96 fuel tank pressure

When removing gas cap there is a lot of pressure being released from the fuel tank. Is the gas cap the source of venting or does it vent in another way. Truck also seems to run a little rough, wondering if this could be due to the venting problem. Any and all help is appreciated.

I would say everything is working fine. If your gas cap does leak or if it's not on all the way, you will have a code set. The cap is a one way valve, it should let air in, but not allow gas vapors to escape. This is the function of the evaporative emissions system circuit. Hope this helped. -Chris