First, check the gauge
Before spending money, establish that the car is actually hot. One XK140 owner on the Jaguar Clubs of North America forum describes the gauge pegging in the Smoky Mountains, and a laser thermometer then showing the engine was running well below what the needle claimed. A cheap infrared thermometer pointed at the thermostat housing settles the question in a minute.
Source: JCNA forum, “XK 140 overheating”.
Read the pattern
The archive's fault database separates XK140 overheating by when it happens, because each pattern points somewhere different:
- Hot in slow traffic, fine on the open road — an airflow problem at low speed.
- Hot on sustained fast runs and climbs — a capacity or circulation problem.
- Worse after a new thermostat, or never warms up — the thermostat and bypass.
- Survives a new radiator and pump — blocked passages in the block.
- Weeping at the pump, rumble at the fan — the water pump.
- Sudden total coolant loss — hoses and the steel bypass pipe.
Each record lists likely causes and what to check, cheapest first, with its sources named.
The thermostat and the pressure cap
Original XK thermostats are of the bellows type, which open by pressure building inside the bellows. Bob Knijnenburg, on the Jag-lovers XK forum, points out that a radiator cap rated higher than standard raises the pressure in the system against which that bellows must work, so the thermostat may never fully open. If a modern high-pressure cap has been fitted to a car with a bellows thermostat, the two need to be considered together. Confirm the correct cap rating for your car against the workshop manual.
Source: Jag-lovers XK forum, “Choosing an electric fan” (Bob Knijnenburg); XK140 Service Bulletin SB 167 as cited in the fault database.
Silt in the block
The most repeated lesson from owners who tried everything else is that seventy years of scale collects in the water passages at the back of the block, where no flush reaches it. Rob Caveney, writing on the Jag-lovers XK forum about his 1955 XK140, fitted a new radiator, an upgraded pump and additives without curing it; when the engine was stripped and the core plugs removed, the passages were almost completely blocked. Removing a core plug or two and probing is a cheaper first look than a new radiator.
Source: Jag-lovers XK forum, “[xk] overheating 140” (Rob Caveney); Jag-lovers XK library, Cooling (Rob Reilly).
Fans
Electric fans are the commonest modern addition and owners disagree about them. Some fit a pusher fan in front of the radiator; others, in the Jag-lovers library, worry that a fan mounted there obstructs the airflow it is meant to help at speed. A thermostatic switch lets the fan run only when needed. On a positive-earth car, check the fan motor and switch suit the car's polarity before buying.
Source: Jag-lovers XK forum, “Choosing an electric fan”; Jag-lovers XK library, Cooling (John Spence).
Radiators
An aluminium radiator is a common upgrade. One JCNA forum contributor reports a noticeable drop over his rodded original, while cautioning that it is not a magic cure. The weight of owner experience above suggests fixing the block and thermostat first, since a larger radiator cannot cool water that is not circulating.
Source: JCNA forum, “XK 140 overheating”; Twyford Moors Classic Cars, cooling system upgrades (as cited in the fault database).