Jaguar XJ8 (X308) Fuel Pump Problems
On the X308 a pump that seems dead is often something cheaper: an empty tank behind a lying gauge, the relay or its socket, or burnt power terminals on the pump. The pump itself does wear out, typically around 100,000 miles, and it lives inside the tank. I'd check the fuel, listen at key-on, bridge the relay and gauge the rail before anyone cuts the parcel shelf.
How bad is it · Garage, carefully
Drivable while it runs, but it can stop dead on the road
A failing pump often groans or stumbles for a while and then quits completely. Keep the tank well above a quarter until it's sorted: the pump relies on the fuel around it to stay cool.
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 worn-out pump in the tank
The pump lives inside the tank and typically lasts about ten years or 100,000 miles. As it wears, the rotor can rub, melt its own plastic and seize, blocking the outlet as it goes. Running on a nearly empty tank makes it hotter. The non-supercharged car has one pump; the supercharged XJR has two, the second cutting in higher up the rev range.
A groan from the boot that comes and goes, a single no-start that cures itself, bad stumbling just after parking — then no hum at key-on and no pressure at the rail. An unrelated code, such as an intake air temperature fault, can show up at the same time and mislead. A dying pump usually gives a no-start rather than a lumpy fire-up. On the supercharged XJR a tired second pump shows as an engine that won't pull past about 3,000 rpm, since that pump only works hard at wide throttle.
With the key on, 12 V reaching the pump feed for about two seconds while the pump stays silent condemns the pump. On the XJR each pump has its own white feed and earths through the body. Striking the tank with a rubber mallet, with at least a quarter tank in it, can restart a motor parked on a dead spot — a clue and a stopgap, not a cure. On the XJR, pulling the main pump's fuse stops the engine within a second, while pulling the second pump's fuse changes nothing at idle. A pump drawing too much current blows its fuse; a bad relay doesn't.
A new pump with a new strainer sock, the short hose in the tank renewed and double-clamped, and a new filter. The original is a Denso; a direct equivalent fits the factory plug or comes with an adaptor lead, but the cheapest pumps are a false economy. Change the filter with the pump: a failing pump sheds debris downstream.
Burnt power terminals on the pump
The feed and earth terminals on top of the pump can arc and burn through. The pump then runs on too little current, so pressure is low rather than gone, and no fault code is stored.
Two or three seconds of cranking before it catches, rail pressure stuck around 25–30 psi instead of climbing to about 40, and an engine slow to pick up revs. A new regulator and cleaned injectors change nothing.
Put a gauge on the rail first; a loud pump is no proof of a healthy one. Low pressure with good voltage at the boot end points at the terminals on the pump.
Repair or renew the leads and terminals, and fit a new pump while you're in there — the old one may still spin on the bench.
An empty tank behind a gauge reading a quarter
The fuel gauge can sit at about a quarter while the tank is dry. The pump still spins, so it passes for a dead pump or even a jumped timing chain.
It stalls on the move with no noise, restarts only roughly with the pedal pumped and dies again; the pump is heard at key-on, yet the test valve shows nothing.
Pour in a few gallons before testing anything else.
Fuel in, and it runs normally. After that, don't trust the gauge near empty.
Not the pump at all: a loose relay under the heelboard
After months parked, the instrument relay under the left-hand heelboard cover can creep out of its socket. The dash stays dark, the pump never primes and nothing fires, while a code reader blames the pump relay with P1230.
None of the instrument lamps light with the key on.
Lift the left-hand heelboard cover and press every relay home before buying a pump relay.
Pushing the loose relay back into its socket; it starts straight away.
The fuel pump relay or the box it plugs into
The pump relay does a lot of work and its contacts wear. A bad contact switches the pump on and off, so the engine cuts out and catches again in an instant, or stops and won't restart. Sometimes the relay is fine and the moulded relay box itself makes poor contact.
It cuts out and won't restart; pressing on the relay brings the fuel back; repeated instant cut-outs that a sticking throttle body wouldn't give.
Swap in an identical relay, then bridge the load terminals in the socket so the pump runs directly. It runs bridged: relay or socket. Still silent: the pump side. With the engine running, wiggle the wiring to find a contact that drops out.
Worth trying a new genuine relay, the brown LJA 6703 AA; relays stamped R6J 4/5/6 or R6K 1–8 are worth renewing along with the ignition and engine management relays. If only pressing the relay helps, renew the box: LNC 2800 AC up to VIN 853935, LNC 2800 AA from 853936. Contact cleaner on the relay contacts, rather than emery paper, has also cured it.
A blocked fuel filter
A clogged filter leaves the pump fighting for flow, so pressure builds slowly and the mixture can run lean.
Several seconds of cranking before it fires, or a lean code such as P0171. Lean codes on both banks with power fading only on long fast hills, or cold starts that get slowly worse.
Find out when the filter was last changed; if nobody knows, assume it's due. The service interval for the filter is about 30,000 miles.
Worth changing before the pump: it's cheap and the pump job is not, and it can shorten cranking dramatically.
A tripped inertia switch
A safety switch cuts the pump after a jolt. It only trips after a knock, but checking it costs nothing.
No pump at key-on after a bump, a pothole or a bang underneath; warning messages can come up with it.
It's a cylinder with a reset button, high on the passenger-side footwell kick panel — on left-hand-drive cars behind the right-hand footwell trim.
Worth pressing the button down and trying again.
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.
Make sure there's fuel
The gauge can read a quarter on an empty tank. A few gallons first.
freeListen at key-on
About two seconds of hum from the boot or at the open filler, then silence until the engine turns. Nothing at all: carry on down this list.
freeLook at the dash
No instrument lamps with the key on: reseat the relays under the left-hand heelboard first.
freeCheck the inertia switch
Passenger-side footwell kick panel. Press the button down.
freeSwap, then bridge, the pump relay
An identical relay first, then a bridge across the socket's load terminals. Pump runs bridged: the relay or its socket. The relay's position in the boot box changed across the years — see the fuse page.
freeMeasure at the pump
12 V on the feed for two seconds at key-on and again while cranking, with no hum, means the pump. Bridge the relay and look for 12 V at the tank plug between the brown/green and the black wires; it's reached through the panel at the back of the boot.
a multimeterGauge the rail
The X308 test valve is smaller than most. About 40 psi that holds is good; low and steady points at the pump terminals, the filter or a tired pump. The test port is on the rear of the right-hand fuel rail. For a fault that only shows on the move, tape the gauge outside the windscreen and watch it as the engine stumbles. Also watch how fast pressure builds at key-on and how fast it falls after switch-off: a quick fall points at the pump's non-return valve or the regulator.
a gauge with a small adaptorChange the fuel filter
Cheap, and worth doing before any pump.
a filterGet to the pump
Quickest is a hole of at least 8 inches cut in the parcel shelf over the pump with snips or a nibbler, no sparks, closed afterwards with a steel plate: about three hours. A tank drop means releasing the plastic lines above the differential and lowering the rear; allow four to six hours first time, and don't rock the tank with the lines connected — they kink, and some are no longer available.
a pump 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) Cranks But Won't StartIf the pump hums at key-on and it still won't fire — flooding, bore wash-down and the crank sensor.
- Jaguar XJ8 (X308) Rough Idle and StallingIf it runs but idles badly or dies at junctions.
- Jaguar XJ8 (X308) Won't Start, No Crank or Car DeadIf the engine doesn't turn over at all or the whole car is dead.
- XJ8 (X308) fuse and relay boxesWhere the fuel pump relay and fuses sit in the boot — the layout changed across the model years.
- 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.