Started driving today, left tank reading 1/2, right tank reading 3/4. Drove for about an hour while driving left tank increased to 3/4 full over that time. When I switched to right tank it read almost empty. I am stumped. When I run the car at idle it he returns seem to be working properly and returning fuel to the selected tank. Seems like a different story when driving. Anyone have a clue as to what is going on? Should I replace fuel tank selector valve and return valves?

 

Yes.

Or you could do some checking and things down a bit so as to possibly avoid buying all three valves.

But I certainly can't explain or understand why being stationary at idle versus driving down the road would have any bearing on the operation of the valves

Mulling required. I'll report back should an epiphany come my way.

Cheers
DD

 

It sounds like the selector valves. The first test, but NOT a cure, is a judicious blow to the valve in/behind the spare wheel well. If that temporarily corrects things, that valve isn't working correctly. There are valves close to the tanks, but, in my experience, they're less likely to cause problems.

Unfortunately, replacing the valve(s) isn't always the end of the story. They are a bit feeble and very sensitive to dirt in the fuel lines, which usually means rust in the fuel tanks...

If I could change one thing in the design of the series 3 XJ, it would be this.

 

Been there done that. I tried all the tests: multimeter to check electrical signal, blowing through the return valves while adding and removing current, etc. The changeover valve had been replaced by previous owner. Defied all logic. This was on my pristine 1987. My old lump 1979 was working perfectly. So I pulled all the valves from the 79 and installed them on the 87. Problem solved on the 87. On the 79 I replaced the changeover valve with a brass “T” fitting and the return valves with brass straight thru connectors.

So I have lived with both systems for a year now. I prefer the modified “one virtual tank” system. The gas gauge shows the total amount of fuel available and I can fill from either side. Some have said that if you park on a hill where one side is lower the gas will overflow on the downhill side. Maybe if both tanks were full to the brim and the hill was really steep that might happen. But it has never been a problem for me.

So if originally is important to you, keep testing, replacing parts, etc until you get it fixed. If your car is a driver and not a show car, trot down to the hardware store and get $10 worth of fittings and be done with it. If later you or the next owner want to go back to the factory system it is completely reversible.

Jeff

 

Hi Jeff, we do like to drive it so the one virtual tank system intrigues me. What exactly do I need to replace to make that happen. I agree with someone who said that this is a flawed system and personally I would prefer to just be able to put gas in it and not worry. On 2 occasions my right tank has overflowed which I can’t figure out why as the return valves seem to work fine when idling and I look into each tank depending on what tank is selected.

 

Replace the changeover valve in the boot with a 1/2" brass barb Tee. It looks like a capital T with push on barbs on all three sides. Then replace the return valves in the rear wheelwells with 3/8" barb connectors which are just a straight pipe with barbed fittings on both ends. In each case carefully tape off the electrical connections for the valves and just leave them there. Technically you only need to do one of the return valves but I did both. That is the short answer.

In reality it’s a bit more complicated because gas will go all over in the boot unless you drain the tanks first. Lots of posts here on the forum about replacing the boot fuel lines. Since you are going to be doing the messy part this is an ideal time to replace all the rubber fuel lines in the boot. I did not drain the tanks. Following advice on the forum I made a bunch of wooden 1/2" pointed dowels to use as plugs and I parked heading uphill and put big pans under the boot vents so any gas that spilled went out the vents and into the pans. With planning ahead I spilled very little gas. Be sure to do it outside (to reduce fire hazard) and not around grass (gasoline will kill your lawn).

When you are done fill up with gas and try the switchover dash switch and you can choose whichever gas gauge is the most accurate.

Jeff

 

When I was working on my fuel system, I added 1/4 turn ball valves in the lines directly out of the tanks. Should minimize fuel spill when working with anything "down stream".

Dave