When I go to fill up my gas tank on a hot day after driving around in my 1968 427 Vette, I get a large gush of hot air out of the tank. It has built up a lot of pressure from the temperature increase so I purchased another new cap (original style from Zip Products) and no change. Is this right and will it cause any issues?

 

what sounds to be pressure escaping from the tank might in fact be air getting sucked into the tank....

just my 2centavos....

Any pressure inside the tank should escape thru the vent line going to the vapor canister.... or thru a vented gas cap...

 

In 1968, there was no canister to relieve vapers and there is no return line as the car was originally a tri-power.

 

It must be vented somehow somewhere..... I don't know enough about the 68 but I'm guessing it's the gas cap......

 

I wasn't aware that those cars did not have a vapor line and canister. I would suspect it's got a vented gas gap or some kind of vent tube. You say you bought a new cap. It really falls on if the original cap was vented or not, and if the new one was in fact a vented replacement.

 

The gas cap allows air into the tank but not out. Like a check valve.

 

You should have a vented gas cap that allows air in or out. In, so that you don't operate with a vacume, and out, to let off pressure. Later, non vented caps will also fit, but will cause problems. The correct cap will solve your problem.

 

Early C3s did not have a vented gas cap.
My 72 has vapor canister and it does build pressure. That is why gas cap says "open slowly".

 

First, the "whoosh" you experience is completely normal.

Late 60's Chevy's had vented fuel caps. "Vented" meaning air was allowed in to the fuel tank to replace spent fuel. For obvious reason, fuel or fumes were not allowed out. Shake the fuel cap. A good one will rattle like a PCV valve.

What is causing the "whoosh", is after you drive the car, the air in the tank can build a bit of pressure beyond atmospheric pressure, caused by agitated fuel and the heat from the exhaust. Of course, a tank that's not "air tight" won't build as much pressure. At least with some "whoosh" you know the tank is in good shape.

Starting in 71 (I believe) Chevy went to a closed system designed as an anti-pollution measure. Turns out the whoosing fumes before fill-ups aren't so great for air quality. Even today, some fuel nozzles at your local gas stations have an umbrella like collar to help prevent fumes from the tank escaping into the atmosphere. Same goal.

The closed system maintains a bit of vacuum in the tank; vacuum sourced from the engine and on a closed system, the fuel cap had no valve. Other components of the closed system were the fuel separator at the tank and the charcoal canister and the lines/hoses that carried the fumes to the induction side of the engine. Pretty simple and no more "whoosh".

 

It should have a vented cap. My 68 427/390 has a vented cap just like my 65. I get no built up air pressure that causes what you describe when you open the gas cap.