Jaguar Repair: 1986 jaguar xj6 gas tanks, 1986 jaguar xj6, tank gauge

Question
my passenger side tank overflows when the car is running and the gas guage on driver side reads below empty? is it just the return valve stuck open? or more? the car sat for 20 years and just recently started doing this. im trying to get it back on the road and i appreciate any help you have to offer. thanks

Answer
Hi Jesse,
The ser III Jag has designed the fuel system so that the left tank can be used if any of the switching valves fail electrically. The fuel supply valve in in trunk is open to the left tank when no power is supplied to it and it switches to the right tank only when power is applied.
The left return valve is normally open and only closes when power is applied. The right return valve is normally closed and only opens when power is applied.

When the right tank overflows when you are using the left tank you need to check to see that the left return valve (at the lower left fender well under a small panel) does not have power to it when the left tank is selected. If that is true you need to then check to see that the valve is not stuck in the closed position and that the lines are clear and not pinched closed. (This is easy to do by just selecting the left tank and open the filler cap and with a light look at the fuel being returned. Then look in the right filler cap to see that no fuel is returning to that tank.)

Next you need to confirm that when using the right tank that the in-trunk switching valve is not stuck in the left tank position and that it is receiving power at the time you have the right tank selected. A quick test of the valve is to have someone switch the tanks while you put your hand on that valve to feel it "Click" each time the tanks are switched to the "Right" tank from the "Left" tank. This is not a 100% test of the valve but a good indicator that the valve is working.

The fact that the right tank gauge is not working does not tell you what is wrong because the tank sending units are prone to rust and stick when left for years. You need to drain the right tank and remove the tail light assembly and remove the sending unit to check that it is not stuck in the low position. I have been able to save about 50% of them with WD-40 and gentle working of the arm.

Most likely you have one or several valves stuck causing the fuel to return to the right tank all the time. If you don't have a diagram of the system and want one, let me know and I will put one up on my web site for you to copy.

Howard