Pressure build up in the gas tank is normal in the fuel injected V-12 engine. The original twist on gas cap is rubbish. I got tired of gas fumes leaking from the gas cap. Exploding in a ball of fire in my Jaguar is not one of my goals. In addition, the original Jaguar mounting bracket is garbage. It's a single piece of metal that holds the filler neck to the body.
The solution to this problem was drastic. Junk the filler neck and replace it.
I went to the junkyard armed with a hack saw and cut the filler neck off a late model Ford that utilizes the modern twist on cap. This style cap is designed with a large "O" ring and seals significantly better the chintzy Jaguar cap. I then cut approx. 3 inches off the Jaguar and Ford filler necks. I welded the Ford neck to the Jaguar filler pipe. (The part that goes into the gas tank).
I removed the original bracket from the new filler neck. I fabricated a new mounting bracket that actually bolts securely to the body. I used sheet metal stock cut diagonally in a star pattern and bent 90% (yellow arrows photograph below). Rotate the filler neck until the drain hole is positioned at the bottom of the neck. This allows excess fuel trapped in front of the "unleaded fuel plate" to drain back into the tank.
The photograph below, taken from inside the trunk, shows the large hose clamp that holds the fabricated flange to the filler neck. This allows the filler neck position to be adjustable. The filler neck needs to be positioned somewhat lower so the new (taller) cap clears the filler door.
I bolted the new filler neck mounting bracket to the body using the existing trim screw locations. (blue arrows photograph below). Note that the filler neck does not reside in the center of the body opening; it’s positioned lower and slightly to the right. Use the rubber filler ring to determine the exact location before bolting the fabricated bracket to body.
Insert the rubber filler ring and align the trim screw openings.
Install the metal trim and secure using the original body trim screws.
Install new twist gas cap and tighten three clicks.
Project complete. This is one of the most useful modifications you can make to your Jaguar. A replacement gas cap is now $10.00 at the local auto supplies store, and unlike the $40.00 Jaguar poorly designed piece of junk, this cap actually works.
