Jaguar XJ8 (X308) Cranks But Won't Start
This page is for an X308 whose starter spins the engine at normal speed but it never fires; if it only clicks or the car is dead, the no-crank page is the right one. On the V8 the usual answer is fuel: a flooded or washed-down engine after a short run or long standing, or failed pumps in the tank. I'd think about the last run, listen for the pump at key-on and watch the rev counter before buying anything.
How bad is it · Garage, carefully
Stranded rather than unsafe, but don't crank it endlessly
Long cranking flattens the battery, overheats the starter and floods the engine further. If the original timing chain tensioners are still fitted, stop and check them before cranking on.
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.
Flooded after a short run or a failed start
A cold V8 runs very rich for its first seconds. Switch it off then, or crank it a few times without a catch, and the plugs end up wet and the engine won't fire.
It was last moved a few feet, it coughed once and stopped, or there's a petrol smell at the tailpipes.
Pull one plug: wet with fuel means flooded. The rev counter will show normal cranking speed.
Charged battery, pedal flat to the floor and keep cranking until it coughs, then ease off as it catches; it can take minutes. Pulling the fuel pump fuse gives a guaranteed dry crank if you're unsure.
Bore wash-down: no compression after standing or flooding
Neat fuel strips the oil film from the bores, and on these Nikasil V8s the rings stop sealing. Compression collapses, so it won't fire even with good fuel and spark. Long storage does the same.
The engine spins unusually freely; it stood for months, or was started and stopped cold several times. 3 to 6 bar on every cylinder, for example.
A compression test. Low on all cylinders points here; one cylinder per bank is enough to rule out valve timing.
Plugs out, about half a teaspoon of engine oil into each bore, crank a few turns with the fuel pump fuse out, then refit and start. Expect smoke while the oil burns off. Repeat if it doesn't catch.
The in-tank fuel pump, or its power lead inside the tank
The pumps sit in the tank and fail most often after a car has stood, with old fuel blamed, or from brush wear at high mileage. The XJR has two. The power lead inside the tank can also burn at its terminals, which looks exactly like a dead pump.
No two-second hum from the boot at key-on; it cranks then dies; it runs for a few seconds on starting fluid; a pump fuse that keeps blowing.
Bridge the relay socket's load terminals: if the pump still doesn't run, the fault is in the tank. Confirm with a gauge on the rail's test valve first.
New pump or pumps with new strainers, and new leads inside the tank. Renew the fuel filter at the same time, keep the tank above a quarter, and disconnect the battery before lifting out a metal tank.
The fuel pump relay in the boot
The pump relay sits in the boot relay box behind the battery. A tired relay can still click and pass nothing, often only when hot.
Hard to restart hot, then fine cold; a code reader may point at the pump relay.
The relays there are unnumbered, so use the chart for your year. Swap the pump relay with an identical one, such as the heated rear window relay, and try again.
A new fuel pump relay.
A poor contact at the airflow meter plug
Corrosion on the airflow meter's plug can stop a start even with fuel, spark and injector pulse all present. It may leave nothing but a readiness code.
Pressure good, spark good, plugs wet, and a new crank sensor changed nothing.
Unplug the meter and try again, then refit the plug firmly.
Unplug and replug the connector; clean both the plug and the socket pins so it doesn't come back.
Slow cranking: the starter or its feed
It turns, but too slowly to fire, and the battery gives up quickly. A worn starter or a burnt starter feed connector does this.
Laboured cranking with no sign of spark, even on a freshly charged battery; or it cranks properly only when warm and dry.
Load-test the battery first. Then look for heat damage at the starter feed connector on the bulkhead.
A new starter, or a new connector if that's where the heat is.
The crankshaft position sensor
Without its signal the engine computer gives no spark and no injection. On the V8 it is a less common failure than on the six-cylinder cars, so I prove it before buying one.
The rev counter stays flat while cranking — it may flick at key-on and then do nothing.
A healthy crank shows about 200–250 rpm on the rev counter. The sensor sits low on the flywheel housing, beside the rubber access plug; the cam sensor is worth a look too.
Worth trying a known-good sensor when the rev counter is dead; replace it rather than clean it.
The fuel pressure regulator or the non-return valve
If pressure bleeds back after shut-off, the rail is half empty at the next start.
Three or four seconds of cranking before it fires in the morning, otherwise it drives normally.
Gauge on the rail: about 35–41 psi running and about 21 psi held after switch-off. A regulator diaphragm should hold vacuum.
Worth checking before a tank drop when pressure falls away, though a weak pump gives the same reading.
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 about the last run
Moved a few feet, started and stopped cold, or stood for months? That points at flooding or wash-down before any part.
freeMake sure it's really cranking
If it only clicks or the car is dead, this is the wrong page — see the no-crank page. A slow, laboured crank is the battery or starter.
freeKeep cranks short
About 20 seconds, then let the starter cool. If the original timing chain tensioners are still fitted, don't keep cranking — a jumped chain can bend valves.
freeWatch the rev counter
About 200–250 rpm while cranking is normal. Flat means no crank signal.
freeListen for the pump at key-on
Two seconds of hum from the boot front wall; have a helper turn the key. Wait about 15 seconds with the key off before the next try.
freePull a plug
Wet: clear the flood with the pedal floored. Dry: check fuel and spark.
freeSpray starting fluid into the intake
If it runs for a few seconds, the fault is on the fuel side.
a can of starting fluidCheck the relay, fuses and inertia switch
Swap the pump relay with an identical one, test the pump fuses with a meter, and make sure the inertia switch hasn't tripped.
freeReseat the airflow meter plug
Unplug, inspect the pins, plug back in firmly.
freePut a gauge on the rail
The test valve is on the fuel rail — on 2001-on cars the front right one. Expect about 40 psi while cranking.
a gauge with a Ford-type adapterProve spark
Turn a coil so a plug fits in its boot, earth the plug on the engine and crank.
a spare plugTest compression
Low on every cylinder means wash-down: oil in the bores, then try again.
a compression gaugeOnly then the tank
With no pressure and the relay bridged, the pumps are next. Have the radio code ready if the battery comes off.
pumps and labourThe 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.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ8 (X308) Won't Start, No Crank or Car DeadIf the engine doesn't turn over at all, only clicks, or the whole car is dead.
- Jaguar XJ8 (X308) Timing Chain Tensioner ProblemsIf the original tensioners are still fitted, or it rattled at start-up before it stopped firing.
- Jaguar XJ8 (X308) Key Fob Not Working, Boot or AlarmIf the dash shows Boot Open or the alarm misbehaves when it refuses to start.
- Jaguar XJ8 (X308) Engine Failsafe ModeIf it starts but the dash shows Engine Failsafe Mode and it won't rev.
- XJ8 (X308) fuse and relay boxesWhere the fuel pump relays sit in the boot — the layout changed across the model years.
- Jaguar radio code generatorFree. For when the radio asks for its code after the battery has been off.
- Jaguar XJ8 (X308) Rough Idle and StallingIf it does start but idles badly or dies at junctions.
- Jaguar XJ8 (X308) Fuel Pump ProblemsIf the pump is silent at key-on or there's no pressure at the rail — relay, inertia switch, pump and how to reach it.
- 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.