I notice every time I remove my gas cap, I get this rush of air into the tank, like the tank is under vacuum. It sucks air for several seconds, not just a quick rush of air. What could be causing this? The car is a 89 TPI 350.

I tried a search, but couldn't find anything that really sounded like the same problem. It is definately not under pressure, and I have checked the tank vent (right in front of the rear axle) and it appears to be fine. Needs a bit of pressure (normal???) to expel air, but it does open.

 

It may have something to do with your vacuum canister, which is located in the opposite corner of your battery in the engine compartment. Just my opinion.

 

If it's the vacuum canister, am I looking for any typical cause or trouble spot, or is the whole thing likely shot?

 

Like I said, it's just my opinion, but:

Mine did/does this, check the connections on the canister.

 

does it really cause a problem? mine does the same thing and i was wondering what it was...

 

Mine does the same thing. When I uncap the gas tank it hisses for acouple seconds. I talked to some other people I know and their 4th gens do it also. I'm not to concerned over it.... my a/f ratio pretty normal