1981 XJ6 stalls, gas tank spews fuel everywhere when the cap is opened.
 

I could use some help! Again.
Today my XJ6 decided to stall at idle. When I restarted it the thing started but would only just stumble along enough to get off the road.
I wasn't certain, but I felt like it was related to one of the tanks but not the other as both times it stalled was running from the right tank but I drove for quite awhile on the left tank in between and all was just fine. I opened the fuel caps and the left tank opened quietly but when I opened the right tank it spewed fuel everywhere.
Is my stalling likely a result of whatever makes my fuel tank spew like a volcano, or am I looking at two unrelated issues?
Any idea how to fix it?

Ok, I may have sorted this out on my own. I filled both tanks after work today and had only driven a couple of miles, but the left tank was down a quarter while the right tank was filled to overfilling. I had changed over the tank with the switch to the right tank shortly before it stalled while idling at a stop light.
I think that when running on the left tank, fuel was returning to the right tank until it was under substantial pressure, then when I switched to the now pressurized right tank the engine ran too rich to idle.
Once I let the pressure out of the right tank (to my great surprise) it idled fine. I took it for a drive for about 20 minutes staying on the right tank and saw no issues.
I'm thinking I need to look at the return valves, the one for the right tank might be stuck open. It just seems super weird to me.
I'm not super confident in my diagnoses, if anybody has any ideas I would be very appreciative.
Also, if anybody knows of any gotchas I should be aware of when taking off those return valves, that would be helpful before I dive into that.

Fuel spilling out of one tank is almost certainly due to a fault with the fuel return. Could be a stuck return valve. Could be an electrical issue. The fact that you can run on the right tank with no issues means that your car by default is returning fuel to the right hand tank, no matter what position the fuel switch is in.

There are 2 return valves. One for each tank. One is normally open (meaning if you dont supply any power to it, it will be open) and the other is normally closed. When you apply 12v to them, they both change over (one opens, one closes )which causes the fuel to return to the opposite tank.

There is a lot more info on this thread, mainly supplied by Doug

https://www.jaguarforums.com/forum/x...-valve-144200/

As you will have read in that thread, the right hand tank (and the return valves) is only set with 12v. So, the first thing I would check would be that when you press the fuel switch in (to select left hand tank) the 12v is removed from both return valves wiring. You can also get an assistant to press the fuel switch back and forth while you listen to the valves. They make a kind of clunking noise when they change.

As for the poor running, I highly suspect you were actually running out of gas. When you have a tank feeding with no return, it doesnt take very long to completely drain that tank. It that state (one tank dry, fuel lines dry) it can take a while for the system to recover once you switch over to the other tank.

Fuel pressure is dictated by the fuel pressure regulator on the fuel rail. Even if your supply line is giving much more pressure than you need, the regulator should be able to cope with it and bleed the pressure off (by returning more fuel back to the tanks) Your tank "pressure" due to the overfilling is before the pump, so I doubt thats the issue.

Until you do some more diagnosis of the valves, keep running on the right hand tank. Gas spilling out of the tank cap and down the rear wing near the exhausts is generally not a good thing.

Thank you, that makes me feel much better. You make a very good point about the fuel pressure regulator, I agree that the pressure in the tank shouldn't have directly caused my stall. I know that the left tank still has at least a half tank in it though. I wonder if the pressure in the tank pushed fuel up through the evaporation system and somehow caused my stall, is this possible?

I had a similar valve issue on my 76 XJ12 (fuel injected). When the tank overflowed the fuel got all the way up to the front of the car and spilled out of the charcoal cannister, making a huge mess. This did not, however, impact the car running so Im not sure if that could be related to your stall.

What you can do is get both tanks half full. Then press the fuel switch in and out. The car should maintain perfect idle throughout. Note : Do not do this for more than a few minutes as obviously all the fuel is going back into the right hand tank so you risk it overflowing again if you run on left hand tank.

I am wondering if maybe you have an issue with your feed valve....... If thats not opening correctly on the left hand tank, you may not have enough fuel to build pressure....... Or maybe there is a blockage somewhere in the feed line from the left hand tank. Do you know the condition of the tanks? Anyway, running the above "both tanks half full" experiment may shed some more light.

The car is new to me, so I don't know much about any of it yet. I'll do the suggested experiments soon and report back.

Got it. These cars are great, but they have some peculiarities, one being the dual tank system you are learning so much about now.

Another scenario that could stop the engine on the left hand tank could be if the left hand return valve is stuck closed. When you select Left Tank from the fuel switch, the right hand return valve (correctly) closes, but the left hand one doesnt open as it is supposed to. This will lead to no return path and massive over fuel pressure, which would surely stop the engine....... Anyway, the half tank test will tell us more...

I looked into the fuel filler caps with the little flap pushed aside and you can see the return line down there with a flashlight. No matter which position the switch was in I could see it running into the right tank. I have a parts car, so I swapped the changeover valves for the return between both cars.
My theory on the stall was that the stall occurred once the right tank was so full that it could accept no more fuel and then it is just as you said, there was no longer a return path, at which point the pressure causes a stall if I stay on the left tank.
In any case, it works great now, those valves were easy to get too and quick to change out.

Ah yes, that makes perfect sense..... completely full tank also means no return path.

Glad you got to the bottom of it.

Odd, but not exclusive. Decades ago, I bought a very nice 85 Ford
F150 4x4. I wanted a Bronco, but, never found what I wanted at the right price. So, the F150 was almost right. Ford's first FI truck. Intrigued me in spite of "first model year" warnings.


I only learned of it's two tanks when I went to my local gas station for a "fillup" Indeed, it had a tank selection control in the cab. A good feature was that if one ran one tank down to empty, the engine continued to run on the second tank, unlike the Jaguar system.


Not long after we bought the 83 Jaguar, we were going to church
one Sunday morning. The Engine went sluggish in the middle of a busy intersection. Instinct took over. I punched the tank selector switch. OK, engine picked up and away we went. Our custom was brunch after church, which we did. But, then, off to the gas station
and top off.


Since then, my practice is to run each tank in turn down to 1/4, the fill each. Alternate selection as I go.


Carl