Hi,
Recently my car just started building vacuum inside the fuel tanks.
I can clearly hear the tank "imploding" while driving, and when I come to a stop and release the filler cap, a strong woooosh followed by the sound of the tank expanding back into place can be heard. I'm afraid of driving the car because eventually the tank will crack from such gymnastics.
I don't know how the pressure relief system works on the XJ6 Series 1, there should be a release valve somewhere.
I'm already thinking of piercing the inside of the filler caps so as to relieve the pressure, but would like to know if there is a better solution.

Thanks!

 

I had that same problem, except my symptom was having the engine stall out, usually in precarious situations, because the fuel pumps couldn't supply enough pressure to overcome the vacuum.

There is information (e.g., jag-lovers.org archives) on the plumbing, which goes to a canister at the front of the car, but I did essentially what you are contemplating. But in my case, I simply disconnected the small lines visible at the front of the filler neck.

BTW, the earliest XJs did have vented filler caps, IIRC.

 

see if your car has a check valve near the charcoal canister, if it has one, remove the check valve and splice the two hoses with a straight-thru connector.

The Series 3 XJ-6, (1979½ to 1987), has that problem and at some point Jaguar did a recall installing an electronic "purge valve", but I have no details other than that.

In my 1984, I simply pushed a 1/8" drill bit through the check valve to break the inner membrane and the problem dissapeared forever. Those check valves in the Series 3 are a GM part which apparently are not correctly calibrated to open and release fuel vapors when a certain pressure is reached.