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    M20 no fuel pump

    Rebuild M20 is all installed and engine turns over but fuel pump fails to come on. Relay tests good, fuse good but no power from relay, have continuity to pin #3 at ECU plug to relay but feel something is telling ECU not to ground 3# and send power to other side of the relay to make the fuse hot and power the pump. We jumped the relay and the pump is working. Checked resistance on CPS and got 519 and 519 at ECU plug #46,47 although everything I read says 48,49 but my plug has no pin at 48! Nor does my running 325i which is the same year. Ordered new relay just because and fuse is hard to make good contact like the board underneath needs pushing up(is this possible?). Can one take the fuse box apart and work on the interior? What am I missing on the fuel side, feel like its something simple Im missing. Thanks

    #2
    If the pump electronics are good and you can jump it to ignite, try checking for spark as well. The m20 DME (Motronic 1.3) does NOT prime the pump before the engine is rotating - nor throw spark if it doesn't sense the CPS signal. Check the CPS and make sure you have ~580ohms on two pins, if you do, check at the ECU connector to be sure.

    Many, many, many times people swap the cylinder identification (wire from the #6 plug wire) and CPS wires since they have the same plug. The engine will NOT run if they are swapped since the DME won't fire the coil, or fuel pump.
    john@m20guru.com
    Links:
    Transaction feedback: Here, here and here. Thanks :D

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      #3
      Checked plugs because I was worried they might have been switched but found the pulse wire and got 519 ohms at the CPS and the ECU plug when connected! is it bad at 519 ?

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        #4
        540 +/- 10% = 486-594.

        Resistance is within spec. How brittle/hard is the wire where it meets the sensor? If it's not pliable, then they tend to snap the wire at the sensor and make intermittent contact. That being said, wiggle the wire while testing, also a good idea to warm the car up and re-test since many times the resistance can be good at room temp, and not so much at operating temp.

        If you swap the wires, there's no ill effects. Countless times I have installed an m20 at work, only to see the fuel/spark failing to ignite, and just swap those two for an instant fire-up. :)
        john@m20guru.com
        Links:
        Transaction feedback: Here, here and here. Thanks :D

        Comment


          #5
          Recently ran into this issue with a friends M20 - CPS measured fine, and had actually been working fine before the engine was pulled and cleaned up, but after going through absolutely everything else trying to get the car started we decided to change it anyway and the car fired right up.

          This was on the 1.3 DME, so no fuel pump or anything the entire time, which is definitely different from the earlier models. You can always test the pump itself by just jumpering the relay pins if you suspect an issue there.

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