Excessive pressure in fuel tank. - Team Camaro Tech
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post #1 of 11 (permalink) Old Aug 15th, 10, 01:56 PM Thread Starter
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Excessive pressure in fuel tank.

Since replacing my gas cap, I have had an usual amount of pressure coming from the tank. Is this normal? Today I blew the 4-5" rubber fuel line that goes from the sending unit to the hard line. The car was previously a 6cyl and now has a bigblock with original lines. Do I need to add a return line or vent the tank? The only flames I want to see are the ones coming off the tires Thanks guys!

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post #2 of 11 (permalink) Old Aug 15th, 10, 01:59 PM
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Re: Excessive pressure in fuel tank.

Is it a vented gas cap?

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post #3 of 11 (permalink) Old Aug 15th, 10, 02:05 PM Thread Starter
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Re: Excessive pressure in fuel tank.

Good question..its a repo SS cap. How do you tell?

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post #4 of 11 (permalink) Old Aug 15th, 10, 02:07 PM
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Re: Excessive pressure in fuel tank.

Pull the cap off and see if you can suck air from the center inside of the cap.

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post #5 of 11 (permalink) Old Aug 15th, 10, 02:13 PM Thread Starter
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Re: Excessive pressure in fuel tank.

Ok, will do. Are there applications for both vented and non?

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post #6 of 11 (permalink) Old Aug 15th, 10, 02:18 PM
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Re: Excessive pressure in fuel tank.

A none vented tank should have a vented cap.... A vented take should have a sealed cap.

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post #7 of 11 (permalink) Old Aug 15th, 10, 02:25 PM Thread Starter
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Re: Excessive pressure in fuel tank.

Ok, I just ran out and checked. There appears to be a one way check valve in the cap. By sucking on it (phlack) I can pull air in, but I cant push air out. I guess I need a vented cap....?

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post #8 of 11 (permalink) Old Aug 15th, 10, 03:22 PM
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Re: Excessive pressure in fuel tank.

Six cylinder had 5/16 fuel lines. a lot of new caps do not have vent due to EPA.
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post #9 of 11 (permalink) Old Aug 15th, 10, 07:33 PM
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Re: Excessive pressure in fuel tank.

normal for a non-return set up. One way to reduce the pressure is to keep the tank full of gasoline.

gasoline as a fluid actually decreases in volume with increased temperature. however, gasoline vapors expand with temperature. more fluid in the tank means less pressure, while less fluid in the tank equals more pressure, with increased temperature. since the tank is valved by the cap (one way valve in only) the tank will not be allowed to build negative pressure and collapes.

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post #10 of 11 (permalink) Old Aug 15th, 10, 09:38 PM
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Re: Excessive pressure in fuel tank.

A vented cap is not going to help, it just allows air in as you use the gas out of the tank.

I would say just put a new piece of hose and a new pair of clamps on.
If you allow the pressure out of the tank fuel vapors will stink up the garage, unless you add a charcoal canister.
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post #11 of 11 (permalink) Old Aug 31st, 10, 12:52 PM
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Re: Excessive pressure in fuel tank.

All our first gens came from the factory with vented fuel caps. The term "vented" means air is allowed into the fuel tank to replace spent fuel. The check valve in the fuel cap prevents fuel and fumes from escaping for obvious reasons.

Some pressure in the fuel tank is normal. It's caused by agitation from driving and heat from the exhaust.

Of course the "whoosh" experienced at the service station when refueling was thought to be polluting and GM (and the rest of Detroit) engineered a "closed" fuel tank system which was installed on 71 model Chevy's. This system used a non-vented fuel cap, a fuel separator at the tank, tubing and hoses to a charcoal canister, through which vacuum is provided by the engine's induction system. This system provided a bit of vacuum in the fuel tank thought to be better for our environment.

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