On my 1991 Z28, I'm getting a lot of preasure in my fuel tank. I know that it is normal to get some preasure but I know I got way to much. If I vent the tank by loosening the gas gap it will hiss for about 5 seconds. If the car is running and the engine is warmed up, I can vent it, tighten the gas cap again, and vent it after aprox 30 seconds once again. The air comming out of the tank is a quite warm. I've changed the gas cap but this is not solving the cause of the problem. If I leave the gas cap loose while driving, I can really smell the gas fumes.
I know the charcoal canister under the hood is meant to handle the tank fumes but I have no idea how to check to see if it is working. If anybody has any ideas, I would like to hear from you. Thanks!

Replace white plastic vent valve over the rear axle, it's probably stuck shut.

Is there any way that I can check this valve to see if it is functioning properly? Is there anything I should look out for or keep in mind while replacing?
Thanks for the reply!

Where can you get one?I even went to the dealer and they said they could not get one.

87tpi,

Go back and ask for P/N 10033125. Thats the vent valve.

91z,

Although the tank valve may be of suspect, I doubt it's the issue. The setting of that valve is actually quite high. It's main purpose isn't for venting pressure, it's to let air into the tank as fuel is taken from it. This prevents a void space from forming and the tank collapsing on itself. I would suspect the EVAP system would get flooded first and fail, before the tanks vent valve bleeds off pressure. What was the outside temperature?

Thanks bro.That helps a lot.

I replaced the vent valve and no difference. I got the valve over the counter at the delearship parts counter.
I do notice that when it is above 80 Degrees F. outside, the preasure builds a bit faster. Three garages said that it is normal to have preasure, not realizing that this isn't normal preasure. I guess you got to own one to know what I mean.
I gotta try to get some EVAP sys. information. The manual I have for the car tells me nothing.

I've had the same problem ever since I started using a Holley blue pump in my car for some reason. My car would even build up silly amounts of pressure with the car not running sitting outside in the sun. Since the car is down for a little bit I took off that "taco bell" valve and it no longer builds pressure but I blew into the valve and it opens/closes without a whole lot of trouble... Might have just needed the cleaning I gave to it prior to putting it in my mouth

In fact, I'm gonna go outside right now and put the valve back on and fill the tank up with new gas... See if it still builds up

The thought had crossed my mind that it could be my fuel pump. As far as I know its the stock pump. The air leaving the tank is really warm so, I thought that my fuel pump could be getting hot and heating up the fuel. There is no way that I am replacing a fuel pump that still works. And, I have no way to really know if that is the problem. A buddy told me that he thought GM had a recall on the 91 for some issue concerning the fuel tank. I think that will be my next step, to see if this is true and see if it was done to my car. ARRRRRG!

The item that you want to check is the fuel tank pressure control valve located in the small 1/4" line just back of the fuel evap (charcoal) canister. It is item #8 in the second picture.

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My purge valve was on backwards!!!!
Hey trickster, thank you so much for the info. If your ever in Toronto, Ontario, let me know and I'll buy ya a beer. Thanks again.