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Thread: 1987 K75C 58,381k Karibic Bleu requesting fuel supply troubleshoot help

  1. #16
    PeteG
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    Quote Originally Posted by PGlaves View Post
    There was a pattern of problems with the electrical connections for the fuel pump. Sometimes it was the connection where the pigtail on the tank plugs in, but often the problem was where the wiring passes through the bulkhead on the bottom of the tank. I would thoroughly bench test the fuel pump out of the tank and also test the wiring through the bulkhead for continuity.

    Best guess: tank bulkhead wiring.
    I will retest both pump and bulkhead wiring. I've done multiple tests on the bulkhead wiring, even wiggling the wires while testing continuity. The pump was out-of-tank tested at least four times. I hear what you are saying Paul, and have never heard, or read of, a fuel pump intermittently failing. I have limited fuel pump experience and the only thing of note when bench tested was that it made a growl when coming to a stop. To me it seemed unusual.

    It is worthwhile testing and trying to create a fault in electrical which I will do. Pesty problem due to it being intermittent.

    Thanks Paul,
    Pete

  2. #17
    PeteG
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    01-18-2020 update: Very good news and a question. 1988 K75C fuel pump electrical

    Quote Originally Posted by PGlaves View Post
    There was a pattern of problems with the electrical connections for the fuel pump. Sometimes it was the connection where the pigtail on the tank plugs in, but often the problem was where the wiring passes through the bulkhead on the bottom of the tank. I would thoroughly bench test the fuel pump out of the tank and also test the wiring through the bulkhead for continuity.

    Best guess: tank bulkhead wiring.
    Thanks Paul. You are 100% correct. Removing and thoroughly testing the sending unit revealed a failure on the positive lead from 4-pin connector to inside tank bulkhead. It was a relief to see the intermittent continuity. I tried grinding off the plastic covering on the outside bulkhead and everything was completely embedded in plastic. I'm opting for a new sending unit.
    On closer tank inspection there was some black grit accumulating below the pump. It is coming from the tank interior that the damper support clip snaps into. Pics attached. The arrows indicate what is now a hard deteriorating plastic that is wedged between the tank side and metal pump housing. The plastic runs vertically almost to the bottom of the tank. Is this a replaceable part? Is it necessary? First picture is the front side of housing. Second picture is rear side of housing.


    Thanks again for pointing me toward the tank bulkhead wiring.
    Attached Images Attached Images

  3. #18
    3 Red Bricks
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    Pete,


    Here's some more info on that sending unit. I'm suprised I didn't mention it earlier.

    https://forums.bmwmoa.org/showthread...893#post936893



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  4. #19
    PeteG
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    Quote Originally Posted by 98lee View Post
    Pete,


    Here's some more info on that sending unit. I'm suprised I didn't mention it earlier.

    https://forums.bmwmoa.org/showthread...893#post936893



    That was very helpful Lee and inspires me to completely remove the plastic and attempt repair. It is identical to my unit. Any opinion on the pictures of the plastic inside of the tank?

  5. #20
    I am not sure what black substance is at issue. I do know that the OEM black rubber damper mount that holds the fuel pump will dissolve in ethanol added fuels. When new it feels like a tire. When damaged it feels soft like a marshmallow. When destroyed it is gooey and sticks to your finger when you poke it. Bob's BMW has commissioned aftermarket alcohol resistant dampers, and has them available. I am not sure who else might also have the aftermarket part.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  6. #21
    PeteG
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    Quote Originally Posted by PGlaves View Post
    I am not sure what black substance is at issue. I do know that the OEM black rubber damper mount that holds the fuel pump will dissolve in ethanol added fuels. When new it feels like a tire. When damaged it feels soft like a marshmallow. When destroyed it is gooey and sticks to your finger when you poke it. Bob's BMW has commissioned aftermarket alcohol resistant dampers, and has them available. I am not sure who else might also have the aftermarket part.
    Hi Paul,
    I dismantled the pump clip assembly. The damper was in perfect condition. This is some type of plastic jammed between the metal housing that holds the pump and the left side of the tank. I've never read anything about this being inside of the tank, but it is clearly there on both the front and rear of the pump housing metal assembly. My initial thought was that it was there for structural support so the pump housing doesn't bend downward and rub against the inner left side of the tank.

  7. #22
    3 Red Bricks
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    Quote Originally Posted by pjar View Post
    Any opinion on the pictures of the plastic inside of the tank?
    IF I get a chance today, I'll check out some of my spare tanks. I do seem to remember something in there to keep the pump shelf from rattleing, since I don't belive the outer edge of the shelf is welded to the tank skin.

    You can reseal the outside of the sending unit with silicone.




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  8. #23
    PeteG
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    Quote Originally Posted by 98lee View Post
    IF I get a chance today, I'll check out some of my spare tanks. I do seem to remember something in there to keep the pump shelf from rattleing, since I don't belive the outer edge of the shelf is welded to the tank skin.

    You can reseal the outside of the sending unit with silicone.




    Thanks Lee. I was going to re-phrase the question as to whether the outer edge of the shelf was intended to be welded to the tank skin. I could see no other reason for needing to have something wedged in there. I too have another K75 (White 1988 that I tried to give to you years ago and glad you didn't accept) that I didn't think to check myself. I will do so on Monday to see what is going on with that shelf. For all I know that "plastic" is really hose that does not meet fuel standard requirements and became hard a brittle.

    Thanks for the silicone tip.

  9. #24
    3 Red Bricks
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    Pete,

    Looked at my spare tanks. All the tanks have it. It is a HARD plastic piece. It is round with a slot cut in it to slide over the edge of the pump shelf. The pump shelf is welded to the tank floor, then the floor is welded to the tank top skin along the lower seams. The plastic prevents rattles.





    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  10. #25
    PeteG
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    Quote Originally Posted by 98lee View Post
    Pete,

    Looked at my spare tanks. All the tanks have it. It is a HARD plastic piece. It is round with a slot cut in it to slide over the edge of the pump shelf. The pump shelf is welded to the tank floor, then the floor is welded to the tank top skin along the lower seams. The plastic prevents rattles.





    Hi Lee,

    Really appreciate you checking that. Look what I found! Your response to this very topic in 2014. https://forums.bmwmoa.org/showthread...-fuel-tank-K75

    If my plastic is completely deteriorated I may be able to fabricate some fuel hose of the correct diameter after making a slit to wedge in there. Just checked my 1987 K75C and it had sleeves exactly as shown in the 2014 post. Much fresher looking than what is left in my 1988 K75C tank.

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