Jaguar XJ40 Overheating, Radiator and Cooling Fans
On the XJ40, running hot is nearly always airflow or a lost hose: a slipping fan clutch, dirt packed between the radiator and the A/C condenser, hardened shroud flaps, or the hidden hose under the intake manifold splitting. I'd check the coolant level and the real temperature first, then the belt fan and the gap behind the condenser, before paying for a radiator.
How bad is it · Garage, carefully
Drive it gently, and stop the moment the needle moves past its usual mark
The gauge sits still over a wide band, so by the time it moves the engine is already hot. A boil-over can take the head gasket and the exhaust valves with it.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
A worn viscous clutch on the belt fan
At idle the engine-driven fan does nearly all the work, and it only pulls as hard as its viscous clutch lets it. A tired clutch lets the fan slip just when the car stands still, and grime caked on the coil at its front makes it worse.
Fine on the move, but the needle climbs in traffic on a hot day and the A/C goes warm; it settles again once you drive off.
Engine hot and switched off, wait a moment and turn the fan by hand. A healthy clutch drags and stops within about a turn; a fan that freewheels has a worn clutch. Cold, it should still feel some resistance. The hand test is only a guide, so a borderline clutch can pass it.
Fit a new clutch; the factory part is EAC8948, and heavy-duty equivalents such as Imperial 215018 (Hayden 2765) do the job. The gauge should settle back at its usual mark. If the yellow plastic fan of early cars shows cracks at the blade roots, renew fan and clutch together; the later black blade is EBC4553.
Dirt packed between the radiator and the A/C condenser
The A/C condenser sits right in front of the radiator. Leaves, grass and insects collect in the gap between the two, out of sight, and the radiator slowly loses its airflow over the years.
Cooling that gets a little worse every summer, worst at idle with the A/C on; the A/C weakens at the same time.
Take off the cover plate over the radiators (four bolts and two cowl clips) and look down the gap with a torch. A lamp held behind the core shows how much light still gets through.
Blow compressed air backwards through the radiator, work a thin air line between the two cores, then rinse from the engine side with a garden hose; a vacuum alone won't shift packed dirt. Keep water off the gearbox cooler lines and don't flatten the fins. Badly blocked fins or tubes mean the radiator comes out to be cleaned or rodded by a radiator shop. Make sure nothing in front, such as a bent number plate, blocks the opening.
Shroud flaps gone hard or missing
The fan shroud carries rubber flaps that close off the opening round the electric fan and only swing open when it runs. Once they turn hard as cardboard or drop off, the belt fan pulls air round the edge instead of through the core.
Runs hot only when stationary or crawling, often after the clutch and thermostat have already been changed. Stiff flaps also let the belt fan draw hot air back from around the compressor.
Look at the shroud behind the radiator: the flaps should be soft, complete and lie flat against it.
Cut new flaps from 1.5 to 3 mm sheet rubber or neoprene, using an old one as a template, drill out the rivets and pop-rivet them back. The belt fan comes off first (four 13 mm nuts), and fan and shroud come out together. Don't simply leave the flaps off.
The hidden hose from the water pump to the water rail has split
Under the intake manifold, a short fat hose carries coolant from the back of the water pump to the water rail. It lives out of sight in the heat, rots unseen and lets go with a split along its length.
A sudden puddle of coolant under the front of the engine, steam from under the bonnet, the low coolant light, then a rising gauge.
Look down between the manifold runners with a torch at the hose under the injector rails. Dried coolant on it, or a wet block below, gives it away. A sudden flood is almost always a hose rather than the pump, which tends to drip; the metal pipe that takes the hose at the back of the pump can corrode through too.
It can be changed without taking the manifold off: remove the air cleaner box, the airflow meter and the air pump to reach it. Cut the old hose and its rear clip off rather than pulling, so the pipe isn't damaged; a 7 mm socket on a universal joint and long extensions reaches that clip. The pre-formed kit hose is the durable choice, fitted straight and not twisted; lubricate it and warm the end to get it on. A Gates 23067 cut to length has been used in its place; the hose is 1-3/4 in bore and 9 in long, and the genuine one has extra plies for the pressure there. While you're in there, do every hose; a full kit is a weekend job.
The auxiliary electric fan has seized
The small electric fan in front of the radiator runs so seldom that its bearings dry out and rust. When it is finally called on, the stiff motor draws too much current and blows the fuse it shares with the A/C clutch.
After a long fast run or a climb, the A/C suddenly blows warm, often with a fuse warning on the dash, and the little fan won't turn freely by hand.
Spin the fan by hand. Then unplug the thermo switch at the bottom of the radiator, bridge the two contacts in its plug with the ignition on and see whether the fan starts briskly.
A stiff motor can often be saved: drill a small hole in the closed end, soak both bearings with penetrating oil and then motor oil, and seal the hole; or strip, clean and grease it. That may only last a season or two, and a new fan is the lasting cure. Renew the fuse once the fan turns freely. On cars up to 1992 the fan isn't wired to the A/C; a fused link with a diode between the A/C clutch relay and the fan relay makes it run with the compressor, which helps in traffic.
No power to the fan: switch wiring, a connector or the relay
The fan is switched by a thermo switch at the bottom of the radiator through a relay; on 1993–94 cars a relay module behind the lower front valance runs both fans. A broken wire at the switch or a burnt terminal in a connector stops the fan as surely as a dead motor.
The fan runs when the switch plug is bridged or when it's fed straight from the battery, but never on its own, even with the gauge well up.
Ignition on, bridge the contacts in the switch plug: the fan should run. If it does, test the switch out of the car in hot water with a thermometer and a meter. If not, follow the wires to the relay and check each connector for voltage and earth with a meter; a test lamp won't show a high-resistance joint.
Repair the broken wire or splice round a burnt connector. On early cars the fan relay must be the five-pin DBC 2483, not the four-pin DBC 2486 that is sometimes supplied. A sticking relay may only need its contacts cleaning. A failed thermo switch is simply replaced; on early cars it is DBC 2145.
The thermostat has stuck shut
A thermostat that sticks shut keeps the coolant out of the radiator. The engine boils within minutes while the radiator stays cool to the touch.
Temperature shoots up soon after a restart or a short drive, steam from the tank, the top hose hot and the radiator cool.
Once hot, feel the top hose and the radiator. Out of the car, heat the thermostat in a pan of water with a thermometer and note when it opens. In an emergency, heater on full hot gives the coolant another way round.
Fit a new thermostat of the right rating, and the right way round. Avoid the cheapest units; a new one can fail within months. If the old one comes out with part of it missing, flush the radiator — the pieces end up there.
Another hidden hose, or the water rail gaskets
More hoses sit behind the intake plenums and at the back of the engine. A small split there puffs out coolant only when hot, and it boils off the engine before it reaches the ground.
The level drops every few days with no drips, no wet carpet and no white smoke; a smell of coolant or a wisp of steam once it's warm. Coolant pooling round the dipstick tube points at the rear water rail gasket; a puddle ahead of the engine mount, at the front one.
Run it up to full temperature with the bonnet up and watch the back of the engine, the hoses round the heater valve and the thermostat bypass hose behind the front plenums. Also the short hose from the thermostat housing to the water rail, the hoses near the fuel rail, and the water-heated valve under it; a coolant smell when the blower first starts suggests the heater core.
Replace the leaking hose or the water rail gaskets; the bypass hose needs the air box and air pump out of the way. If one hose of that age has gone, the rest aren't far behind.
A cracked header tank or radiator end tank
The coolant header tank and the radiator's end tanks are both plastic. Heat and age crack them, often along a seam or at the inlet pipe, and the leak can hide under the tank's label.
The low coolant warning at start-up and a cupful needed every week with no drip found; or overheating with a weep at the side of the radiator. A tank run nearly dry through a broken recovery pipe shows as a high gauge with only lukewarm air from the heater — and the low coolant light may not come on.
With the engine warm and the system under pressure, look along the header tank seams and under the label, then at the radiator's inlet and end tanks. White crystals along a seam mark a leak that dries as fast as it weeps. Check the small hose from the radiator top to the header tank, gently — its spigot is thin and snaps.
Renew the tank or the radiator. Epoxy won't stick to the glass-filled nylon, but a crack can be plastic-welded with a low-wattage iron and a cable tie as filler rod to keep you going. Keep the level topped up meanwhile. A snapped bleed spigot can be replaced by tapping the stub and fitting a brass or barbed spigot set in sealant, without tightening hard into thin plastic. If the tanks are sound and coolant still vanishes without a trace, try a new cap.
Both fans run all the time, even from cold
On 1993–94 cars the fans can be called by the relay module, by the thermo switch or by the A/C high-pressure switch. A stuck relay, a faulty pressure switch or a previous hard-wired link keeps them running with the engine cold.
Fans start with the ignition, or keep running after the car has been switched off and cooled down.
Unplug the A/C pressure switch (black two-pin plug): if the fans stop, it's the switch. A tap on the relay module that stops them means a sticking relay. Look at the relay base for joins between wires that aren't factory. With the A/C off, unplug the three-pin temperature sensor at the bottom left of the radiator: if the fans stop, the module is not the fault. The module is the long green unit on the charcoal canister under the left front wing, behind the undershield.
Replace the pressure switch, which means evacuating and regassing the A/C; renew or clean the relay; or remove the added link. The fan control boxes also contain transistors that can fail and run the fans erratically. On 1992–94 cars water wicks into the module along its earth wires: clean it and seal the earth eyelets on the left inner wing.
The thermostat stuck open, so the gauge reads low
A thermostat can also fail open. The engine never quite reaches temperature, the gauge sits low and the heater stays lukewarm.
The needle is slow to rise or stays in the cold band, especially in winter; now and then it reads right, then drops back. Weak heat from the heater in winter goes with it.
Watch the gauge from cold: with a good thermostat it reaches its usual mark within minutes and stays there. A gauge that sometimes reads correctly points at the thermostat rather than the sender. Test the old one in hot water; one that is open well below its rating is finished.
Fit a new thermostat; for the six-cylinder engine EBC3621 is the 88 C unit with a jiggle pin, and a 192 F unit also works. Some new ones are a fraction too big for the recess in the housing and need the rim taking down evenly. On early digital-dash cars, with an 88 C thermostat the reading moves between half scale and one bar below.
The water pump: a weep-hole drip or a slipping pulley
The pump seldom fails outright. It usually leaks first from its shaft seal through the weep hole; less often the pressed-on pulley slips on its unkeyed shaft or a rusted impeller stops moving the water.
Slow coolant loss with a drip under the front of the engine, a squeal or grit in the coolant; with a slipping pulley, running hot with no leak at all.
Look at the weep hole from underneath, cold and hot. Engine hot, squeeze the top hose while the engine is revved: you should feel a surge. Mark the pulley and shaft to see whether they move apart. The weep hole often leaks only with the engine running and dries at once, leaving a white crust: run it, switch off and feel underneath.
Worth trying: replace the pump, or just its front half, which carries the bearing and seal. A single dose of leak sealant can hold a weep-hole drip for a while, but repeated use clogs the system. After a pump change, refit fan studs of the original length; longer ones can bind on the pump. Make sure a squeal is really the pump and not a belt or the air pump.
Coolant weeping from the side of the head: a core plug
The cylinder head carries several core plugs. One can rust out or go missing on the exhaust side, hidden behind the manifold gasket until the manifold comes off.
Coolant dripping from the head on the exhaust side, getting worse when the area is cleaned.
With the exhaust manifold off, look along the head for an open or rusted core plug, and check the others while you are there.
Worth checking before suspecting a cracked head: fit new core plugs.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Check the coolant level cold, and look for leaks
Top the header tank up cold and note how much it takes over a few days. Look under the front of the engine, down between the manifold runners and at the water pump weep hole.
freeMeasure the real temperature
The gauge can sit on its usual mark even when the engine is boiling. An infrared thermometer at the thermostat housing tells you the truth. The gauge sender is the single-wire sensor there; the two-wire one feeds the engine computer.
a cheap infrared thermometerSpin the belt fan hot, and look at the shroud flaps
A fan that freewheels with the engine hot means a worn clutch. While you're there, check the flaps are soft and all present, and the blades aren't cracked.
freeLook between the radiator and the condenser
Take off the top cover plate and shine a torch down the gap. Clear any debris by blowing and rinsing from behind.
freeTest the electric fan
Turn it by hand: it should spin freely. Ignition on, bridge the thermo switch plug at the bottom of the radiator; the fan should start. If it doesn't, check the wiring and the relay.
freeFeel the hoses and test the thermostat
Hot engine, cool radiator: the thermostat is stuck shut. Slow to warm and a cold gauge: stuck open. Test the old one in a pan of water with a thermometer.
a thermostatCheck the cap and pressure
The cap should be the 16 psi (110 kPa) type with a sound seal. With the engine warm, look along the tank seams and at the radiator's plastic tanks. A tired cap vents coolant into the overflow tank in the left wing and overboard, leaving no trace.
a capLast, the water pump and the radiator itself
Squeeze the top hose with the engine revving for a surge, and mark the pump pulley against its shaft. A radiator that is blocked inside goes to a radiator shop to be rodded; a flush won't clear it.
pump or radiatorThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar fuse tables, 1990 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Head Gasket Failure, Low CompressionIf coolant vanishes with no leak and bubbles rise in the tank.
- Jaguar XJ40 Air Conditioning ProblemsIf the A/C itself blows warm while the engine stays cool.
- Jaguar XJ40 Heater Not WorkingIf the gauge stays low and the heater is cold, rather than running hot.
- XJ6 (XJ40) fuse boxes and relaysFor the fan fuse and relays; other years from the same page.
- Jaguar XJ40 Transmission Problems, Limp ModeIf gearbox fluid spits from the breather when the fans aren't running.
- Jaguar XJ40 Belt Squeal, Crank Pulley WobbleIf a squeal from the belts came first: the balancer can stop driving the pump.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.