I have ande Elky with a recent TPI swap. Everyting is running great, but I noticed yesterday that when I went for Gas, there was a significant amount of pressure ( not vacumn) in the gas tank when I unscrewed the cap. I was expecting some vacumn, due to the canister purge. Any thoughts? Did I hook up the canister purge to the wrong spot on the throttle body ( hooked it to the second (lower) outlet on the passenger side of the TB).

 

Some pressure is normal. I couldn't say how much is significant.

JamesC

 

Sounds normal to me. My tank has hissed more than any other car I've owned, right from brand new. On really hot days (100°), it will sent a wave of heat out after taking the gas cap off.

I had to laugh getting my gas cap pressure tested for emissions testing. It's obvious that it's going to pass.

 

If it wasnt originally FI (I presume not by your info, says 85) the fuel sending unit does not have a provision for a vent. Did you swap out your sender? The stock setups in the F-body have the charcoal canister line and a line running to a fuel tank vent valve, besides the fuel pressure and return so there are a total of 4 lines.