I am getting a large vacuum pressure in my gas tank which I think may be the reason I'm stalling after about 10 minutes of driving. Every time I unscrew the gas cap It will suck in air for a good 15-20 seconds. I know a little bit of pressure is normal but I'm sure this goes beyond it. I just replaced the fuel filter so I have ruled that out. I'm really unsure as to what would cause this.
I'm a new firebird owner and this is the first car I have ever really spent time under so any thing of relevance would be helpful. thanks.

 

What year? Most have a tank pressure check valve and purge valve for the EVP canister. If the canister is plugged, the vacuum line on the purge valve may be drawing a vacuum on the tank. However, a functioning fuel cap should have a vent valve which prevents building a vacuum.

 

its a 91 v6.
I will check the purge valve tomorrow. The guy I bought it from also took a valve off that was located under the fuel tank, the name is escaping me at the moment, it had a white plastic housing and plugged into rubber line running with two others. I cleaned it up and plugged it back in. It appeared to be working fine, just had some dirt and oil in it.

 

Well, I changed out all the dry cracked vacuum hoses, cleaned the egr valve and ran some seafoam through the gas tank. Seems to idle better now but still is not fixing the negative pressure in the gas tank and the car is still stalling.

I was searching around other posts and someone mentioned that the stalling may be due to rust in the gas tank clogging the screen or "sock". Could this be the cause of the problem?