Jaguar Repair: fuel overflows from right tank, return valves, return valve

Question
Hello I just replaced the fuel changeover valve, and both return valves at the rear wheels new fuel filter, and when on the passenger side right side the car will run but overflows alot out the filler cap, Runs fine on drivers side and fuel is returning to drivers side while running. when i change to the right tank i hear the click but cant look in the tank  cause it starts overflowing immediately Many thanks, and need a jag mechanic thanks again

Answer
Hi Jerry,

First I need to tell you how the system operates and with that information you will see what is going wrong.

The fuel switching valve in the lower rear of the trunk draws fuel from the right tank only when powered and blocks off fuel from the left tank. Only when power is cut off for any reason, usually when you switch to the left tank on the instrument panel or a loss of power due to a break in the circuit.

The return valve on the right tank (in the right rear wheel well) Opens only when power is applied. (by the instrument panel switch) When this valve has no power, it is normally closed.

The return valve on the left tank is normally open and is only closed when power is applied.

Most of the fuel pumped forward is returned by way of these return valves however, the fuel picks up heat by being forced forward and from the road heat and engine heat which expands the fuel so the return fuel is cooled by the air-conditioning system. A fuel cooler mounted on the Harrison compressor.

If you have the right tank filled to the brim and operate the right tank with an AC system not running you could be pumping heated fuel back into that tank thus an overflow. To test the system to see that it is a failure of the return system and not heated fuel causing the problem you should drain a little fuel out of the right tank so you can see (with a flashlight)the returning fuel in the right tank.

You can drain a little fuel by the drain plug in the bottom of the tank under the large plastic cover on the bottom of the tank.

Then turn on the left tank and confirm returning fuel in the filler neck by opening the small flap and go to the right tank and confirm that NO fuel is being returned.

Then turn on the right tank and confirm returning fuel in the right tank and check the left tank to see that NO fuel is returning there.

If you still see fuel returning to the right tank when the left tank is selected, You first test both switching valves for "no" power then use a fuel block-off clamp to clamp off the right fuel tank supply. (BE careful as the large hose can be old and brittle and crack when pinched off) This test is for a stuck valve. (I found several of those)

Then confirm that you have "NO" power to the right and left return valves. These valves also can stick in one position or the other (found several of those too)

Next confirm that you didn't get two of the same return valves or that you didn't get them mixed up as they look alike.

Good luck,
Howard