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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 Member

    Sweetwater, TX
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    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 We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    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.
     
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  3. May 20, 2020
    scoutpilot

    scoutpilot Member

    Asheboro, NC
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    What he said.
     
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  4. May 23, 2020
    gromit

    gromit New Member

    harrisburg, pa
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    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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    Not that I know of. You could always leave the cap loose to find the problem.
     
  6. May 23, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 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 If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 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
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  8. May 29, 2020
    mickeykelley

    mickeykelley Well-Known Member

    Republic of Texas
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    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.
     
  9. Jul 27, 2021
    Peter Dorey

    Peter Dorey Member

    Vista, CA
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    Bringing this back since I have a similar question:
    Should the fuel cap be 2-way vented? Mine builds up pressure, causing vapors to come out from the fuel tank sender under the seat.
    I purchased a new Stant 10625 vented fuel cap, but it still builds up pressure. I believe these only have a Belleville type spring/washer inside to relieve negative pressure (vacuum) inside the tank.

    I just re-sealed the fuel tank sender and don't want to blow the cork gasket out...
     
  10. Jul 27, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Year/model/equipment? Put that in your sig and it will appear with every post.

    All the under-seat tanks were vented as far as I know. This is a labyrinth in the cap, no valve of any type.

    Looking at your profile - A '56 will dribble out gas on side hill if the tank is full. No pressure seal.
     
  11. Jul 27, 2021
    Zoomer

    Zoomer eJeeper (walking)

    Minnesota
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    Should not build up pressure except with increase in temperature. As you draw gas out it should lower the pressure in your tank rather than increase it.
     
  12. Jul 27, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    All we sold for these Jeeps at the dealership for this application was a Stant cap. It leaked a lot. They stocked it because it was cheaper to buy than from the Jeep parts network.
     
  13. Jul 27, 2021
    Peter Dorey

    Peter Dorey Member

    Vista, CA
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    The fuel tank is an original reproduction with the 6 hole fuel sending unit and no tank return line.
    I am in San Diego, so the tank gets hot during the day from the sun and will create pressure inside the tank if not vented.
    The new Stant 10625 has no labyrinth that I can see, or the labyrinth holes as so small I cant see them... and air can't get through them...
    It looks just like the old cap I removed, but without all the cracks in the rubber and rust on the top.
    Is there a different part number I should look for?
    [​IMG]
     
  14. Jul 28, 2021
    Twin2

    Twin2 not him 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Virginia Beach, VA
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    cant help you with a part number
    but a vented cap will have a very small hole
    DSCN0672.JPG
     
  15. Jul 28, 2021
    Dave B

    Dave B Frankenjeep '67

    Northern Minnesota
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    This one is supposed to work. With some research or a phone call you may find assurance.

    >>>New Black Fuel Tank Gas Cap Fits 46-71 CJ-2A, 3A, 3B, 5

    It states this in the ad:

    About to order a new gas cap; the black gas cap for a '61 CJ5 (Item No. 640594) is vented, correct?
    Yes, it is vented. Thanks
     
  16. Jul 28, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    The Stant cap of today is not the same as the Stant cap we sold in the day. The design is different (black painted, not plated, and more cylindrical) with the vent in the center of the cap.

    Note that RockAuto shows 3 makers for these caps, Motorad, Stant and Gates. From the pictures, they all look the same. Motorad says their cap is not vented, while Stant and Gates say vented. They all show a warning sticker on the cap, which is nonsense for a vented cap.

    JMO - all these sellers are offering a cap they have determined is "good enough" and all are likely the same product. The parts sellers are often clueless about such issues, and are probably taking their compatibility listings from a third-party cross-over index like the Hollands Guide that junkyards use. Caveat emptor.
     
    Last edited: Jul 28, 2021
  17. Jul 28, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    This looks like the Stant cap we sold in the day. It's obviously vented. Worth a shot, though I would expect it to leak from slosh or side-hill.
     
  18. Jul 28, 2021
    Keys5a

    Keys5a Sponsor

    Florida Keys
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    The black cap linked to Kaiser Willys is a reproduction of the original AC brand cap, also later made by Eaton. This should be a vented cap.
    -Donny
     
  19. Jul 28, 2021
    Dave B

    Dave B Frankenjeep '67

    Northern Minnesota
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  20. Jul 28, 2021
    Peter Dorey

    Peter Dorey Member

    Vista, CA
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    I took the smallest drill bit in my chest and punched a small hole in the copper/brass spring that applies pressure to the rubber seal. The hole is inside the rubber seal and allows fumes to vent to the underside of the cap, where they can escape out under the cap. The hole is only about .018", which is plenty for air to escape but shouldn't allow too much fuel to escape during turns.
    I re-installed the cap and checked the tank a few hours later. Seems like it's working because it's the first time I have removed the gas cap without air either sucking in or blowing out of the tank :)
    [​IMG]
     
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