Jaguar XJ6 (X300) Cranks But Won't Start
On the X300 a healthy starter with no catch is usually the crankshaft position sensor, the ECU plug it feeds, or the fuel pump relay. I'd watch the rev counter while cranking and listen for the pump at key-on before buying anything; if it last ran for only a few seconds, it's simply flooded.
How bad is it · Garage, carefully
Stranded rather than unsafe, but it may cut out on the move
Crank sensor, relay and ECU plug faults often start as cutting out while driving. Once it has refused to start, don't trust it in traffic until the cause is found.
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 crankshaft position sensor
It sits behind the crank pulley and reads the timing ring. Its fine-wire coil cracks with heat and vibration, so it fails hot and intermittently and stores no code of its own. No pulses, no spark and no fuel pump.
The rev counter stays at zero while cranking, or it won't restart hot and then starts next morning.
A healthy one reads about 1,300 ohms, but a sensor can measure right and still fail. A needle that moves while cranking is a good sign, not proof. P0727 points this way.
A new LHE1640AA with its lead tied clear of the belts. A cheap generic one is fine; at 100,000 miles or ten years I'd just replace it.
Fuel pump relay, injector relay and their fuses
The ECU earths the pump relay for a second or two at key-on, then only while it sees crank pulses. Relay contacts arc and carbon up, the socket can push out of its frame, and a blown fuse to the injector relay kills all six injectors.
No short buzz from the rear at key-on, no fuel at the rail, or it cuts out on the move and restarts after a few minutes.
Pull the relay and bridge the two big pins, 30 and 87, through a 15 A fuse: if the pump runs, the pump is fine. Then look for the key-on pulse on 86 and an earth on 85. Test fuses with a meter, not by eye.
Clean the contacts or fit a new relay, reseat a loose socket, replace the blown fuse. The pump relay is usually in the boot box on a green base, but on some cars it's in the right-hand footwell box; the XJR has two.
Corroded pins in the engine ECU plug
The ECU sits low in the right-hand footwell and water from blocked sunroof drains runs onto its black plug. The crank sensor input pins rot first, so it looks exactly like a dead sensor.
Starting gets harder over weeks, the rev counter is dead while cranking, and a new crank sensor helps for a week or not at all.
Unplug it and look at the pins themselves, including under the inserts on top. Dust on the harness hides the damage.
A new connector or new pins rather than cleaning, or a known-good ECU swapped in to prove it. Clear the sunroof drains so it doesn't come back.
Fuel hose off the pump inside the tank
The pump outlet hose or its clip lets go inside the tank. The pump hums as normal but nothing reaches the engine.
Drove normally, then cranks fast and won't fire, with the pump audible and the rev counter moving.
There's no ready test valve: put a gauge on the rail with an adapter, or crack a line at the engine; it should hold about 40 psi. A hiss at the filler cap proves nothing.
Drop the tank and refit the hose and clip, or fit a new pump with a flared outlet. On the XJR check the primary pump feed at the Y-pipe; check new lines and O-rings before refitting.
Flooded after a very short run
The AJ16 runs very rich for the first seconds of a cold start. Switch off then and the plugs are wet and the bores washed. Pressing the pedal while cranking makes it worse.
Last time it only moved a few feet, it smells of petrol, or it fired once and died.
Think back to the last run before buying anything.
Charged battery, pedal flat to the floor and crank for about 20 seconds (full throttle cuts the injectors), then ease to about a fifth and release when it fires. Afterwards never switch off a cold engine within a few minutes.
Several weak ignition coils
The AJ16 will run roughly on four of six coils without a code. With more of them dying it simply won't catch. Counterfeit coils fail early.
Cutting out and misfires before it stopped starting; a new crank sensor changed nothing.
Lift one coil off the cam cover, still plugged in, put a spare plug in it and earth the plug on the engine, not the cam cover. Crank and watch for a spark.
A full set of good-quality coils, with the worn screw holes repaired with inserts.
Weak battery or a loose main feed at the battery
The ECU needs steady voltage while the starter pulls hard. The two 250 A main fuses and their spacers on the battery positive work loose and drop it under load.
Worse in the cold, after standing, or relays chatter while cranking.
Wiggle the positive cables at the main fuses in the boot. Load-test the battery; a voltage reading alone isn't enough.
Clean the studs and spacers and refit the fuses tight, or a new battery.
Long cold cranks: a gummed throttle body or the wrong plugs
Oily grime in the throttle body and idle valve upsets the cold-start mixture, so the engine behaves as if flooded.
Cranks four or five seconds before firing on cold mornings, then a slight misfire; fine hot.
Take the intake off and look inside the throttle body, idle valve and airflow meter.
A proper clean of all three. A change of plug type is worth trying if it persists.
Cam sensor housing out of line
The cam sensor lives in a round housing with a sight window, separate from the crank sensor. If it shifts, the ECU can't find which stroke it's on.
Fires only on the second or third attempt, or spins five to ten seconds before catching; the dot on the housing doesn't line up with its window.
Look through the oil filler while cranking: if the cam turns, the engine is turning. Then check the alignment mark.
Reseat the housing on its mark. A sensor fitted 180° out needs re-timing.
The airflow meter
A faulty meter sends a reading that stops the engine from holding a start, even with good fuel, spark and crank signal.
Idle went erratic and stalled for days first; now it fires for a second and dies.
Unplug the meter and try again. If it starts, even roughly, that's your answer.
A new meter, then a battery-off relearn if the idle sits high afterwards.
Water or rust in the fuel
A blocked drain under the filler flap lets rain sit by the tank. Rust gets into the fuel and the filter breaks up; in winter, water freezes in the lines.
Won't restart hot, or won't start in a hard frost and then cures itself.
Open the fuel filter and clear the filler drain.
Filter, regulator, rail and injectors cleaned or replaced and the lines blown through. A fuel-system water remover is worth trying in winter.
The fuel non-return valve losing prime
If the valve leaks, the rail drains overnight and the short key-on prime isn't enough to refill it.
Long crank after standing, or it fires on the second turn of the key.
Turn the key on and off two or three times, listening for the pump each time, then crank.
Worth trying the key routine first; if it helps every time, the valve is the next part to look at.
The fuel pressure regulator
A split diaphragm either drops rail pressure or lets fuel into the intake and floods a hot engine.
Hard hot restarts, or a smell of fuel.
Pull the vacuum hose from the regulator to the intake: wet with fuel means it's leaking.
A new regulator is worth trying if the hose is wet.
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.
Watch the rev counter while cranking
About 200–250 rpm means the crank sensor is talking. A dead needle points at the sensor or its ECU pins.
freeThink about the last run
A few seconds of running, or a petrol smell, means flooding. Clear it with the pedal floored before anything else.
freeListen for the pump at key-on
A short buzz from the rear. Turn the key on and off a few times before cranking.
freeRead the codes
Worth doing, but the crank sensor and weak coils often leave nothing stored.
a code readerTest the pump relay and the fuses
Bridge 30 and 87 through a 15 A fuse; check the key-on pulse and the earth. Test fuses with a meter. Wiggle the main fuses at the battery.
a multimeterProve fuel at the engine
Crack a line at the rail: it should hold about 40 psi. Unplug the airflow meter and try once more.
a rag and a spannerProve spark
Coil off the cam cover, still plugged in, plug earthed on the engine.
a spare plugLook at the ECU plug
Right-hand footwell. Unplug it and look at the pins; clear the sunroof drains.
freeOnly then buy parts
Crank sensor first, as it's cheap and the usual culprit; coils or a tank drop only once the tests point there.
a sensor, coils or a pumpWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Won't Crank, Just ClicksIf the engine doesn't turn over at all, or you only hear a click.
- Jaguar XJ6 (X300) Immobiliser Problems, Won't StartIf the check engine lamp goes out a few seconds after key-on and stays out.
- Jaguar XJ6 (X300) Rough Idle or High IdleIf it starts but stalls at junctions or cuts out on the move.
- XJ6 (X300) fuse boxes and relaysWhere the pump and injector relays sit — 1996 and 1997 from the same page.
- Jaguar XJ6 (X300) Fuel Pump ProblemsIf the pump is silent, weak or whining — relay, the hose in the tank, or the pump itself.
- 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.