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I thought I read a thread on this topic a few months back but my search did not turn up any thing.

Anyone else get pressure build up in their gas tank after going for a ride? When I go to remove the gas cap ther's a pressure relief and I hear the tank make that oil can sound.
I'm running the stock set up with the 1/4 inch return line and a brand new vented gas cap.

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1969 Camaro RS/SS 350
 

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I would check to see if that cap is working correctly. See if you can suck air through the center vent. I had a tank partialy collapse because it would not let air in to replace the gas as it was being used in the engine. I fixed the cap and solved the problem.

-Mark.
 

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Yep,

Sounds like gas cap. My Camaro is fine but had same issue with my 60 Vette and had to use one of those cheep locking gas caps because they suppiled more vent. I vacume was so strong it was sucking gas out the overfill hose. I think it was my high volume fuel pump that suppiles food for it's 383/425hp?

Andy 1969 Pace Car and 60 Vette 383/425hp
 

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My car does it, but it is a pressure release when I remove my cap. I can feel the vapors coming out of the tank, not going in. I think that my mufflers, and exhaust heat up the fuel in the tank and it builds pressure in the tank. The vented caps are designed to let air into the tank when the tank is at a lower pressure than the surrounding air. I used to have a cap with a hole drilled in it because I didn't like the rush of air coming out, but I always had a gas smell in my garage after I parked the car. Since I replaced the cap with the hole in it with a new vented cap I no longer get the gas smell in the garage. I guess the cap keeps the pressure contained until the gas tank cools down. I can live with it this way. If your car is sucking air into the tank, then you have a problem with the vented cap.

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Mark Canning
1969 Indy Pace Car
350/300HP RPO Z11
My 69 L48 - 350/300HP Engine
 

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Mine had a crossflow muffler exhaust leak heating up the tank. I could hear the fuel boiling when I stopped the car!
Scary!
The stock cap should retain pressure or vapour would come out all the time and some your fuel would be lost to evaporation. Plus, it would allow fumes to escape into your garage and possibly cause a fire.
I'm not sure what the pressure retained is, I think it's around five pounds or so. Smog stations here have a cap tester, but it's pass/ fail, does not read in PSI.
David

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Check my web page for First Gen Camaro suspension info:
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67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer
 

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Interesting topic-- I've noticed a "sucking" sound like a vacuum in the tank when I fill up too-- like air was never allowed in as gas was sucked out. Should the cap let air into the tank? I don't recall if these tanks had a vent line, do they? -Dan
 

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When I had a holley carb, it used to leak out the butterfly linkage and be hard to start when it was warm and had pressure built up. I put on a new cap and it solved the problem

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69 convertible
LT1 4L60E
 
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