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Thread: Fuel cap leak

  1. #1

    Default Fuel cap leak

    Just wondering if anybody has had a fuel cap leak? My friend’s did and stained his fabric... Just wondering how prevalent they leak and of any preventative measures in the future. Thanks.


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  2. #2
    Administrator DesertFox4's Avatar
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    Default Re: Fuel cap leak

    A couple things can cause that big of a leak. First the gasket may be damaged or aged and no longer seals well.
    That is a somewhat common issue after sometime in service. Years ago a solution was discovered that takes care of this issue. Rubber gaskets are used with the stock caps. They lose elasticity over time and don’t seal well. I’ll attach photo of a permanent solution. It is a blue gasket available through Mercedes dealers. The photo has the part number on the package. This gasket is somewhat thicker than the stock gasket and you may need to adjust the tabs on the caps due to the thicker blue gaskets. This blue gasket does not seem to age and lose shape or elasticity or be affected by fuels. These gaskets have solved many cap leaks over the last 18 years or so. I installed a set in my first Kitfox 16 years ago and they are still in service with no leaks. I hope they are still available from Mercedes under part number 140-471-00-79-493
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    Secondly you can J.B. Weld shut the underside of the crimped rim of the cap and the four tiny “vent” holes found on the rim of the cap. Not the ram pressure tube hole in the center of the cap that is there to help pressurize the fuel system in flight. Either one or both of these fixs should end the fuel leaking during flight. Likely the first fix will take care of the problem. Hope this helps. I do these modifications to my caps and have never had cap leaks.


    DesertFox4
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    7 Super Sport
    912 ULS Tri-gear


  3. #3
    Senior Member jiott's Avatar
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    Default Re: Fuel cap leak

    I agree; have those gaskets on my Kitfox also.

    One other possibility-it could be overflow from the vent tube caused by:
    • Parking on uneven surface with full tanks, possibly in hot weather.
    • Flying with full tanks and ball not centered or one wing low. A friend of mine was following me one day as I flew carelessly with ball not centered and full tanks, and he observed a steady stream of full coming out of my vent tube. Quite a stain on my wing when I got home.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #4
    Senior Member Dave S's Avatar
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    Default Re: Fuel cap leak

    Jim & All,

    Very much agree that a person can get fuel to exit the cap pitot even though the cap gasket is perfect.

    I discovered a couple additional ways to dump fuel out the pitot.

    1) Maybe this won't happen with a tailwheel taxiing; however, on a trigear if a person makes a tight turn at low taxi speed with full tanks, it is easy to squirt a bit out the pitot due to the centrifugal force on the fuel in the tank.

    2) On a maximum slip with full tanks - ditto. Wondered why I smelled fuel once following the slip - The stain on top of the wing pointed directly to the tail access cover - even a few stains on the TAC evident. The natural air flow back from the tail cone guarantees the odor will be pulled forward.

    Oh well....
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  5. #5
    Senior Member jiott's Avatar
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    Default Re: Fuel cap leak

    You're right on Dave; I have had that happen to me several times in a slip.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  6. #6

    Default Re: Fuel cap leak

    Yeah, it was weird. First leg full tanks no leak. Second leg 2/3 down to a 1/4 by the end, but more yanking and banking it leaked. Pretty bummed, 10 hours on the plane and now have stains on the Oratex wings. Thanks for all the info, I appreciate it.
    Last edited by littlecricket; 03-27-2019 at 08:18 PM.

  7. #7
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    Default Re: Fuel cap leak

    Sorry about the stains. Anything you can do? Did you wax the wings with Oratex wax? Wondering if that helps at all? What color are your wings?
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
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  8. #8
    Senior Member jiott's Avatar
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    Default Re: Fuel cap leak

    I wonder if this staining thing is the Achilles heel of Oratex? Everything else about it seems to be so good.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #9
    Senior Member efwd's Avatar
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    Default Re: Fuel cap leak

    If I'm not mistaken, fuel stains painted planes as well.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  10. #10

    Default Re: Fuel cap leak

    Yeah, the wings had fresh wax. Only 9 hours on the plane... Always hangared. Lars thinks the stains may fade over time... Going to try a few more things to remove the stains. If stains stay, might do a light dusting of paint to cover them up. Not sure about white wings if to do again...

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