Fuel Tank Pressurization

While troubleshooting my current "sudden death" syndrome (http://www.naxja.org/forum/showthread.php?t=68356) I noticed that the fuel tank was not pressurizing like it used to.

My current driveway anchor is a 2000 Cherokee Sport with 4.0 engine. The fuel tank has the top-mount all-in-one fuel pump module.

Can anyone tell me:
- What, if anything, pressurizes the tank? (EVAP system? How?) Or, is this just from the vapor pressure of the fuel?
- What is normal pressure for the tank? (I'm guessing only a few psi or less.)
- Does the fuel pump module affect or regulate tank pressure in any way? (looks like all it controls is fuel pressure, not tank/air pressure)
- What will happen if the fuel tank does not pressurize? (i.e. - would leaving your gas cap off make your XJ stall out/undriveable?)

I'm going to drop the tank (again) anyway, so if I remove the filler neck and hook it up (with the gas cap) to the pulled tank, I should be able pressurize the tank and check for leaks. If the tank and fittings hold up, that would leave the EVAP system.

- Would a leak or lack of pressure cause a fault or failure with the EVAP system? One that would cause the engine to shut down?

Thanks again...

There is a evap pump to pressureize the tank, it's only for emissions and will not have any effect on how it runs.