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Gas Cap Question

Discussion in 'Early CJ5 and CJ6 Tech' started by Michael Wedin, May 20, 2020.

  1. May 20, 2020
    Michael Wedin

    Michael Wedin New Member

    Sweetwater, TX
    Joined:
    Mar 20, 2020
    Messages:
    11
    Should all early cj5 gas caps be vented or are they vented. I seem to be getting a vacuum in my tank when running and engine dies. I had a 1930 Model A ford that did that and wondered if under the drivers seat tanks need to be vented.
     
  2. May 20, 2020
    timgr

    timgr Jeepin' Nerd 2020 Sponsor 2019 Sponsor

    Medford Mass USA
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    Aug 10, 2003
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    20,940
    All under-seat tanks have a vented cap, at least they should. You'd only use a sealed cap with a vapor recovery system (charcoal canister) and that system was never used with the under-seat tank.
     
    Beach66Bum and scoutpilot like this.
  3. May 20, 2020
    scoutpilot

    scoutpilot Member 2019 Sponsor

    Asheboro, NC
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    Jun 7, 2014
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    What he said.
     
    Rick Whitson likes this.
  4. May 23, 2020
    gromit

    gromit New Member

    harrisburg, pa
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    Nov 6, 2018
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    16
    I want to use what appears to be an original gas cap. To isolate problems, is there a reliable way to test/fix old ones for proper venting?
     
  5. May 23, 2020
    scott milliner

    scott milliner Master Fabricator

    Seattle Wa.
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    Dec 17, 2002
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    Not that I know of. You could always leave the cap loose to find the problem.
     
  6. May 23, 2020
    timgr

    timgr Jeepin' Nerd 2020 Sponsor 2019 Sponsor

    Medford Mass USA
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    The usual complaint is that they are leaky, not too well sealed. The caps I've seen use a labirynth to stop the liquid and pass the vapor. Obvious by inspection. Testing, dunno ... that's an unusual request. Possible you could try pressurizing the tank slightly through the fuel lines and see if it leaks out at the cap. An aquarium pump or similar would be suitable. Or if you have a Mity Vac, try to pull a vacuum through the gas lines.
     
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  7. May 28, 2020
    PeteL

    PeteL Member 2020 Sponsor 2019 Sponsor

    Hills of NH
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    I think some may have a one-way valve in the vent, IIRC.

    I ran over one once and flattened the cap slightly. My gas tank imploded from the vacuum.

    Scared the poop out of me.

    I fixed the tank with compressed air. (I think the Air Force calls this a fuel-air bomb...)
     
    Last edited: May 28, 2020
    gromit likes this.
  8. May 29, 2020 at 9:22 AM
    mickeykelley

    mickeykelley Well-Known Member

    Republic of Texas
    Joined:
    Oct 10, 2015
    Messages:
    1,611
    My original cap had the gasket finally dry and crack only to eventually split in half. I couldn't find a replacement gasket so i ordered the one from Walcks and it has be working fine for over 2 years. It looked just like the original so I'm not sure if it was NOS or not. But I can't find it on his site now.
     

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