Pressure in fuel tank

When removing fuel cap there's a lot of pressure
suddenly released almost blow the gas cap out of
my hand?  Also engine rpm is very slow returning
to idle when letting off gas pedal, checked the
throttle body butterfly returns normally but it's a  
couple of seconds before the engine idles down.
Checked everything, everywhere I could for
vacuum leak by spraying carb. Cleaner around ,and
nothing no engine rpm change. PCV is clear and
drawing good vacuum, vacuum line to fuel
regulator good, but when I removed the vacuum
line from the fuel regulator there is no change in
how the engine is running. It starts and runs good,
but the idle changes at times and the engine
races,well runs up to about 2000 rpm for a couple
of seconds on it's own once in awhile.
I'm thinking coolant temperature sensor to
computer not the gauge. Does it control the purge
valve? I know it controls fuel mixture and timing.
I checked the engine wiring and sensors, the
coolant tempture sensor is can't get a constant.  
Thanks for any advice