We have a 1997 Pontiac Sunfire. It builts up pressure inj the gas tank to the point that when you open the cap it blows off like opening a hot radiator. It also makes a vacuuming sound when the car is running. The gas gage randomly floats to full and then to empty. It will go back to the correct position (I think anyway) Can anyone tell us what this could be and if they are related?? Thanks for any help.

Have you tried a different gas cap? Also the fuel pump could be causing this.

Sorry Twink, wrong again. A gas cap seals the fuel system; he's not having a problem with no pressure, he's having a problem with excessive pressure. A new gas cap won't change anything.

Freddie: My car will build up pressure in the tank and make that same noise if I run the tank low on fuel. Do you hear the "vacuuming" noise only when the car is running or as soon as you turn the key on? Does it seem as if it's coming from the rear of the vehicle? Your fuel pump could be getting weak and making more noise; a bad sending unit would be the reason your gauge is floating back and forth. However, neither of these will cause pressure to build up in the tank.