Jaguar XK8 Turns Over But Won't Start
When the V8 spins, sometimes too easily, and never catches, the usual culprit is fuel-washed bores after a short cold start — too little compression to fire. Oil in the plug holes brings it back. After that I'd check the battery, the fuel cut-off switch, the fuel pump and its connector, the gear position switch and, rarely, the timing chain.
How bad is it · Garage, carefully
Usually recoverable — but don't keep cranking blindly
Long cranking on washed bores floods the engine further and flattens the battery. If the engine suddenly spins much more freely and quietly than usual, or rumbled before it stopped, stop cranking: the secondary timing chain may have broken.
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.
Fuel-washed cylinders after a short cold start
A cold V8 runs very rich for its first seconds. Stop it before it warms and the fuel washes the oil off the bores; the rings stop sealing and there's too little compression to fire.
It followed moving the car a few feet, or a start that died at once. It spins fast and evenly; plugs come out wet; backfires can blow the air box apart.
Charge the battery first. Then the flood-clear: ignition on, press the pedal slowly to the floor and hold it while cranking — that cuts the fuel — in bursts with rests, until it sounds like catching, then let the pedal up. Pulling the fuel pump fuse or unplugging the injectors does the same. Still nothing: plugs out, a tablespoon of engine oil in each cylinder, spin it over, fit new or dried plugs and try again.
Oil in the bores, new plugs and a few long crank cycles; expect smoke at first. Often the floored-pedal crank on a charged battery is enough; a later engine software update is said to stop it recurring.
The fuel pump or its connector
The boot pump fuse is live only briefly at key-on and while running — no voltage there during cranking is normal. What fails is the pump, or the connector at the pump. From 2003 the 4.2 cars are returnless: a module by the right rear pillar drives the pump with a pulsed feed, so a test light glowing on both pump wires is normal, not a missing earth.
No hum from the tank at key-on; no pressure at the Schrader valve on the fuel rail.
Bridge the pump relay socket in the boot and press the fuel rail valve. If the pump stays silent, check its connector before buying one. The 4.0 XKR has two pumps. A humming pump isn't proof of flow — crack the line at the filter inlet. Try a second relay of the same type. On a 4.2, check the drive signal with an oscilloscope, and don't test with a pump from a 4.0.
Replace the broken connector at the pump, or the pump itself if the connector is sound. After a winter laid up, a pump can stick rather than die.
The gear position switch reading the wrong gear
The engine computer takes gear position into account. A failing switch that reads Drive in Park or Neutral can stop a hot restart.
Won't restart hot, with an engine failsafe message; fine after 20–45 minutes. A neutral/park code right after a failed start.
Try starting in Neutral; read codes straight after a failure, before they clear.
A new gear position switch; starting in Neutral gets you home meanwhile.
A broken secondary timing chain
Early 4.0 engines used plastic chain tensioners. A broken tensioner can jam and snap the secondary chain on one bank, and the engine can't start.
It ran as if on one bank and died, rumbling or rattling first; cranking now sounds uneven or unusually free.
Stop cranking. Take that bank's valve cover off and look at the chain.
Major engine work on the timing chain.
The immobiliser not enabling the engine computer after new keys
Programming keys and enabling the engine computer are separate jobs. If only the keys were done, the engine cranks with full fuel pressure but gets no spark and the engine computer stores P1260.
Straight after keys were added or a module changed: cranks, fuel at the rail valve, no spark. Code P1260.
Switch off, then on, clear the codes and try another key — P1260 clears only after a key cycle. If it stays, the immobiliser set-up routine has to be run with a Jaguar diagnostic tool.
Running the immobiliser set-up; it can take more than one attempt before P1260 clears. A P1000 afterwards clears with driving.
The fuel cut-off inertia switch has tripped
A safety switch cuts power to the fuel pump in an impact. A kerb strike, a hard bump or bodywork at the rear can trip it, and the engine then cranks normally and never fires.
No pump hum at key-on and no fuel at the rail valve, after a knock, a rear-end repair or work near the passenger footwell.
Pull off the trim panel at the end of the dash by the passenger door and press the reset button on the switch. Check the pump fuses in the boot and the power module connections while you're there.
Worth trying first: resetting the switch costs nothing.
A battery that cranks but can't run the electronics
The starter turns on a battery that sags under load; the engine computer and ignition need more, and a weak battery throws spurious messages.
ABS or ASC 'not available' while cranking; the scanner won't connect; lights dim slightly.
Load-test the battery before anything else; you want about 12.6 V at rest. If the battery tests good, measure the voltage drop along the main positive cable and across the earth straps while cranking: a corroded crimp or an earth strap fitted to an uncleaned surface can lose enough to stop it.
Worth fitting a new battery first; it can also cure a scanner 'no link' and wipers switching on by themselves.
A crank position sensor clogged with debris
The sensor is magnetic and collects metal crud on its tip. Without a crank signal the engine is dead — there's no fallback, unlike a failed air-flow or cam sensor — and it can stop working before it sets a code.
Code P1336, or no code at all, with fuel pressure present and the engine cranking at normal speed.
With the car raised, the sensor is at the back of the block, right side, below the throttle body, held by one 10 mm bolt. Pull it, clean the tip and check its connector, which joins under the ABS cover next to the power steering reservoir.
Worth trying: clean and refit it; replace it only if P1336 returns.
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.
Think back: was it started briefly and switched off?
If yes and it spins freely, go straight to clearing the bores.
freeLoad-test the battery
ABS or ASC not available while cranking often means a weak battery.
free at most parts storesKey on: listen for the pump
The pump fuse is live only during priming and running — no voltage while cranking is normal.
freeNo pump hum after a knock? Reset the inertia switch
Behind the trim panel at the end of the dash by the passenger door.
freePress the valve on the fuel rail
Fuel under pressure: the pump side is fine.
freeFloor the pedal and crank
Battery charged, pedal held to the floor to cut the fuel, crank in bursts with rests until it catches.
freeClear the bores: oil in each cylinder, crank with plugs out
Then dry or new plugs; expect smoke at first.
freeHot-only no-start? Try Neutral
Read codes straight after a failure.
freeThe 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, 1998 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.
Circuit descriptions from the Jaguar fuse tables, 1997 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 XK8 Won't Start, Just ClicksIf the engine does not turn over.
- XK8 (X100) fuse boxes and relaysThe fuel pump relay and fuse in the boot — other years from the same page.
- Jaguar XK8 Timing Chain Tensioner ProblemsIf the engine stopped suddenly and cranking sounds uneven or too free — check the chain before anything else.
- Jaguar XK8 Nikasil Engine ProblemsIf it keeps refusing to restart after short runs — how to tell whether the bores are worn.
- 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.