Jaguar XJ6 (X300) Overheating, Cooling Fans
On the X300 six, overheating in traffic is usually the fans: a blown fan fuse, a loose fan plug, corroded fan wiring or the relay module. Sudden coolant loss is more often a split hose — the pump-to-block hose or the heater hoses at the back of the head — than the pump or the head gasket. I'd measure the real temperature first, then check both fans run with the A/C on.
How bad is it · Garage, carefully
Don't drive it hot — tow it if the coolant has poured out
The gauge on this car reads Normal across a wide range and goes blind once the coolant is gone, so by the time it shows red the engine has been hot for a while. A burst hose empties the system in minutes.
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 fan fuse blown, a fan unplugged or corroded fan wiring
Two fans do the work: in series at low speed, in parallel at high speed, fed through two fuses in the left-hand engine-bay box and a relay module. On these cars the wiring lets the fans down more often than the fans themselves: a blown fan fuse, a fan plug that has come off, a feed or earth wire corroded under old tape.
Fine on the open road, hot in traffic with the A/C on, and one fan or neither turning. New fans and a new relay module change nothing.
Check both fan fuses and both fan plugs first. Then bridge the radiator switch terminals with the ignition on: if the fans still don't move, the fault is between the fuses and the fans. The purple feed at the relay module should sit near battery voltage; for example, 11.3 V there led to a wire gone green inside. A fan that runs straight off the battery but not in the car points at its earth wire.
Replace the blown fuse, refit the loose fan feed, or cut out the corroded length of feed or earth wire and splice in new.
Fan control relay module failed or stuck
The module behind the bumper under the left-hand outer headlamp holds two relays and three diodes that switch the fans between series and parallel. A failed relay loses a fan speed; a stuck one runs the fans with the key out until the battery is flat.
No slow speed: both fans come on at high for a few seconds and stop. Or the fans keep running for hours after the engine is off, and only pulling the fan fuse stops them.
Tap the module while the fans are stuck on — a stuck relay can let go. Unplug the radiator switch: if the fans keep running, it's the module holding them on. If disturbing its connectors cures it, a poor contact inside is the likely cause.
Fit a replacement series/parallel fan relay (DBC11617 is the number quoted for it), or clean the contacts and connector if the fault comes and goes.
Burst hose: the pump-to-block hose under the intake
A short, thick hose carries coolant from the water pump into the side of the block, out of sight under the intake. When it splits, the system empties in minutes — often on a rev spike or a hard downshift. Pattern hoses are thinner than the reinforced original.
Steam, the low coolant light and coolant pouring from the middle of the undertray, or a jet of water just right of the pump. Weak cabin heat can come a week before. The needle may never move.
Refill and look underneath with the engine running and a light. With the hose off, look at the alloy spigots for corrosion.
Fit the genuine hose with clean spigots, and do the lower multi-branch hose at the same time — on the XJR the dismantling costs more than the parts.
Split heater hoses at the back of the head
Behind the engine, the heater hose at the top rear of the head (MNA6728AB) and the two-part hose from the heater valve to the bulkhead age and split. They sit where nobody looks, so a smell comes long before a puddle.
A whiff of coolant now and then for weeks with nothing on the ground, then a stream hissing from the back of the head — or smoke, a big puddle and an almost empty reservoir with the gauge still normal.
Look down the back of the engine with a torch, and from below by the bulkhead. The hose is easier to cut off than to pull off. A pinhole where the heater pipe joins the back of the head can leak only when hot and running, so nothing puddles: look with the system pressurised and the car up in the air.
Replace the hose. Trimming the split end or splicing in a two-size coupler gets you home, nothing more.
A worn fan motor drawing too much current
A worn motor draws more current than its fuse will carry, so the fuse blows — sometimes days after it was replaced. A dead or shorted motor in the series pair can also send the other straight to high speed. X300 and X308 fans are the same, so a used one from a later saloon fits.
Fan fuse blowing in hot traffic; fans cycling far more often and jumping straight to high; one fan never turns although it spins freely by hand.
Feed each fan straight from the battery. Measure each motor's current with an ammeter: motors drawing well over 25 A are the ones that blow fuses. If neither fan runs at low speed, see that each one runs at high: in series, one open-circuit motor stops both.
Replace the worn fan, and the relay module too if the short has damaged it.
Water pump seized or its bearing failing
The pump on the six is not a common failure, but its bearing wears and it can seize — a car that has stood unused is a candidate. A failing bearing weeps steam before water shows at the pump's tell-tale hole.
A rattle first, then a squealing belt on cold starts for a week or so; then the needle goes to red and coolant comes out of the tank.
Belt off, turn the pump by hand: it must spin freely and not lock when turned back. Look for staining under the pump. The bearing is sealed, so there's nothing to grease; a weep from the tell-tale hole is the warning to renew the pump.
Fit a new pump and drive belt.
Thermostat sticking
A sticking thermostat makes the engine run warm and keeps the fans working. Aftermarket ones vary; the genuine part has a jiggle valve and seals on a tapered, coated metal gasket that a flat one may not match.
Runs warmer than it used to with both fans healthy, or the fans run on after every drive. The same follows a new thermostat fitted upside down.
Drop the old thermostat in boiling water: it must open fully.
Fit a genuine thermostat with a new metal gasket. It goes in the same way round as the old one, with the small bleed hole where it was.
Air trapped after a coolant job
After a hose or coolant change air stays in the system, and the XJR's intercooler circuit traps it most easily. The level reads high when hot, the surplus is pushed out of the header tank, and it looks like a head gasket.
The low coolant light flickering after a long hot run, a green ring round the header tank, a small regular loss and nothing on the ground.
Check the level only cold, just under max. Hot-weather expansion pushes surplus out once and then it settles — topping up hot keeps the cycle going.
Bleed it: on the XJR burp the intercooler circuit at its top nut, then idle with the heater on high for five minutes.
Radiator twin thermal switch failed open or shorted
The switch in the lower left-hand radiator tank has two stages, low speed and high speed. It can fail open, so the fans never step up, or shorted, so they never stop.
The gauge climbing with the A/C on while the fans stay slow; or fans running on long after shutdown with the engine cold.
Unplug it: if stuck-on fans stop, the switch is shorted. Ground each of its wires in turn with the ignition on — the fans should come on at low, then high; if they do, the switch is at fault. Off the car, hang it in boiling water and check both contacts close with a meter.
Worth trying: replace the radiator switch once the bridge test has cleared the wiring and the module.
Slow weeps at hose joints and the thermostat housing
Small losses usually come from joints that weep only when hot: the branches of the lower multi-branch hose, corroded aluminium spigots under the clamps, or the gasket under the six's thermostat housing adapter. The coolant evaporates off the hot metal before it can drip.
The level drops a little every week or two, a coolant smell after a run, white or green crystals at a joint. A chafed lower multi-branch hose can pass for a failing head gasket.
Look for white deposits or pinholes at each join. Pressure-test the cold system and leave it 30 minutes; if it holds perfectly, suspect the cap. Run a finger along the underside of every hose joint; the narrow seam under the thermostat housing bracket and the back of the pump are worth a look too.
Worth doing: clean the spigots back to bare metal, refit with good clamps, and replace the weeping hose or gasket. Sealant only slows a weeping thermostat housing gasket; it needs a new gasket. The header-tank cap must seal for the system to hold its 15 psi, so renew a doubtful one with the thermostat.
A silted or blocked radiator core
The fans sit diagonally behind the core and rely on rubber flaps that close at low speed, so they pull air through the fins instead of drawing hot air back in. Silt inside or debris in the fins cuts the flow however well the fans work.
Runs hotter than it should with both fans turning, no coolant lost and a healthy thermostat; a fan fuse that keeps blowing can come with it.
Look at the fins from the engine side for packed debris and bent blades. Feel across the core with the engine warm for cold patches.
Worth trying: disconnect the fans and back-flush the core from the engine-bay side with a plain garden hose, comb the bent fins and pick out debris once a year. Keep a power washer away from it. Flush the inside too and refill with the right coolant mix, not water.
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.
Know what the fans should do
From VIN 761570, and on earlier cars after bulletin TSB 14-06, both fans run slowly whenever the ignition is on — that's a factory link, not a fault. They come on at about 88 C, go to high at 100 C and may run on after a hot drive. Fans running for hours with the key out are a fault.
freeMeasure the real temperature
The gauge reads Normal across a wide range, and with the coolant gone the sensor has nothing to read. Use an infrared thermometer on the thermostat housing or a scan tool. A needle that jumps to hot when revved can be a poor earth at the bulkhead post. The gauge has its own one-wire sender, separate from the ECU sensor: earthing its lead with the ignition on should peg the needle at H, which proves the gauge and its wire. Neither sensor switches the fans.
freeCheck both fans, fuses and plugs
Warm engine, A/C on: both fans should turn. If not, both fan fuses in the left-hand engine-bay box, both fan plugs, then bridge the radiator switch, then the purple feed at the relay module.
freeFind where the coolant goes
Level cold, just under max. Look underneath with the engine running for a jet near the pump, and down the back of the head for the heater hoses. White crust at a joint marks a slow weep. Run a finger under each hose joint.
freePressure-test
Pump the cold system to working pressure and leave it 30 minutes. A weep that only shows under pressure gives itself away; no drop at all points at the cap.
a hand pressure testerThen the pump and thermostat
Belt off, spin the pump by hand. Boil the old thermostat. Fit genuine parts and bleed the air out afterwards.
a pump, belt and thermostatThe 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, 1995 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 (X300) Battery Drain, Flat BatteryIf the fans keep running with the key out and the battery is flat by morning.
- Jaguar XJ6 (X300) Heater Not WorkingIf the needle sits low and the heater is weak — or the heat went just before the coolant did.
- Jaguar XJ6 (X300) Air Conditioning ProblemsIf the A/C goes warm standing still — the fans and the A/C pressure switches work together.
- XJ6 (X300) fuse boxes and relaysTo find the fan fuses in the left-hand engine-bay box; 1996 and 1997 from the same page.
- 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.