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I bought a new GM vented gas cap for my '69 Camaro last year. I drove the car and after parking it I heard a high pitched hissing sound. I followed the sound to the gas cap. I pulled the gas cap off and heard a big whoosh of air rushing into the tank. Apparently the new "vented" gas cap is not venting. Does anyone know how to remedy this issue?
 

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Normal effect. Cap allows air in, not out. Pressure on tank keeps fuel pump primed.
Cap keeps fumes inside tank, not in enclosed garage.
 

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Discussion Starter #3
The problem is the cap is not letting air in. The only pressure developed on the fuel pump is a vacuum in the tank. The vacuum being caused by the fuel pump removing fuel from the tank. This doesn't seem normal to me. Am I wrong?
 

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Normal effect. Cap allows air in, not out. Pressure on tank keeps fuel pump primed.
Cap keeps fumes inside tank, not in enclosed garage.
x2:thumbsup:
 

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Thanks for pointing the situation out. Yes, a simple check is to suck on the inside of the cap, cap should allow air to pass into the tank.
 

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mmmm nothing like the taste of 89 octane!

Try seven year old 93 octane. That's what my last mouthful was. Exquisite on the palette with an intriguing bouquet. No ethanol so it didn't even count as one of my vegetables for the day. But it did actually fire up in the car.

Northstar, I bought a vented cap from AutoZone recently and it seems to work fine. It was a Stant I believe. My tank definitely pressurizes. The cap has a warning "Open Slowly" and believe me you have to. It blows little gas sprinkles out if you crack it too fast.

The vented caps do only let air in but if the tank pressure gets high enough they act like a radiator cap and will let air out. It's quite possible that's what you heard as the exhaust heat will cause the fuel/vapors to expand. Next time you drive it for awhile, hold a well worn dollar bill by the edge of the cap while you open it and see if it gets blown back or sucked toward the edge.
 
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