Fuel cap hissing
 

After i drive my truck on for a few hours and then go to fuel it up it will sound like air is getting sucked into the tank when i crack open my fuel cap, is this common? Or is it air escaping from a pressurized fuel tank? The truck runs fine

Can you tell if it is air getting sucked in, or out?

It's normal for air to blow out. Never heard of a tank sucking in though, unless there was actually something wrong.

Are you noticing a loss of fuel pressure or overall power?

Is this a new problem, or has it done it since day one?

Most newer style fuel systems are a sealed system, this is part of the vehicle emissions, I've had more than a couple make this hiss, never been a problem.

Try a new fuel cap. If the vent gets clogged it will build excess pressure in tank. My brother-in-law's tank would start to pull the sides in until the pressure was released.

Thanks for the replies. Its done it since i bought it, i think youre right about the the cap needing to be replaced. No loss in power, cant tell if im losing fuel pressure, i dont have a gauge but it runs fine. I know that rubber hose thats part of the return line that goes behind the fuel filter, its burst a few times and man does that thing dump fuel from a pin sized hole, one day it soaked my whole under carriage before i caught it.

ther is a bad vent valve on fuel tank. ther are other posts somewhere here describing.