water pump? (Bob Grossman, George, Yves)

36. Runs well from cold, then after anything from ten minutes to three quarters of an hour it falters, backfires, drops onto fewer cylinders and stops; leave it to cool and it starts and runs again, and the cycle repeats, sometimes worst just after you stop when airflow ceases.

    IgnitionInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Coil breaking down at operating temperature; Gene Burda attributes it to coil overheating, Ron and E.W. Blake report coil replacement curing identical symptoms on an XK150, and Nick_53_XK120_OTS confirms coils are negatively affected by heat and that XK drivers routinely carried spare coils.
    2. Coil position and radiated heat; Rob_Reilly observed his failures occurred when the engine was hottest after stopping, when airflow ceased and radiated heat increased toward coil locations.
    3. Coil suppressor in the wrong place; SB 186 (Apr 56) attributes coil failure to the suppressor in the HT lead between coil and distributor being in the end of the coil instead of in the centre terminal post of the distributor cap.
    4. Thermal ageing, Andrew_Waugh cautioning that electrical devices all have a thermal life.
    5. Loss of internal dielectric oil; Rob_Reilly notes his coils had a screw plug in the top for filling them with dielectric oil and RJBasso suggests shaking a coil to check for internal fluid, but neither confirmed it as a cause.
    6. Not the coil at all, but a distributor cap or connection failing at temperature, or points limiting coil recovery.
    7. Fuel side heat soak or vapour lock presenting similarly.

    What to check · Cheapest and easiest first

    1. Check where the suppressor is, a free check with a documented factory basis in SB 186.
    2. When it dies, check for HT at a plug lead before anything else, which splits ignition from fuel in one move.
    3. Feel the coil and note its position relative to the manifold and exhaust, remembering Rob_Reilly's coils felt warm but not excessively hot, so touch alone is not diagnostic.
    4. Measure primary resistance across the CB and SW terminals as one input and not a verdict; Rob's read 3-4 ohms, the running-problems page discusses 4.0-4.4 ohms primary and 2.9 A stall current, and Mike_Balch warns that none of the three or four separate coil tests is conclusive in isolation.
    5. Test the coil hot, not cold: Rob_Reilly's removed coils later functioned normally when tested cold, and Charles (cb1) describes heating the coil about five minutes and retesting for breakdown at operating temperature.
    6. Check points and condenser before buying a coil.

    Applies to: All XK140. SB 186 (Apr 56) is the specific factory item. On ballast, D_J2 states they didn't use ballast resisters till the '60s and Mike_Balch that the Lucas coils do not need a separate ballast resistor because one is built into the Lucas coil - relevant if a modern coil has been fitted, because the ballast question changes with the part.

    Source: XK140 Service Bulletins 1954-57, SB 186 (Apr 56), jag-lovers XK, Ignition Coil Failure (Rob_Reilly, Nick_53_XK120_OTS, Roger_McWilliams, Robert_Wilkinson, Mike_Balch, Andrew_Waugh, Charles (cb1), D_J2, Ed_Nantes), jag-lovers XK library, XK140 running problems (Gene Burda, Ron and E.W. Blake, RJBasso, Klaus Nielsen, Ken Boetzer)

37. Either overcharging - a battery needing topping up constantly or smelling, bulbs failing in runs, headlamp brightness visibly fluctuating - or undercharging, with a battery flat after a few outings despite the engine running well; sometimes an audible chattering from the control box on the bulkhead.

    ElectricalInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Regulator contacts dirty, pitted or maladjusted; the RB310 is a three-unit box covering voltage, current and cut-out, and Jim Heavner notes it has three sets of contacts so there is more to go wrong than in a simpler regulator.
    2. Regulator contacts sticking, giving continuous full output; Steve McDonald describes the regulator sticking and causing a full charge into the battery, and Dave Quirt reports the same, often preceded by noticeable fluctuation in headlight brightness.
    3. Cut-out relay not closing, giving no charge, or not opening, so the battery discharges back through the dynamo at rest.
    4. Regulator coil degradation, which Matt Kennedy assesses as unusual unless the lacquer on the wire has degraded or corrosion and dirt cause current bypass.
    5. Clearances in the box out of specification.
    6. Riveted internal connections gone high-resistance, which Rob Reilly suggests soldering for reliability.
    7. The dynamo itself faulty, and Cleo Bay warns a sticking regulator can burn out the generator's armature, so an uninvestigated regulator fault becomes a dynamo bill.

    What to check · Cheapest and easiest first

    1. Watch the ammeter and the headlamps at different revs and loads, since fluctuating headlamp brightness is Dave Quirt's early warning.
    2. Look at the battery, since needing frequent topping up or smelling is overcharging, full stop.
    3. Open the box and look at the three sets of contacts; Rob Reilly cleans them by dragging a strip of sandpaper through them and Pat Harmon suggests very fine emery, 400 grit or finer, and alcohol.
    4. Follow the test procedure in the Service Manual, since this is a box with specified clearances that should not be guessed at.
    5. Test the dynamo output independently before condemning the box, which is Matt Kennedy's point.

    Applies to: All XK140 as built (positive earth, dynamo, Lucas RB310 control box). Cars converted to an alternator will normally have had the RB310 removed - Limora's instruction sheet states the RB 310 voltage regulator can now be discarded after the alternator rewiring.

    Source: jag-lovers XK, Lucas RB310 Voltage Reg Problems (Gezzler, Matt Kennedy (Kendoo), Nick_53_XK120_OTS, Rob_Reilly), jag-lovers XK, Voltage/Current Regulator Lucas RB 310 (Jim Heavner, Pat Harmon, Gary King, Shane (SWA1)), jag-lovers XK library, Ammeter (Steve McDonald, Dave Quirt, Cleo Bay, Neville Laing, Rob Reilly)

38. Following a conversion from positive to negative earth, the ammeter needle goes the wrong way, the radio is dead and sometimes smells, the battery does not charge, and occasionally nothing works properly at all.

    ElectricalInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Ammeter connections not reversed; Simonpfhc says the only wiring that needs to be changed is to swap over the ammeter connections, and Limora's sheet states that a car with an ammeter will need it reversed or it will read backwards.
    2. Dynamo not re-polarised, since without flashing the field for the new polarity the dynamo will not charge.
    3. Coil primary connections not swapped.
    4. A polarity-sensitive radio destroyed, the best-documented casualty - Len_Brighton reports he fried his radio in the process and Jaginabox1 reports the same, two independent reports in one thread.
    5. Electrolytic corrosion of alloy components, which Roger Payne raises as referenced research on the grounds that the factory earth straps were positioned for positive-earth operation.

    What to check · Cheapest and easiest first

    1. Establish first whether the car has been converted at all, since an XK140 is positive earth as built.
    2. Check which way the ammeter deflects with a known load on and the engine off.
    3. Check whether the dynamo charges at all, and if not suspect polarisation before the regulator.
    4. Check the coil primary connections.
    5. Identify every non-original electrical accessory - radio, clock, electronic ignition, electric fan, electronic fuel pump - and check each for polarity sensitivity before switching anything.

    Applies to: All XK140, which are positive earth as built; Limora's alternator-conversion sheet states All XK's were Positive Earth and identifies the standard ammeter as the BM4. Brad's XK140 pump was specifically a dual polarity type.

    Source: jag-lovers XK, Convertsion from positive ground to negative ground on XK140DHC (ex_jag1, Len_Brighton, Jaginabox1, Mike_Eck), jag-lovers XK, Positive or negative earth? (Rob Reilly, Simonpfhc, Roger Payne, Phil.Dobson), Limora, XK140/XK150 alternator conversion instructions

39. Following a dynamo-to-alternator conversion the ammeter pegs, reads implausibly, or the owner is told the gauge cannot be used; or an owner researching the conversion cannot find out whether the kits work.

    ElectricalMonitorConfidence: Medium

    Likely cause · Most likely first

    1. Alternator output exceeding the original ammeter's capacity; Kirbert identifies this as the primary technical obstacle, since modern alternators exceed the original gauge's roughly 35-40 A range.
    2. Wiring loom capacity, with Limora careful to claim only that their own kit will not blow out the standard BM4 ammeter nor exceed the wiring loom's maximum capability, implying that not every conversion can say that.
    3. The RB310 left in circuit when it should have been removed.
    4. Polarity, since Rob Reilly notes alternators were made only for negative earth, which is why the E-type switched systems, so an alternator conversion on an XK140 usually drags the negative-earth conversion in with it.
    5. Unknown product quality; Etch reports finding no reviews for Dynamator or Coventry Auto Components units.

    What to check · Cheapest and easiest first

    1. Find out what the car actually has: dynamo with RB310, alternator, or a Dynamator type unit in a dynamo body.
    2. Identify the alternator's rated output and compare it with the ammeter's range and the loom's capacity.
    3. Confirm the RB310 has been removed or correctly bypassed if an alternator is fitted.
    4. Confirm the car's earth polarity and that the ammeter is connected for it.

    Applies to: All XK140 as built, and a large proportion of surviving cars since the conversion is common. On the thread found, the contributor asking about the conversion abandoned it: Etch decided instead to rebuild the original dynamo for about $200 after consulting an auto-electric technician, and the thread contains no substantiated complaints about control boxes, wiring, warning lights or belts - the objections are cost, ammeter capacity, and not knowing whether the products are any good.

    Source: jag-lovers XK, Alternator Conversion on XK140 (Kirbert, Etch, Len Wheeler), jag-lovers XK, Positive or negative earth? (Rob Reilly), Limora, XK140/XK150 alternator conversion instructions

40. Odd, apparently unconnected faults together - dim lamps, a gauge reading wrongly, an indicator that will not flash, one rear lamp abnormally bright, instrument lights out - and things that improve if you press on a connector or shake the harness.

    ElectricalInvestigate soonConfidence: High

    Likely cause · Most likely first

    1. Poor or missing earth at a component, so its return current finds a path through another circuit, which is why the symptoms look unconnected.
    2. Engine-to-frame earth strap corroded, loose or missing; on the XK140, sk99 places one end at the starter and the other where the engine mounting bolts to the frame.
    3. Earth via a component's case; Eric Capron notes the DB10 flasher relay's ground path runs through the relay case and the relay must be correctly grounded for the relays to work.
    4. Corroded internal contacts in the chromed tail lamp assemblies causing shorts.
    5. Gauges earthing through each other; Bill Burke diagnosed a grounded backing plate behind the gauge cluster causing cross-gauge interference, and removed all the gauges to earth each while isolating them from one another.
    6. Dirty connections and fuse clip corrosion, the routine cause of lighting circuit failures.
    7. An intermittent short; Larry Schear isolated a bad harness by systematic circuit disconnection and flags shorts in instrument lighting circuits as a fire hazard.

    What to check · Cheapest and easiest first

    1. Find, clean and remake every earth before diagnosing anything else, the cheapest and highest-yield electrical work on the car.
    2. Inspect the engine-to-frame strap at both ends for corrosion and tightness; Roger Payne notes his book Jaguar XK140 Explored details both earth straps and their exact locations.
    3. Look inside the rear lamp bodies for corroded contacts.
    4. Look for an unexpectedly bright lamp, the classic signature of current returning through the wrong path.
    5. Check the fuse clips and the main connector blocks for corrosion.
    6. Confirm the gauges are each earthed and each isolated from the others.

    Applies to: All XK140. peder reports that some XK140s have an extra earth cable between the fuel tank sender and the chassis that does not appear in the official wiring diagrams, so absence from the diagram is not proof it should not be there.

    Source: jag-lovers XK, Brake lights and rear directional not working (Rob Reilly, Phil Dobson, Eric Capron), jag-lovers XK library, Ammeter (Bill Burke, Larry Schear), jag-lovers XK, Engine to frame grounding strap location xk140 (sk99, Roger Payne, Mike Eck)

41. Indicate while braking and the brake light on that side goes out, or the indicator does not flash on that side; brake lights dead but indicators fine or the reverse; one side working and not the other; rear indicators that stop flashing when the lights are on.

    ElectricalInvestigate soonConfidence: Medium

    Likely cause · Most likely first

    1. The arrangement itself: the brake or stop function and the flashing indicator share the bright filament of the rear lamp, switched between them by the Lucas DB10 double relay, so brake and indicator cannot both use that filament by design and interference between them is the normal behaviour of a healthy car.
    2. DB10 relay failed, or not correctly earthed, the earth running via the relay case.
    3. Flasher unit failed.
    4. Direction indicator switch shorted or working on one side only.
    5. Brake light switch failed.
    6. Corroded internal contacts in the chromed tail lamp assemblies causing shorts.
    7. LEDs fitted; Roger Payne notes the Lucas flasher unit requires bulbs of a specific resistance to work, Clive Jervis identifies LEDs failing with Lucas flasher units for that reason, and paultg's actual fault turned out to be reversed LED polarity, corrected by rewiring.

    What to check · Cheapest and easiest first

    1. Before diagnosing anything, establish the design, since an owner reporting that the brake light goes out while indicating may be describing a correctly functioning car.
    2. Check for LEDs, and if fitted check their polarity and the flasher unit's compatibility before anything else.
    3. Check the bulbs and the lamp body contacts for corrosion.
    4. Check the DB10's earth, via its case, and then the relay itself.
    5. Work through the flasher unit, the indicator switch and the brake switch.
    6. Use an XK140-specific wiring diagram; the XK140 diagram in the jag-lovers library labels the rear units rh/lh stop tail and direction indicator lamp as combined assemblies.

    Applies to: All XK140. One of the jag-lovers cases was a cascade of simultaneous aged failures - a hairline crack in the light switch with damaged plastic contactors, a defective voltage regulator, a shorted directional switch, a failed flasher unit, a faulty brake switch and corroded tail lamp contacts - so on a long-stored car, finding one fault does not mean you have found the fault.

    Source: jag-lovers XK, Brake lights and rear directional not working (Rob Reilly, Phil Dobson, Eric Capron on the DB10 tag routing and shared bright filaments), jag-lovers XK library, XK140 wiring diagram (combined stop, tail and direction indicator rear lamp units), jag-lovers XK, 140 directional indicators (Roger Payne, Clive Jervis (clivejer), paultg)

42. Ammeter needle sitting on the wrong side, or moving the wrong way, or the owner unsure which side means charge; fuel gauge stuck at full or empty, or slamming to the top when fuel sloshes and falling back, or reading full with the ignition off.

    InstrumentsMonitorConfidence: High

    Likely cause · Most likely first

    1. Ammeter: connections reversed, usually after a polarity conversion; Mike Morrin states plainly that with the needle to the minus side the battery is being discharged.
    2. Ammeter: a non-original gauge fitted; Larry Martz found his Mk IX marked C on the left and D on the right, the opposite way round to Rob's XK140, so the face markings are not a reliable guide across cars.
    3. Ammeter: the gauge is telling the truth and the charging system is at fault, or there is an added load the owner has forgotten - in Rob's case a large auxiliary electric fan, with George Badger diagnosing a charging problem or excessive draw.
    4. Ammeter: alternator output beyond the gauge's range.
    5. Fuel gauge: loss of contact between wiper and wire-wound resistor in the tank sender.
    6. Fuel gauge: corrosion in the sender after long storage.
    7. Fuel gauge: a leaking float, with fuel inside the float the confirmation.
    8. Fuel gauge: a bent float arm fouling the tank.
    9. Fuel gauge: loose or shorted connections at either end, such as the power wire on the wrong terminal at the voltage regulator, or one or both of the other wires shorted to ground at the sending unit.
    10. Fuel gauge: earth problems, including an extra earth cable between sender and chassis which peder found exists on some XK140s despite not appearing in official diagrams.
    11. Fuel gauge: gauge internals out of adjustment, Rob_Reilly identifying magnet adjustment nuts inside the gauge that clamp the magnets in position, which is where peder's stuck gauge was resolved.
    12. Fuel gauge: gauges earthing through one another at the cluster backing plate.

    What to check · Cheapest and easiest first

    1. Ammeter: switch on a known load with the engine off and see which way the needle goes, since Neville Laing notes the behaviour with the engine off tells you whether the gauge is connected correctly.
    2. Ammeter: confirm the car's earth polarity and that the gauge is wired for it.
    3. Ammeter: total up the actual electrical load, including anything added such as a fan, lamps or a radio.
    4. Ammeter: check dynamo output before concluding the gauge is wrong.
    5. Fuel gauge: the two-wire test - disconnect both sender wires and the gauge should read empty, earth both to the chassis and it should read full - which separates gauge and wiring from sender.
    6. Fuel gauge: check for power at the gauge with the ignition off, where there should be none.
    7. Fuel gauge: check the earth at the sender, including any additional strap.
    8. Fuel gauge: check resistance through the sender across its travel.
    9. Fuel gauge: remove the sender and check the float for fuel inside it and the arm for bends.

    Applies to: All XK140. The standard ammeter is the BM4 (Limora). The extra sender earth strap is present on some XK140s only and is not in the official diagrams, which is a real trap.

    Source: jag-lovers XK library, Ammeter indication (Mike Morrin, George Badger, Larry Martz, Bill (XK140 OTS)), jag-lovers XK library, Ammeter (Neville Laing, Steve McDonald, Dave Quirt, Cleo Bay, Bill Burke, Larry Schear, Rob Reilly), jag-lovers XK library, Sticking fuel gauge