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Troubleshooting Diagnosing problems done here.

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  #1  
Old Feb 19th, 06, 06:18 PM
Cossack Warrior Cossack Warrior is offline
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Default Excessive gas tank pressure

I just purchased a recently restored 69 big block. Yesterday after a short drive I heard a strange noise coming from the rear of the car. I isolated it to the gas tank. I started to remove the gas cap and was very surprised to have the cap shoot out of my hand and go back about three feet! I was almost knocked over by the pressure release. What is going on? The cap says it is "vented", but it sure fits tight. Any thoughts?
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  #2  
Old Feb 19th, 06, 07:32 PM
Camaro Dave Camaro Dave is offline
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Default Re: Excessive gas tank pressure

Try replacing the cap. It might be bad.
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Old Feb 20th, 06, 04:39 AM
Everett#2390's Avatar
Everett#2390 Everett#2390 is offline
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Default Re: Excessive gas tank pressure

The cap is vented, but to allow air to "enter" the tank as fuel is used.

If tank is pressurized, the exhaust is heating up the fuel tank, thus building pressure inside. Gas cap is a one-way valve, let's air in, but not out.
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Old Feb 20th, 06, 10:35 AM
OAKLYSS OAKLYSS is offline
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Default Re: Excessive gas tank pressure

I have been working on the same problem. I have 2 new GM gas caps, both work the same. They have a 2 way valve that allows air in as gas is used, and they vent out excessive pressure. You can check by blowing hard on the center hole inside of the cap. I am trying to determine that release pressure in P.S.I. I guess about 5 psi. But it is not low enough because it causes the car to cut out a little under load when it pressurizes. The valve can be moved to one side with a small nail and it will prevent pressure build up, but will also allow gas fumes to escape-not good. So I am working on a way to lower the release pressure in the valve.
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Old Feb 20th, 06, 11:52 AM
Camaro Dave Camaro Dave is offline
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Default Re: Excessive gas tank pressure

Be careful and don't overlook the gas cap when it comes time for a Smog check. They will fail you for that and charge you a $59.00 rip off fee.
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Old Feb 21st, 06, 09:14 PM
Straight-line-69 Straight-line-69 is offline
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Default Re: Excessive gas tank pressure

A little pressure is normal. You don't want fumes or fuel to escape.

If it's balooning your tank, you have problem and Everette (above) is probably right about the exhaust causing the trapped air in the tank to expand.
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  #7  
Old Feb 28th, 06, 12:58 PM
Sully Sully is offline
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Default Re: Excessive gas tank pressure

I have had the same problem with my 69 Big Block
Camaro and Nova. New gas caps made no difference.
The pressure would either leak out the tank at the
fuel line entrance or out the top of the tank at the
sending unit. Made no difference that tank wasn't
full. I guess it could be heat from the exhaust.
It can really make a mess.
I am at the point that I may drill a small hole in the
cap to see what happens.

Sully.........
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  #8  
Old Feb 28th, 06, 09:22 PM
Straight-line-69 Straight-line-69 is offline
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Default Re: Excessive gas tank pressure

Sully,..a hole in the gas cap will surely leak fuel, especially on acceleration when fuel moves to the rear of the tank. Sounds like you'd add another leak to the one you have on top of your tank,..which is likely a bad or mis-aligned o-ring for the sender.
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  #9  
Old Feb 28th, 06, 11:40 PM
Anth Anth is offline
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Default Re: Excessive gas tank pressure

Yeah, this is a common problem - happens on my car all the time - the above is true about it being a one-way valve, the crossflow muffler heats the gas in the tank and it builds up pressure; shouldn't be too big on an issue if you release it every once in a while (after each cruise) - this is just something I think you (and I/us) have to live with... if you drill a hole in it you'll certainly have a gas trail following you... ever seen The Coyote carrying a bag of gunpowder?... it'd be like that...

Anthony
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  #10  
Old Mar 1st, 06, 07:54 AM
OAKLYSS OAKLYSS is offline
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Default Re: Excessive gas tank pressure

The gas caps are a 2 way valve-you can check this yourself by blowing thru it from the inside of the cap. It just has too high a release pressure.
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  #11  
Old Mar 1st, 06, 11:41 AM
400bird 400bird is offline
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Default Re: Excessive gas tank pressure

Quote:
Originally Posted by Camaro Dave
Be careful and don't overlook the gas cap when it comes time for a Smog check. They will fail you for that and charge you a $59.00 rip off fee.

smog checks dont go back past 76, here in cali
so both you and i dont have to worry about it
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