on my 88 iroc 350 tpi everytime i go to the gas station to fill up when i take the cap off there is this intense pressure and noise coming from the gas cap i have never heard another vehicle do this before in my life.A mechanic told me i have a fuel injector stuck open but i can't see it doin that?????has anyone else had this problem and how do you fix it?

 

It is somewhat normal to hear a hiss when opening the fuel tank - but it is not a good thing.

Emissions laws dictate that the fuel tanks evaporative emissions be contained (ie. the fuel tank is sealed - not vented to the atmosphere) and sent to the charcoal canister where they are then sent into the engine during certain conditions where they are burnt up with the rest of the incoming fuel and 'cleaned' up by the catalytic converter. So car manufacturers run the vapors through the engine and it's emissions control systems, where it will (idealy) be turned into harmless emissions instead of raw hydrocarbons (gasoline vapors). The system that controls this is called the EVAP system. So with the tank being sealed, it will tend to have a vacuum created in it when the fuel in it is used up. This vacuum is extremely unwanted as it makes the in-tank fuel pump work much harder than it should (or usually is capable of for very long).

So to attack this problem GM put a one way vent for the fuel tank to allow air to come into the tank as the fuel is used up to take the space that the fuel previously occupied. This vent has a tendancy to clog up as it is positioned near the fuel tank on the underside of the car and can get covered in things like mud or undercoating. So if you are getting a loud hiss every time you open your tank, that 'vacuum relief valve' is likely clogged.

It is possible that the EVAP system is not purging and letting out the contents of the charcoal canister when it is supposed to and in that case, the hissing you hear is pressure escaping as the fuel vapors have no place to go and are slightly pressurizing the tank until you open it.

So either way, a loud hissing is not a good thing. It is going to be somewhat normal for it to hiss a little bit on days when the temperature is extreme, but it should not be very obvious.

 

matt87gta good call. That is most likely what it is. The ventilator is located on the driver side back seat tub. It is near the brake lines and is facing towards the ground. It is black with a white top. I would take this off and clean it out. It should have two springs and a plastic valve. The carbon canister filter may also be clogged and might need to be changed. You should take care of this before you start getting vapor lock.