Jaguar XJ6 Series 3 Overheating, Losing Coolant
When a Series 3 runs hot in traffic, I'd start with the small aux fan in front of the radiator — join the radiator switch leads and see if it runs. If it does, look at the engine fan and its viscous clutch, then at a radiator clogged inside or packed with debris at the front. If it is losing coolant, find where it goes — an overfilled tank or a tired cap, a hose that only leaks hot, the thermostat housing gasket or water inside the inlet manifold — before you suspect the head gasket.
How bad is it · Garage, carefully
Drive gently and watch the gauge
A few degrees over 90 °C in hot traffic is no harm, but a needle that keeps climbing means the cooling is losing. A cracked plastic engine fan can burst and wreck the radiator and the bonnet.
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.
The red aux fan relay
The small electric fan low in front of the radiator is switched by a red Lucas relay on the top rail, left of the shroud. When the relay dies, the fan stays still just when the car sits in hot traffic and the gauge climbs. A relay of the wrong pattern does the same — the socket needs one with the 87A layout, and an ordinary one shorts and blows the fuse.
Fine on the move, climbing past 90 °C in stop-and-go traffic with the A/C on; the aux fan never heard; the A/C going warm at idle.
Ignition on, pull the two leads off the radiator switch and join them. The red relay should click and the fan run. If not, bridge the relay's power wire to the fan wire: fan runs and the fuse holds — relay or its connection; fuse blows — the fan motor or its wiring.
A new relay with the same pin layout — a generic one at about $10 does the job of the Lucas part (DAC1028). If one has been swapped in before, make sure it is the 87A type. The opposite fault, a fan that never stops even once the radiator is cold, is usually the same relay stuck closed: unplug it, and if the fan stops, renew it.
The radiator thermo switch or its connector
On the six-cylinder the switch is the big brass nut low in the right-hand radiator tank, seen past the power steering pump; on the V12 it sits on the water pump housing. It closes at around 95 °C. A dead switch, or crud on its terminals, leaves the fan off however hot the water gets. The switch can also be the leak: a slow weep where it screws into the radiator tank looks like a radiator fault.
The fan runs as soon as the switch leads are joined; the gauge slips past vertical in traffic while the fan stays silent. A switch that closes only near 100 °C instead of about 95 °C lets the needle creep before the fan helps.
Join the two switch leads with the key on — if the fan runs, the switch or its contacts are the fault. Pull and refit the leads to scrape the terminals. To prove the switch, take it out and heat it in water with a thermometer and an ohmmeter: open when cold, closed near 95 °C.
Clean the terminals and refit the leads; if the switch fails the hot-water test, fit a new one. The later switch is metric, not tapered, and seals on a soft washer, so no PTFE tape. Back it off a touch before tightening, to be sure it isn't corroded into the tank; a stubby spanner past the power steering pulley reaches it.
The aux fan fuse, a hidden in-line fuse or a lost earth
The fan takes a battery feed through its fuse in the under-bonnet fuse panel on the left wing, so it can run on after the key is out. Two small diodes sit on a board taped into the harness left of the radiator, and the black wires must be earthed. A melted in-line fuse tucked under the fuse box by a previous owner, or a block connector with no earth, kills the fan with relay and switch both good. The aux fan itself fails too: its motor windings cook and it runs slowly, and pebbles collect in the stamped dish of the tray under it and jam the blades. A seized fan motor can burn the relay and its wiring without blowing the fuse.
Relay and switch test good, yet no power or no earth at the fan; the fan worked for a week after a repair and stopped.
Check the fan fuse, then look under and around the fuse box for an added in-line fuse. Feel for the taped diode lump on the left of the radiator and test the black wires to earth. Spin the aux fan by hand and look in the tray beneath it for debris. If it turns slowly when powered, check the brushes and measure across opposite commutator segments for an open winding. Make sure it pushes air through the radiator towards the engine — one had been fitted backwards.
Replace the melted fuse; repair the connector or add a second earth wire. Clear the debris and drill a drain hole in the dish so it can't collect again. A slow or seized motor needs cleaning and new brushes, or a replacement; a burnt relay socket means checking the fan before fitting a new relay.
A tired fan clutch, a cracked fan or poor airflow
The main fan is plastic on a viscous clutch bolted to the water pump pulley. A worn clutch slips, so the fan moves too little air at idle. After decades the plastic yellows and cracks at the hub, and a cracked fan on a seized clutch can burst and take the shroud, the radiator and the bonnet with it. In extreme heat the stock shroud itself can hold back the airflow.
Hot at idle and in traffic, better on the move; A/C weak when standing; hairline cracks at the blade roots; the fan doesn't turn with the engine running or won't spin freely with it off.
Engine off, spin the fan by hand and look at every blade root. A window fan stood in front of the grille at idle shows whether more air cures the heat and the A/C. A good clutch turns by hand with a slight drag and stops within about one turn when flicked; one that is stiff or freewheels is worn. Most old fans show four fine, evenly spaced lines at the hub — moulding lines; the cracks that matter are at the blade roots.
Fit a new S3 fan, EAC3266 — an XJS fan has a different offset and diameter and won't fit the S3 shroud — and a new clutch if in doubt. Early S3 cars, before the mid-1982 update, use a different fan and mounting, so check before ordering. Electric fans in place of the clutch fan cure it too: power them from battery +, never from the coil +, or the gauges swing at key-off and the car may not restart.
A clogged radiator
Scale and debris settle in the bottom of the radiator, right where the fan switch sits. The gauge reads the water rail on the engine, so a blocked radiator keeps the switch cool while the engine runs hot — the fan never cuts in even with every part good. Leaves and dirt packed between the A/C condenser and the radiator do the same from the outside.
The aux fan coming on more and more often, or never at all with the engine past 95 °C; cold patches on the radiator core when warm. Brown, muddy coolant when you drain it.
Engine warm, feel across the core for cold spots. Lift the top rail and look between condenser and radiator. A flow test can pass while the core is still poor.
Take the radiator out and clean it properly — inside and through the fins — or have it rodded or recored; after that the gauge stays at or below 90 °C. A recore beats a cheap copy radiator, and electric fans only help at idle on a clogged one.
Coolant lost from the expansion tank or its cap
You fill and check the coolant at the brown expansion tank on the left of the engine bay; the cap on the radiator header stays shut after the first fill. Anything above about half a tank is pushed out through the vent into the atmospheric tank hidden in the left front wheel arch, and from there onto the ground. A cap that no longer holds its 15–16 psi lets the coolant boil near 100 °C instead of about 120 °C, so the car loses water and runs hot. Even a new cap leaks if the seating face in the filler neck is corroded or bent, or its collar crushed. The brown plastic tank can split along its seam or crack round the through-bolt in the middle, and a new plastic tank can crack the same way. Cars built up to about mid-1981 keep the Series 2 layout, with the reservoir under the front wing.
A puddle just ahead of the left front wheel after a run; the low-coolant lamp coming on every week or two with nothing dripping in the garage. Coolant weeping at a brand-new cap before the engine is even warm; a wet seam on the tank.
Check the level cold only. Fill to about half, run two heat cycles and mark where it settles — that is your level. Have the cap pressure-tested, look for cracks round the through-bolt of a fibreglass tank and pin-holes in a steel one, and check the rubber grommet of the low-coolant sensor in the tank. On later cars the level settles about a quarter of an inch below the bar in the filler neck; fill above it and the extra just goes to the overflow. With the engine cold, loosen the cap: no hiss of pressure points to the cap, and a new one is a cheap test. Look at the seating face in the neck as well.
A new cap of the right rating and the level kept at half stops the loss. A cracked fibreglass tank is best replaced with a stainless one painted black; a sensor grommet that seeps takes a new grommet and sealer. A split seam can be sanded and patched with epoxy as a stopgap; a stainless tank also lets the cap seal airtight. A worn seat in the neck needs a new neck, not another cap.
A heater hose clamp or the throttle-body hose leaking only when hot
The system only leaks under pressure, about 15 psi, so a loose clamp at the heater matrix or a soft hose drips when hot and looks dry once cool. The throttle body is warmed by a loop of two small hoses: water rail to throttle body, then a long one back to the water pump, EAC29287. The edge of the clamp on the short red hose can chafe through the long one.
Coolant on the floor under the bulkhead slightly right of centre, or round the throttle body on the exhaust side, gone by the morning. Coolant going steadily with nothing on the floor: a pin-hole in a small hose sprays onto the hot engine and evaporates.
Engine warm and idling, bonnet up: watch every clamp, the heater hoses at the bulkhead and the small hoses at the throttle body. A hired pressure tester finds these in minutes.
Tighten the heater hose clamps. Renew both throttle-body hoses with 6 mm (1/4 in) fuel hose and small worm clamps — the original hose is no longer made, and the inside diameter is what matters. Change them with the engine stone cold.
An old radiator hose split or pin-holed
Rubber hoses age from the inside with every heat cycle: they swell, then split, and the top hose a few inches from the thermostat housing is a typical spot. A pin-hole may only open above a certain pressure, so the hose looks dry when cold and sprays when hot. Once the pressure is gone, the coolant boils at about 102–103 °C instead of above 120 °C.
A fine mist of coolant over the air cleaner; no pressure left in the system after shutdown; the low-coolant lamp coming on during a long trip; a hose that looks bulged.
Engine warm and idling, run your eye along the top hose and the hoses underneath and to the heater, and feel for soft or swollen spots. If a lower-pressure cap makes the loss stop, a hose is opening under pressure.
A new hose — a generic top hose cut to length does the job. On a car of unknown history I'd renew the whole hose set and the coolant at once: kits are sold, it is under a day's work, and cheaper than a tow. A weaker cap only hides the leak.
The thermostat housing gasket
The thermostat housing on the front of the water rail seals with a paper gasket, and the small bypass hose from the water pump runs right below it, so a drip there passes for the hose or the pump. The long top bolt corrodes into the aluminium and snaps if you force it.
A slow drip at the front of the engine after a drive; a leak that survives new hoses and clamps.
Dry the area, run it warm and watch the housing joint rather than the hose. Before undoing anything, soak the bolts in a proper penetrant — not WD40 — and use heat.
Redo the housing gasket with new bolts and copper slip on the long one; avoid stainless bolts in the aluminium. If a bolt snaps, drill it out, or pull the water rail and free it on the bench.
The water pump belt off or slipping
One belt turns both the water pump and the power steering pump. If it comes off, coolant stops moving through the block and the gauge goes to the top within miles, with steam and coolant from the overflow. Steering that suddenly goes heavy at the same time is the giveaway. After years in storage the pump impeller can rust loose on its shaft, so the belt turns and nothing flows.
Heavy steering, then the needle pinned and coolant boiling out of the overflow.
Stop at once and look for the belt. Turn the pumps and the alternator by hand. A belt that keeps squealing after tightening can point to a loose harmonic balancer rubber. After a cold start, squeeze the big top hose: it should go firm within about half a minute, and the radiator hose should heat once the thermostat opens.
Refit and tension the belt correctly and top up. Before trusting the engine again, a compression test and a cooling-system pressure test, then watch the tank for bubbles.
The thermostat stuck open or the wrong type
A thermostat that will not close lets the engine run well below its rating: the gauge barely leaves the stop, the heater stays lukewarm and the injection stays rich. The correct Jaguar thermostat also blanks the housing bypass when fully open — an ordinary one lets coolant skip the radiator and can cost up to 15 °C of cooling.
The needle near 60 °C on a long drive and a top hose slow to warm; or running hotter after a thermostat change.
Infrared thermometer on the water rail. If it reads below the thermostat rating and the sender agrees, the thermostat is the only cause. One can look fine on the bench and still not seal.
A new genuine 82 °C thermostat brings the gauge past 80 °C within a mile. 82 °C suits most cars and hot climates; an 88 °C one can give a little more response on a 4.2.
The gauge or its sender reading wrong
The sender sits on the water rail above the inlet manifold with a single green wire. New senders tend to read a little hotter than the old ones, so 95–100 °C in stopped traffic may be the gauge. Some cars also have an in-line resistor between sender and gauge. A dry joint in the spade connector at the sender makes the needle wander, and with the coolant low the sender sits in air and reads nonsense.
Readings changed after a sender swap, or a needle that drops and recovers; the car never boils or pushes coolant out.
Key on, earth the green wire: the needle should swing fully right. Then point an infrared thermometer at the thermostat housing and compare it with the gauge.
Re-make the spade connector at the sender. If a new sender reads high, a resistor in series at the sender trims it — for example, 15 ohms brought one gauge true at 100 °C.
Coolant leaking inside the inlet manifold
Coolant passes from the head into the inlet manifold through gasketed water ports, and two small hoses from the water rail and the water pump warm the throttle neck. Coolant dripping from the small vacuum drain nozzle at the end of the manifold means the water is already inside it — a failed port gasket or a cracked throttle neck. Stop-leak can build up in the ports and keep the gaskets from sealing. Some of the head's waterways are blanked off by the manifold itself and its gasket, so a failed manifold gasket leaks coolant inside the manifold rather than into the cylinders.
A steady warm drip from the end of the inlet manifold, often where the rubber cap on the drain nozzle has rotted; the expansion tank empty and the low-coolant lamp on.
Find the drain nozzle and see where the drip comes from. Fill with plain water and see whether it still drips with the engine off; a pressure test shows it as well.
Worth doing: take the inlet manifold off, clean out any stop-leak and renew the water-port gaskets, and look at the throttle neck for a crack. While it is off, check the steel coolant pipe along the engine — it can rust through, and copper pipe or hose replaces it.
The head gasket or a weeping head stud
Coolant can leave past the head gasket or round a corroded head stud nut, more often on post-1984 cars. The loss is invisible: a quart or so per 100 miles, or a tank dry in a day or two, with no drip, no white smoke and often a clean dipstick. A weep at a stud nut boils off too fast to see once the engine is hot. After years of standing, the head itself can crack — one car that overheated and ran very rough after storage needed a replacement head.
Coolant vanishing with nothing on the floor; a rough engine that smooths out when warm, or one plug fouling; milky deposits inside the breather or the oil filler cap even with a clean dipstick.
Look inside the breather and filler cap for mayonnaise. From a cold start, before the coolant warms, see whether the tank already holds pressure — if it does, combustion gas is getting in. Look over the head nuts right after a short cold run; UV dye in the coolant helps. An overnight pressure test catches a small leak, and a combustion-gas test on the coolant settles it.
A head gasket job, with the head and block checked for flatness, corrosion and cracks — a gasket that fails again points to a warped or cracked face. The head studs go to 55 ft-lb. Don't pour sealer in: it doesn't fix the gasket and it clogs the manifold ports.
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 and the gauge
Top up cold at the brown expansion tank, to about half. Compare the gauge with an infrared thermometer on the thermostat housing: 88–90 °C is normal, and a few degrees more in hot traffic with the A/C on is no harm.
freeCheck the cap and its seat
Engine cold, loosen the cap: a hiss means it holds pressure. Look at the seating face in the neck for corrosion or a dent, and at the tank seam and through-bolt for a crack.
freeFind where the coolant goes
A puddle ahead of the left front wheel is overflow from a brimmed tank. A drip under the bulkhead, at the throttle body or at the thermostat housing is a leak — idle it warm with the bonnet up and watch the clamps. A drip from the drain nozzle at the end of the inlet manifold is water inside the manifold. Look along the top hose for a bulge or a slit.
freeLook at the belt and the engine fan
Engine off: the pump belt on and tight, then spin the fan by hand and look at every blade root. Any crack, replace the fan before driving far.
freeJoin the radiator switch leads
Ignition on. Fan runs — the switch or its terminals. Nothing — move to the relay.
freeTest the relay and the feed
Bridge relay power to the fan wire. Check the fan fuse, any added in-line fuse and the earths.
freeClear the front of the radiator
Lift the top rail and clean out between the condenser and the radiator. Feel the core for cold spots.
freePressure-test the cap and the system
Test the cap on its own, then pump the system up and look for hose, tank and sensor-grommet leaks. If coolant still vanishes, a compression test and a combustion-gas test on the tank tell a head gasket apart. An overnight pressure test catches a slow one; milky deposits in the breather point the same way.
a hired testerCheck the thermostat
If the gauge sits low or it ran hot since a thermostat change: infrared on the water rail against the rating, and a genuine Jaguar thermostat only.
a thermostatRod or recore the radiator
When the fans work and it still runs hot. Do this before fitting electric fans.
rodding or a recoreThe 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, 1987 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 XJ6 Series 3 Air Conditioning ProblemsIf the A/C goes warm in traffic as well — the condenser airflow and the gas.
- Jaguar XJ6 Series 3 Heater Not WorkingIf the gauge reads low and the heater is lukewarm — the thermostat and the heater valve.
- Jaguar XJ6 Series 3 Alternator Not ChargingElectric fans load the alternator — check its output before adding more.
- XJ6 Series 3 fuse boxesWhere the under-bonnet fuse panel sits.
- Jaguar XJ6 Series 3 Misfire, Rough RunningIf it misses and one plug fouls along with the coolant loss.
- 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.