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72NOVA454
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922 Posts
Discussion Starter · #1 ·
Ok, here we go, another automotive 101 question that I don't know.

How do "vented" gas caps work?

1) do they allow airflow in both directions?

2) are they set to open at certain pressure - similar to a radiator cap, or are they always open to the atmosphere

thanks

Lee
 

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They are designed as a one way valve. When the gas is pumped out of the tank to your engine it creates a low pressure area in the tank. Air is allowed into the tank so a vacuum doesn't form. Gas fumes cannot escape out the vent because it is one way. Some times they freeze up here in the winter causing problems. If you suspect is is frozen up or plugged with dirt just turn the cap open and if it is plugged you will hear air rushing into the tank. Hope I explained it well enough.

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Philippines Cowboy
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home brew said:
...just turn the cap open and...you will hear air rushing into the tank.
Or, in some cases, rushing OUT! I saw a Chevy mechanic open a fuel line on a '56 Corvette and the fuel came pouring out as if an electric pump was going full tilt. The owner, who happened to be standing nearby, saw what was happening and came running over to remove the filler cap.That solved the problem.

A little trivia some might find interesting: Back in the fifties, somebody marketed a clever little plastic cap to seal the glass pop bottles used then after the metal cap had been removed. It was shaped like a figure "8" with one end an open ring, to slip over the bottle neck, and the other end designed to snap over the open end of the bottle. Well, some engineer at Chrysler thought it was so clever that he'd design a similar filler cap. Owners would never again have to worry about misplacing the filler cap. And, in the process, Chrysler would save a lot of money! (Remember, this is before the days of government regulations concerning such matters.) Some of these plastic caps were molded and a number of cars were fitted with the tanks with the required "pop bottle" filler tubes. Many tests were conducted, both at Central Engineering and at the Chelsea Proving Grounds, and no faults could be found. So, the cap was about to be released into production when one of the VP's came out of his office long enough to hear about it. His reaction: You're not going to put that on MY Imperial! End of a clever idea.
 
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