Got the gta running finally. While driving I noticed gas is leaking at the filler neck. Gas cap is some replacement cap. Figured I would check here and see if you guys have any recommendations.

 

PROPER stock replacement is all you need. They are non-vented on 3rd gen EFI engines.

I had a 91 RS that would spew fuel out the cap ever time you nailed it with a full tank. Turns out some dingleberry screwed on a cap from a Ford Thunderbird. No idea why, but it leaked worse than if I had just stuck a rag in it. Put on a proper aftermarket replacement cap (Stant) and never an issue after that.

 

I second the Stant gas cap. I bought a locking one for mine though (had issues with someone "borrowing" gas in the middle of the night) and have no issues out of it.

 

Make sure the filler neck is round and not distorted into a teardrop shape from the weight of the gas pump.

 

Are you sure about the non-vented part?

My '83 has the vented metal cap, it's carb'd now but was originally a cfi car. no issues with leakage.