92 Lumina Gas Tank Pressure...

My son owns a '92 Lumina. He called to tell me he was hearing a sound like escaping air after he turned the car off. It lasted about 20 sec. I told him to check the gas cap/filler the next time he drove the car. Calls me back and says if fact the noise was comming from the gas cap and when he unscrewed it a woosh came out. What could cause this pressure build up? I know the fuel pump is in the tank. Thanks

If his car has the ethanol conversion, there is a vent valve on the fuel tank to release the excess pressure(check the SPID label in the trunk for code N14 which indicates the car has the ethanol conversion). If it doesn't have the N41 code, then the purge valve on the evap canister in the engine compartment regulates fuel tank pressure.

1hh

Thanks, his car does not have an Ethanol conversion. I'm thinking it might be the charcoal cannister(EVAP).