1991 Jaguar XJ6 fuel tank

Never heard of a tank collapsing but i'm sure the tank is collapsing first as there is a vent issue which works the pump harder in turn fries the pump with heat and starvation.

So what vent should I be looking at?

On the tank there are a few lines, I assume its fuel injected? You have a supply line to the rails, you have a return line to the tank, you have the filler neck, (now the gas cap plays a part in the venting as well) and you have a breather vent line that goes to a charcoal canister to vapor the fumes, my first step would be to loosen my filler cap allow the tank to recieve air. Some tanks have another tube that runs up next to the filler tube which also vents, I would trace lines and seek their locations, also inspect for collapsed line. Two to the motor and your other is the breather. Once locate un hook and blow compressed air into and check that it goes in tank by sound if not its plugged or collapsed. Hope this helps you i'll see if I can find a diag for you.

Okay did a little research for ya. If your vin # is 629285 or lower you have a non return valve between your fuel pump and filter so its a backflow preventer more or less, now it also said that you have a fuel tank pressure control valve which bleeds the tank to a charcoal canister. I cant seem to find if the tank is steel or plastic. From the way I read it the fuel capbreathes in not out yet the tank build pressure releasing it through the pressure valve to the canister.I am assuming the pressure valve has a set relief psi so it will only let air to char can and not back in tank, so the cap breathes in and if thats faulty then pump is sucking all air and fuel out of tank which causes collapse. It makes sense but just another engineering overkill migrane! If the vin # is629285 or less cap # is cbc5292-oe if 629286 on up # is njd5942bb and can order them on jagbits. Com also you might find more forums for info and discussions w/other jag enthusiast. I'll keep the "vette". 74

Thanks for the info. Its a new cap from the jag dealer in St Louis so it should be ok. I will start checking the lines. By the way it is a steel tank which makes it even more odd but I did notice there was no swoosh when I took the gas cap off so there was definitely a pressure issue.