My car hasn't been running for 10 years. I've been in the process of getting it road worthy,and right now I'm rebuilding my throttle body.

The car was almost out of gas,so I put about 4 gallons inside. I continued to do some work on the throttle body after I put fuel in. I went to open the hood to get ready to put the throttle body on,and I realized the fuel line was leaking fuel all over the engine bay and my parents' garage floor. I threw a silicone hose over the fuel line,and plugged it up for now. I went to go siphon some fuel out to stop the fuel line from leaking,so I could try to put the throttle body back on (I still haven't),and either air shot in the filler neck,or air shot out of it. I'd go with the latter. I haven't had the key in the ignition for 2 weeks,so it couldn't be the fuel pump actually turning on. It's not hot inside the garage;in fact it's about the same temperature as outside. I wanna know why there could be fuel pressure build up without doing anything aside from adding some more fuel.

 

The fuel was probably cooler than ambient temp when it was put in. After sitting in the tank, there was vapor expansion from warming, causing pressure in the tank.

 

Oh I forgot about the vapors. That makes sense I guess. Thanks.

 

NOW would be a good time to verify proper fuel tank venting into the charcoal canister, and proper canister purge valve operation.

You need a proper, Genuine GM service manual for this vehicle.