65 Mustang Gas Cap Gasket

JeffMustang65

New Member
Aug 13, 2012
23
0
1
New York
Hey guys. I have a question about the gas cap on my 65 Mustang. It has a rubber gasket that seals the cap to the filler tube. Should the rubber be thick enough to completely seal the cap, to the point where I hear a hissing noise of pressure releasing when I take the cap off when filling the tank? The rubber gasket that came with the new gas cap seemed kind of wimpy and thin, so I cut a thicker piece out of rubber and installed it. I'm wondering if it will make it hard for the motor to suck the gas to the carb if there is a completely sealed tank.
 
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I don't think it is supposed to be sealed like that. New cars have an evaporative emissions system that keeps the gas vapors from getting too hot and expanding in the tank. These old cars did not have that and most likely just didn't seal well at all. How bad is it? I get a little "pff" on hot days when I open the cap on my truck.
 
Is this a new gas cap? Make sure the gas cap is properly vented. I replaced my gas cap and a lot of the caps being sold were non vented. I managed to find one that was vented. You can add a small line coming off the filler neck behind the gas capand run it to the outside of the car, keeping it away from moving parts and heat sources.
 
Did you have this problem with your old cap?

Look for a small hole or something on the inside of the cap. I bought the lockable key cap because that was the only one I could find that said vented in the description.

My cap had a little valve looking thing on it, and I blew through it to make sure it was vented.

If you have a well enough ventilated area and the proper containers, you can drain some gas out of the tank by either the drain plug or the fuel line into a container, look in the trunk, and see if the tank is trying to collapse into itself.

If you look at this picture, the lower right close up has the vent in the center of the cap.
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