I'm in the process of trying to determine why my car stalled and won't start...

I removed the gas cap and had a very strong positive pressure come out that almost forced the cap to blow off.

What is causing this and does it have to do with the engine stalling and not be able to start.?

 

Your Rochester Valve is blocked. The fuel tank is being overpressurized because it cannot vent itself.

I'm trying to figure out why it would cause your car not to start? Does it start after you remove the cap? Is there a Check Engine Light?

 

See page 60 and X of the Jaguar 801s Doc

801S TITLE (jagrepair.com)

the Rochester plumbing is on the doc to make sense of what's going on

The Rochester is not used on all X300 models and a quate from page Y

A percentage of 1996 MY ON Sedan Range vehicles are equipped with a twin canister enhanced evaporative emission system that provides reduced evaporative emissions and enhances the system’s on-board diagnostic capabilities

To know if you do have a rochestor it will have 1 square vapor canister fwd of the left rear tire

If you have 2 vapor canisters , 1 on left and 1 on right you will not have Rochester but instead a canister close valve that is electric controlled by the engine ECU

Editing

In the end if you have too much vapor pressure pressing the fluid against the fuel pressure regulator inlet side it will take care of it and reduce it to normal fluid pressure

But there was someone that stated by changing the Rochester valve they " recovered " their idle , idle being different then starting but there my be a relationship somewhere like the FPR return line not being able to return to a pressurized fuel tank.