Jaguar XJ6 Series 3 Fuel Tank Switch, Changeover Valve
On the Series 3 the pump draws from whichever tank the changeover valve selects, and the return valves send spill fuel back to the tank shown on the switch. When the valve sticks or goes in backwards, one tank runs dry while the other overfills, and the engine starves with fuel on board. It's usually dirt from rusty tanks, so I'd look at the fuel in the filter before buying a valve or a pump.
How bad is it · Garage, carefully
Get home on the good tank, then fix it
An overfilling tank pushes fuel out through the vent and the charcoal canister, so a strong fuel smell means switch tanks and stop soon. A stuck valve can also leave you stranded on an empty tank with fuel in the other.
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 changeover valve stuck by dirt from the tanks
Three solenoid valves live in the boot: the changeover valve on the feed and a return valve for each tank. When the changeover valve sticks, the pump keeps drawing from one tank while the spill fuel goes back to the tank you selected, so one fills to the brim and the other runs dry. A valve that worked for years and then sticks has usually caught rust or gum from the tanks, and a jammed valve can take the pump with it.
One gauge climbs to full or pegs while the other falls to empty; a strong fuel smell, fuel at the rear vent or the charcoal canister; moving the switch changes nothing. It dies on the road and then only runs with the revs held up. No click from the boot when the switch is moved.
Have someone work the dash switch while you listen at the valves; each should click. Switch out (right tank) puts 12 V on all three valves, switch in (left tank) leaves them unpowered, and a valve that loses its feed falls back to the left. If a valve clicks on the bench but not in the car, suspect the dash switch contacts dropping the voltage. Drain the main filter into a clear jar: brown sediment means the tanks are the real cause. If a tank starts overflowing on the road, select the other one to get home.
Fit half-inch pre-filters (for example NAPA 3299) between the tanks and the changeover valve, then clean the changeover and return valves in carb cleaner; the original valves often outlast new ones. Bench-test any replacement, because new valves can stick or leak across straight away. Don't fit a new pump until the dirty fuel is out. Some cars do without the valves altogether: a T piece on the feed and one on the return, so both tanks are used together, or one pump per tank switched by a relay. The valves are mainly there to stop the lower tank overflowing when parked across a slope.
A changeover valve fitted the wrong way round
The valve's wires fit either terminal and the hoses fit either port, and the car still runs. Fitted backwards, it draws from the opposite tank to the one the switch and the gauge show. The engine runs on the wrong tank until it's dry, then starves with fuel left in the other.
Trouble that starts after a new valve or new hoses, often after a previous owner's work. It stops with the gauge showing fuel and fires at once on the other setting; the pump delivers only a dribble into a bottle.
With the switch out, confirm 12 V at the valve, then take the tank hoses off the valve one at a time and see which tank actually flows. A weak flow from the pump inlet into a container points at a blocked or wrong feed; prove the pump itself only with a pressure gauge.
Refit the valve so the tank it draws from matches the switch position and the gauge.
Rust, water and sediment in the tanks
The twin steel tanks rust inside, and water gets in through a perished drain hose behind the filler cap hinge or a tired cap seal. The dirt settles in the tank sump and blocks the in-tank strainer, the pre-filter or the main filter. A roller-vane pump will jam on fine rust even behind a good strainer, and relined tanks can still shed it.
Stumbles and cuts out at low speed, runs short of fuel above about 3000–3500 rpm, loses power at motorway speed and recovers later. Water in the fuel shows as a rich smell, fouled plugs and the odd puff of brown smoke. Power that comes back the moment you change tanks points at one tank's strainer. Starting on a squirt of starter fluid but not on petrol, with the pump humming, is water in the tanks until proven otherwise.
Change tanks as soon as it starts to starve; if it recovers, the fault sits on one tank's side. Open each tank drain and see what comes out: water, black muck or clean fuel. Take the filters off and blow through them onto a white cloth; a pre-filter you can't see through hides its own blockage. If the pump spurts, feed it from a can of clean fuel: a steady flow then puts the restriction upstream. Watch rail pressure as it dies; falling pressure is fuel, steady pressure sends you to the ignition.
Drain and flush the tanks, clean the sock strainer above the drain and fit new filters; expect to change the pre-filters a few times until the loose dirt is caught. Clear the filler drain hose and renew the cap seal if water is getting in. A leak at the drain takes a new sump seal (C26310) and, with the sender out, a new sender seal (ARA1502). Opening the drains every few years keeps water and silt from building up. A drain bolt that weeps wants a new seal and a smear of sealant rather than more torque. A rusty tank kills a new pump quickly, so the pump comes after the clean-out, not before.
Every gauge dead at once, a bad contact on their shared feed
The fuel, oil and temperature gauges share one green supply wire through one fuse. A poor contact there stops all three together while the warning lights and panel lighting carry on.
Fuel, temperature and oil gauges all lying at rest at the same time; warning lamps normal.
With the key on, look for voltage on the green wires at the gauges. Check the gauge fuse and its holder by measuring across it rather than by its look.
Clean and reseat the fuse and work along the feed wiring until the contact is restored.
A tank sender, the dash switch or the gauge wiring
The Series 3 fuel gauge has no voltage stabiliser: battery voltage on a green wire, and a second wire to the dash changeover switch, which routes it to the right sender (light green/red) or the left (light green/slate). The switch that moves the valves also picks which sender the gauge reads. A float full of fuel, an arm jammed in a dented tank, a worn track, a weak earth or a corroded wire all make it lie.
Reads empty with fuel on board, drops from three-quarters to empty in one go, or reads differently on each side; a new sender changes nothing.
First make sure the gauge isn't right: a valve leaking across moves fuel from tank to tank. Earth the sender wire at the switch for that tank and the needle should swing to full, which clears the gauge. Measure each sender from the switch to earth: roughly 20–30 ohms full and about 220–300 empty; zero is a short, no reading a broken wire. Sweep a new sender slowly by hand on the meter before it goes in, and look for jumps.
Replace the sender or the dash switch that fails the test; on some cars the gauge read true only after both senders, the switch and the valve were renewed. Bench-test a replacement gauge with its case earthed as it would be in the car.
A blocked tank vent holding the tank under vacuum
Each tank breathes through a vent line to the charcoal canister. If that line clogs, the pump pulls the tank into a vacuum it can't draw against, and the engine starves after a run even with fuel on board. A vent wrongly joined to engine vacuum does the same and can pull the tank in.
Dies after 15–20 minutes or after a long hot run, restarts after a pause and dies again; a rush of air when the filler cap is opened.
Drive with the filler cap loosened; if the fault goes, the vent is blocked. Open the caps after a run and listen for the hiss. Then trace the vent hoses to the canister and blow through them, and make sure none of them reaches a manifold vacuum source.
Worth trying: clear or replace the blocked vent line and refit the canister hoses as built; a fuel-soaked canister can be refilled with fresh charcoal. A tank already pulled in has to be replaced, since it starves the pump and confuses the sender.
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.
Change tanks and note what happens
When it starts to starve, select the other tank at once. If it recovers, the trouble is on one tank's side: a strainer, a hose or the valve.
freeListen for the valves
Key on, flick the dash switch and listen in the boot: the changeover valve and both return valves should click. Silence means checking the voltage at the valve, then the switch.
freeLook at the fuel
Drain the main filter into a clear jar and open the tank drains. Rust, water or black gum puts the tanks and filters ahead of the valves and the pump.
freeFind which tank really feeds the pump
Take the tank hoses off the changeover valve one at a time and compare the flow with the switch position.
freePre-filters, then the valves
Fit half-inch pre-filters ahead of the valve, clean or bench-test the valves and fit a new main filter. Then check fuel pressure raw and regulated.
pre-filters, a main filterThe pump last
Only once the fuel is clean. A pump that has run on rust may still need replacing after everything else is done.
a pumpThe 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, 1987 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 XJ6 Series 3 Won't Start, Cranks But Won't FireIf it turns over but won't fire at all: the pump, its relays and the valve at start-up.
- XJ6 Series 3 fuse boxesThe fuse panels of the Series 3, for the gauge feed.
- 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.