Everytime I go to fill up my car I hear a HISS as if pressure built up. I only hear it when I open the gas cap. Im assuming that some kind of ventilation is clogged. I dont know if the cap has a built in vent. or if there is some kind of tube by the filler neck. any ideas?

Are you hearing a vacuum hiss or a pressure hiss? Vacuum in the tank is normal. Next time you hear the hiss, look for a lot of fumes coming around the cap WHILE it's hissing. If you see a lot of fumes, your EVAP system isn't functioning properly. I suspect everything is normal, however. Do you have a check engine light on?

no check engine light is on. ever. it sounds like it could be a vaccum hiss. considering i never smelt or saw anything. but it could be that i was just too fast on it. heard the hiss and removed the cap without a second thought.

The EVAP system pulls a vacuum on the tank for a few reasons. Mostly to keep an eye on holes in the tank and to evacuate fuel vapors. I'd say there's nothing wrong..

ok i just wanted to check. i thought maybe something was up because it happened like the last 3 times. but its alright i guess...next time i fill up i'll check for fumes or anything. see if its pressure or a vaccume. thnx for the help