For reference the car is a 1989 Z-28 305 TPI.

I don't know what code it's throwing exactly, but my gas tank has a ton of pressure built up whenever I take the gas cap off, even if it's after just a quick 10 minute drive.

I was driving it home from class tonight and the service engine light came on and the car was unable to rev above 2.5/3k. when I unscrewed the gas cap and let the pressure out and simply screwed it back on the light turned off and the car started running normally again.

It still has the stock charcoal canister, but I replaced the EGR valve and cleaned out the plenum and EGR holes to get rid of carbon build up and put new gaskets on all of it. It also has a new cat.

Any help would be appreciated - thanks.

 

This is a common problem. pull up google.com and type thirdgen.org gas tank pressure. you will find tons of info. There are a few relief valves, canister may have one, there is one under car by gas tank I think. there is also a fuel return line back to the gas tank. several sources that can feed gas back to the tank or not allow pressure to escape. honestly, I don't know what they are, but I'd know pretty darn quick if I ran that google search.

 

If you have a charcoal canister with the electric solenoid, I'd look there. Either the vacuum lines are off or broken or the solenoid is malfunctioning.

Or possibly the line from charcoal canister to tank is plugged or broken.

That solenoid controls the engine vacuum being applied to the tank to evacuate the vapors that are building up. When the fuel gets hot over time from exhaust heat and wamer unused fuel being returned from the engine, you can get high pressure build up in the tank which can affect how the fuel pump operates.