This should be an easy one. On my 78 454 powered Barth, the gas cap is unvented and when I stop for gas there will be a significant amount of pressure to release when I remove the cap. Is this normal or should I have a vented cap?

Good question. The very early vehicles had vented gas caps. Later models (like my 86 Regal) had one way sealed caps which allowed air in but no fumes out. These had a suction line from the tank running forward to some hefty carbon filters in the front right wheel well, to collect fuel vapors which were then sucked into the carb and burned.

I'm not sure exactly when they switched caps and vent designs. I think it was around 1980. When in doubt, the vented cap will always work. (By now the carbon filters are probably not working anyway.)

We have 3 inches of snow... Overcast and 23. Time to head back to Florida!

Good morning you guys! I second Steve VW’s reply. If you ever experience a vacuum though that’s an indication of a vent failure in the cap and it will starve the engine of fuel. Happy Trails!

My dad told a story on one of his brothers, a motorcycle enthusiast. Seems he thought the chrome cap for cycles tank was an improvement over the family's car gas cap. Grandad didn't get too far before the engine quit. The fuel pump pulled a vacuum, dimpled the tank against the baffles. I don't know the rest of the story but I think the original cap went back.

How things have changed. Nowdays we get a Check Engine light and a P0455 or P0457 error code to tell us the gas cap is leaking!

On mine the buildup is pressure, not vacuum. When I stop after a long drive the pressure escapes when I take the cap off, often with embarrassing audio results.

quote:

buildup is pressure

That seems like the EVAP system has an issue....
Matt