Major pressure build up in gas fuel lines!?!?!?

[kpc_0721]

So everytime I went to get gas and was unscrewing the gas cap there is always a big build up of air and it will hiss for about 5 seconds while all the air is escaping out......wat would cause this to happen? The lines I'm using are pretty small so could this have anything to do with it?

[gumby74]

I think thats normal. Hope actually.

Mine does the same thing often, not every time, but often enough that I am used to it happening. More frequently on hot days or when I drive around for a while without turning off the car.

[kpc_0721]

Yea I know its normal to have some pressure but I have alot of pressure built up everytime I run my car, dont know if it will harm anything if I run the car for a while and keep letting the pressure build up? Dont some gas caps have a little vent or something to help let pressure out?

[gumby74]

You know, now you have me thinking.

I believe the factory fuel cap is vented. No idea how much pressure its rated to contain though.

Heres to hoping someone with a definite answer chimes in.