This is my first post here, so I'm very sorry if this is formatted incorrectly.

I have a 1991 Firebird, base, automatic V6.

Cutting to the chase - I am currently getting 13.5mpg in town, and roughly 15.8mpg highway - minimal idling. This is tested over the course of 3 tanks of gas. I have only had this car for a couple weeks, and would have filed this in my brain as normal, however I can very clearly smell gas when I'm driving with the window open (ac doesn't blow cold). At first I thought it was the gas cap, as it had been leaking from the cap a small bit. I replaced the cap, and that small leak stopped however the smell is as strong as ever. Aside from the leaking cap there is no visible/substantial leaking of fuel that I can find. The smell does not persist after shutting the car off, and it doesn't smell after being left to sit for 5-10 mins.

I haven't noticed any hesitation when accelerating at all, and there was only one time when the car stuttered when idling immediately after starting, but that's it. Idles at 700rpm consistently.

My thoughts are maybe fuel injectors leaking? Maybe the fuel pressure regulator diaphragm is leaking? I think its probably not the 02 sensor, as that wouldn't account for the smell. Anybody have any "usual suspects"? I'm entirely unfamiliar with this engine, and I'm used to working on carburetors so I'd love any/all insight.
I will check plugs for fouling tomorrow if I have time after work, anything else I outta look at?

 

Had an issue like this myself, it was the fuel pressure regulator. You can disconnect the vacuum line going to it and either smell it or suck on it to see if it's letting fuel through. Just a heads up if you suck on it you're gonna get gas in your mouth. You could always use a vacuum pump to check

 

How is your EVAP system functioning? Another thing to look at. Also fuel tank vent valve. Kind of sounds like by pressurizing the tank even more by fixing that gas cap it got worse. check that tank valve. I don't even know what my gas milage is, but its not very good.

 

My thoughts as well. I had similar problems and for me it ended up being the injectors, the little UFO valve by the fuel tank, and the gas cap. OP I can drop some diagnosis pages out of the ‘88 Firebird service manual if you’d like. It won’t be identical to your car but similar enough to help you find your problem.