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Thread: Pressure builds up in gas tank

  1. #1
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    Pressure builds up in gas tank

    I just had a new tank, filler neck and vented cap installed on my 55 GMC. I have noticed that pressure builds up in the tank when I remove the cap. Now I'll shown my ignorance, I thought a vented cap would not allow that to happen. I am not sure if this is normal. Has anyone else had this issue.
    Thanks.
    Don

  2. #2
    Registered Member BamaNomad's Avatar
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    It depends on how the fuel system is designed. Some are designed for 'unvented' caps while others are for 'vented caps'... Which cap did you buy? (I think you said 'vented', but maybe it's not working properly?

    What other updates have been made to your fuel system?

    UPDATE: What follows is some explanations/information I found on other sites:

    Fuel tanks prior 1971 were vented through the fuel cap on our old Chevys. There's a one-way check valve in the FC that allows air into the tank to replace spent fuel. Fuel or fumes would not be allowed to escape for obvious reasons.


    Rattle your fuel cap. If it rattles similar to a PCV valve, and if the rubber gasket is properly sealing, your fuel tank is vented.

    Yes, the tank can pressurize a bit from the heat from the exhaust, agitation from driving, and changes in atmospheric pressure. Perfectly normal, and why sometime you'll experience a "whoosh" when you remove your fuel cap.

    After 1971, the EPA decided that this "whoosh" wasn't good for the environment and the closed evac system was employed which included a fuel separator at the tank, a charcoal can, a bit of vacuum from the induction system, and the plumbing.

    A bit more information which might help:

    Yes the tank needs a vent. As said, there are several schemes. If the filler is above the tank, a vented cap will work. If the filler is low, the tank should have a vent somewhere on it and an unvented cap is used.

    Some stock setups have a small tube that vents into the filler neck, like '57 Chevys. Some use a vented cap, some vent to a charcoal canister.

    Last edited by BamaNomad; 03-19-2022 at 09:44 AM.

  3. #3
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    Simply said, your "vented cap" is not venting.

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    Chevy truck gas caps interchange from the 40s until 70. The elaborate evap system on my 72 Blazer was rusted and totally jacked up. When I installed a new plastic tank, I took the fill pipe out and cut it in half and welded a top half off a 69 Chevy truck and made my own 69/70 Blazer fill pipe. Does your truck have the OEM tank or a late model one, if later one.

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    Sorry I did not get back sooner. The truck has an external electric fuel pump. I have a new stock 18 gal take with Filler Assy kit from Jegs for 55 Chevy/GMC truck which included a vented cap. I am still unclear on whether a cap for a 55 year model truck should vent both in and out or just in. Thanks for the replies. Don

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    Stock system for a 55 vents both ways.

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    The newer non-vented caps will not fit the early fill necks, they are smaller.

  8. #8
    Registered Member 55 Rescue Dog's Avatar
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    The fuel tank needs to breathe in and out. The fuel filler door on my 55 had been deleted, so I had to put a fuel filler neck in the trunk with a sealed screw on cap. The Rick's tank I used has a separate bung just for the fuel vent which I have connected to a hose with a fuel filter on the end of it high up in the quarter panel for a vent.
    IMG_9443.JPG

  9. #9
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    Fourty years ago my 67 Camaro SS 350 used to lose gas out the cap on launch with slicks when racing. I removed tank pulled sender and brazed a piece of brake line into sender and looped the line in a couple circles. It never sloshed gas on launch again. Still has tha modification in place.

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