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Should normal tank pressure be negative (sucking in) or Positive (pushing out)? I have a brand new vented cap and whenever I pull up to the fuel pump and remove the the cap, there is a pressure release and the tank makes that oil can sound. Is this normal? I tested the cap and it seems to vent properly.
 

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I thought 69's had a return line? I could be wrong. You may have the wrong type of cap, one vents pressure, and one has a check valve to control vacuum in the tank. Otherwise the tank would swell like a watermelon or collapse like you crush a can.
 

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Not all 69's had a return line.
 

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I have a new cap, and it has pressure only when its warm out, to a limit of course. Once it gets to a certain amount of pressure it vents.
 

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The 69 Camaro uses a vented cap, but the cap lets air into the tank to make up for the gas that is being used by the engine. It doesn't let the pressure out caused by the muffler and tail pipes heating up the tank . Thats usually why the tank makes that hissing noise when you remove the gas cap. It's air coming ou, not going in.
 

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Same for me, new vented cap no return line on a 69. Its kind of scary hearing the tank inhale when removing the cap, is this really normal.
 
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