Well you might want to see this....
This is just a theory & I am not saying this will work 'nor to do it, but by looking at the setup, even if everything was working properly, it would still end up like this(considering all the talk about these cars & the tank leakage, "whooshing", etc). These are just my thoughts as to a possible source of tank problems.

Mind you this is on a EURO spec 1985 XJ6 & only entails the rear tank area, as the euro does not have the luxury of a closed evap system. This is also a "NEW" aftermarket tank CAC 55221. That was installed in 2003. Car only had 5000 miles put on since.

I had trouble with the car stalling after an decent amount of drive time. I would pop the cap & spray would blow out to about 6" high & would hear the tank flex & the car would run fine & then just stall again at any given time.

These are my observations while removing all components.
I didn't bother with photos of the drivers side(left tank) I gave it to my friend to recycle. But it was already welded once & about 2" from the bottom was completely rusted from front to back. I took it out & tossed it into the bed of his truck.
This is the one that was in better condition & of prime concern. This is what happens when your ventilation system on the tanks is clogged or not working properly.

For those who do not know, as your tank uses fuel, air has to replace the space that was occupied by the fuel, if air cannot replace it soon enough/at all, you will get a vacuum & in turn the implosion you will see in the photos(if you ever hear your tanks"pop"..I can almost guarantee you will get this soon, if not already). This also causes a lack of fuel issue, because the pump would not be able to suck fuel from the tank & in turn..stalling.

I believe here is where the problem lies......
On the filer cap if you look between the cap & body you will see a hose running into the body. On the cap housing there is a VERY small hole..maybe .5mm..yes THAT SMALL. This is the hole that is supposed to "vent" the tank(goes to the EVAP system on USA cars).

I do believe that if the cap is removed..very easy to do takes about 5 minutes with a screwdriver & the hole drilled out a bit bigger, this would allow the tank to take in more air & prevent the IMPLOSION you see here.
I cannot test this as like I said I removed the entire fuel system & am installing a fuel cell.

Here are pics of the tank & the "vent" hole I am referring to.

 

Geez, you weren't kidding when you said a it was tiny!

 

Here is what I have done for years to solve this problem.

I simply open the gas caps (or "cap" on the XJS) and lay a rag carfully around the upright lid and over the filler opening.
I choose a spot in the metal part of the inside centre. I want to be a little concentric with my target, and then with a 1/16 drill bit, I simply make a hole, being careful not to allow any filings into the gas tank. Also being careful not to go though the outer cap itself.
This takes five minutes including finding your drill and bits. Problem solved. No more imploding petrol tanks.

JustV12.

 

Drilling a hole in the underside of the cap is a simple solution, but will allow gas fumes to escape. On the jaglovers forum, the popular solution to this issue is to use a 1/8" drill bit to poke through the inside of the one-way valve in the right front wheel well near the charcoal canister. The valve tends to stick closed and blocks off the flow to that small hole pictured in the filler neck above. By puncturing the diaphragm inside the one-way valve and making sure the tiny vent hole is clear with a paper clip pokey tool, the fumes will still be absorbed by the charcoal canister.

The valve is located behind the right front wheel, and has a small hose on one side, with a larger hose on the other:


Here's a long-winded discussion about it:
http://forums.jag-lovers.org/av.php?1315250l38

 

bbarcher, notice I said this was for the Euro car. I think all the stateside peoples know about the one way valve.
But you got that right about long winded...One reason I left that forum. Plus the fact that if you use anything other than JAG parts, or go against their "thinking' they shun you.

Euro's do not have all that stuff up front.

From everything I gathered while taking it all apart, the EURO cars do not even have a "venting" system as it is ALL contained. The small holes that are on the filler neck go to small canisters in the rear pillars & then back to the tank...it makes absolutely no sense.

There is no where that I found to where it is vented to the atmosphere & I have everything that is stock on this car out of it.
Here si the exact routing that I found...

It goes from the filler cap to the small canister. From the canister back to a 90 deg fitting return to the tank. Both of the small canisters are connected together by a hose.

here are some pics of one of the small canisters. I am also using the canisters in the venting of the fuel cell.

 

Yes I overlooked the fact it was a EURO or failed to remember it didn't use the charcoal canister. Sorry about that, just wanted to throw that info in here for people in the US I guess.

I did some quick research on the venting systems of EUROs and it sounds like the two pipes that are connected together on yours are supposed to be open to atmosphere through a hose or pipe that comes out somewhere behind the rear subframe. If they are connected together with a sealed hose, then there is no venting as you said. There should be a tee to an open hose under the car, or two open hoses- one for each side.

I made a drawing a while ago that shows the vapor tubes and how they connect for the US cars; on EUROs the one labeled canister should be open to atmosphere, with a hose allowing it to vent outside the car. Might be a good idea to put a filter on the vent hose to prevent debris from entering the tube.