Jaguar XJ6 (X300) Fuel Pump Problems
On the X300 a fuel pump that seems to have died is more often the pump relay or a hose blown off the pump inside the tank than the pump itself. I'd listen for the short whir at key-on, swap the relay and put a gauge on the rail before anyone quotes for a tank drop.
How bad is it · Garage, carefully
Drivable while it runs, but it can stop without warning
A worn relay or a loose hose in the tank tends to let go all at once, often after a stop. Don't plan a long trip on it until the pressure has been proved.
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 fuel pump relay
The pump is switched by a cube relay on a green base in the boot fusebox, by the battery. Worn contacts add resistance, so a good pump gets too little voltage and the rail never reaches pressure. A cheap copy relay can fail within days.
It only starts after the key has been cycled a few times, dies after standing, or refuses and then fires on a later try with nothing changed. If bridging the socket makes the pump run but the relay never does, look at the base: carbon marks on the socket mean the base needs replacing too.
Swap it with the identical relay beside it. With the relay working, the pump side should read close to battery voltage; for example, 9 V against a 12.5 V battery means tired contacts. The ECU runs the pump for about a second at key-on, then only while the engine turns. On the bench, 12 V on pins 85 and 86 should make it click, with continuity between 30 and 87 only while it's powered — a normally-closed relay of the same shape won't do.
A new genuine relay; filing the contacts only buys time, so keep a spare in the boot. To get home, pull it and bridge 30 to 87 so the pump runs, then fit a new one.
A worn-out pump
The original Denso in-tank pump typically gives around 100,000 miles. The XJR carries two: the second cuts in only at high revs, around 3,000–4,000 rpm, through its own module beside the second pump relay.
Very rough running and then a stop, sometimes with transmission code P1775 because the engine ran so badly. On an XJR, a flat spot only at the top end. A pump can also jam outright when sticky residue builds up inside it.
Prove it with a gauge on the rail before buying anything: around 40 psi that holds steady clears the pump. Then look for 12 V at the pump plug with the key on. A healthy pump draws about 8 A, measured through its plug with a 20 A meter.
A new pump (JLM12204) with a new strainer and hose while the tank is out, then clear P1775; it shouldn't return. An in-tank replacement is less work than an external pump, which needs its own pickup and is noisy.
A blocked fuel filter making the pump loud
The filter sits on the frame rail under the car. When it clogs, the pump labours and whines long before the engine runs badly.
A whine from the rear that's louder than it used to be, while the car still starts and drives normally. A clogged filter can also show as a rough, lumpy idle.
Find out when the filter was last changed. If nobody knows, change it before blaming the pump.
A new filter brought the pump back to its normal note. A pump that stays loud afterwards can still run on for a long time, but budget for it.
The hose or seal off the pump inside the tank
A short rubber hose joins the pump outlet to the pipe leaving the tank. It rots from the inside, and when it blows off or its clip loosens, the pump whirs as normal but nothing reaches the engine. A torn rubber seal between the pump and its housing does the same.
It quits on the move or won't start after standing, the pump is audible at key-on, and no fuel comes out of the rail hoses.
Fit a gauge or crack a hose at the rail and crank. A pump that still throws a strong jet on a battery once it's out is healthy, which points straight at the hose.
Worth trying: trim the hose end and re-clamp it if the rubber is sound. Otherwise new 8 mm full-immersion high-pressure hose, 20–30 cm of it, and a new strainer; with the tank out, many fit a new pump too.
Not the pump at all: the crank signal or the fuel cut-off
The ECU only keeps the pump running while it sees the engine turn. A failing crank sensor, a wet plug on it, or the safety fuel cut-off switch stops the pump just as a dead one would.
You hear the short whir at key-on, then nothing while cranking.
Listen through the rear trim panel and look for 12 V at the pump plug with the key on. If it primes, the fault is forward of the tank. The inertia switch is on only when pressed down; popped up, it cuts the pump.
Work through the crank sensor and ECU plug checks before anyone touches the tank.
The fuel pressure regulator
It sits at the front of the fuel rail. A split diaphragm lets petrol into the intake and lets the rail drain after switch-off.
A strong petrol smell on the first start of the day and longer cranking, getting worse over a season.
Pull the small vacuum hose off the regulator: petrol in it means the diaphragm has gone. A gauge fitted in line shows a weak regulator better than one teed in. The regulator works against manifold vacuum: at idle the rail reads in the mid-30s psi, and pulling the vacuum hose should lift it to about 38–40. Measure cold and hot — the engine computer flags neither a pressure fault nor a leaking injector.
A new regulator is worth trying if the hose is wet. If the smell comes only after standing, follow the feed pipe too: it can rust through under a plastic clip. With petrol in the intake and a dry vacuum hose, take the rail off, pressurise it and look for a dripping injector; if the smell is outside the engine, check the vapour canister lines, the filter and the pipe joints.
Water or debris in the fuel
Water reaching the pickup starves the engine, and rust or bits of a failed pump block the strainer.
It loses power after a long run and recovers after a stop, with whitish-grey vapour from the exhaust while it's down on power. A tank left standing empty corrodes inside, and the rust packs the strainer round the pump.
Look at what comes out of the old filter. Bad fuel can be pumped out at the rail by powering each pump relay output in turn.
Worth trying: water remover through several tanks in a row. After a pump change, clear out the old pump's debris or the poor running stays. With rust in the tank, have it cleaned and fit a new pump and strainer.
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.
Listen at key-on
Put your ear to the rear trim panel. A quiet whir for about a second is normal; it stops until the engine turns.
freeSwap the pump relay
Boot fusebox, green base: swap it with the identical relay beside it and try again.
freeMeasure the relay output
Close to battery voltage on the pump side is right. A few volts short means worn contacts.
a multimeterCheck 12 V at the pump
At the pump plug with the key on. Voltage and a silent pump means the pump; no voltage means the relay side or the ECU.
a multimeterProve pressure at the rail
Around 40 psi that holds steady clears the pump. Pump runs, no pressure: the hose in the tank. At idle, with the regulator's vacuum hose on, expect a little less.
a cheap gaugeChange the fuel filter
On the frame rail. Cheap, and it quietens a whining pump.
a filterOnly then open the tank
The tank comes out after releasing the feed and return pipes above the differential with a quick-release tool (310-044 or a plastic clip tool). Allow three to four hours in a workshop and a full day first time. A hole cut in the rear parcel shelf reaches the pump from above; the sender hole is too small. Changing the pump through the sender and pump openings of an empty tank has been done, but it's very fiddly and the baffles get in the way; tank out is the usual route.
a tank dropWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Cranks But Won't StartIf the pump whirs at key-on and it still won't fire — crank sensor, ECU plug and flooding.
- Jaguar XJ6 (X300) Rough Idle or High IdleIf it runs but stalls at junctions or hunts at idle.
- Jaguar XJ6 (X300) Immobiliser Problems, Won't StartIf the pump never primes and the security light behaves oddly.
- XJ6 (X300) fuse boxes and relaysWhere the pump relays sit in the boot box — 1996 and 1997 from the same page.
- Jaguar XJ6 (X300) Misfire, Ignition Coil FailureIf it stumbles on one cylinder rather than losing power across the board.
- 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.