COMET CENTRAL

Few years ago my 64 was making the "swoosh" sound when I opened the gas cap. I wasn't entirely sure whether it was pressure or vacuum. Then on our way to Pidgeon Forge on a very hot day after driving 200 miles, we stopped for gas. The tank was low on gas, maybe a quarter or less. I happened to have the trunk open before I opened the cap and I could SEE the floor of the trunk was bulged. When I opened the cap, the swoosh happened and I saw the spare tire drop as the tank top sucked back down to level.

So if you want to confirm whether its pressure or vacuum perhaps open the trunk and put your hand on the tank top while you open the cap. For me, with not that much gas in the tank and a hot day the pressure was enough to bulge the tank visibly. I took a pick and ran it around and under the gasket on the gas cap which apparently was dry rotted and blocking the vent? At any rate I haven't had the problem since.

AS loose as my cap fits,,,it couldn't build pressure if you built a bon fire under it......good thing it has a door over it///been that way forever !! pop

thanks guys, I loosed the gas cap n ran fine, my initial trouble was the car would stall and not restart. took cap off (with a whoosh) could tell if it was vacuum or pressure. let car sit a couple minutes and it started right up. got a vented cap, no more troubles. Thanks again guys