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    Odd random misfire issue....

    I've been trying to tackle a odd misfire issue I've been having on my '89 M20. Happens randomly but very rarely.

    First time it happened (9/2014)
    Driving after about 20 minutes, then all of a sudden- loss of power, backfiring. Limped home about 15 miles and all of a sudden in 3rd gear- boom, started running perfectly. No CEL. Normal temp.

    What I tried-
    1. Seafoamed
    2. New O2 sensor
    3. Checked for vac leaks

    Then it worked perfectly fine for 1,500 miles and about a year....not one issue at all. I thought it was just a faulty o2 sensor and fixed my issues.

    Second time it happened (7/2015)

    Started the same issue, about 10 minutes into my drive- back to misfiring. Let the engine cool down for an hour and tried to start it again, huge hesitation on startup, but eventually got it going. Hard to keep idle, limped all the way home again. Then boom, pulling into my driveway and it went right back to normal. No CEL. Normal Temp.

    1. Installed new NGK copper plugs (old ones were OK and only a year old, had a bit of oil on them..but nothing horrible)
    2. Ran compression- all good numbers in 160-165 range.

    Other things that I have replaced:
    -New Distro rotor, new cap, new wires.
    -Rebuilt M50 injectors (and SSSquid tune) (All injectors are seated well)
    -Verified all fuel connections, holds pressure. (all new lines, FPR, filter etc)
    -AFM, ICV- all of the intake stuff has been cleaned and verified of working correctly
    -No vac leaks. I've gotten a smoke test done at a shop and I do not have any issues.
    -Battery Grounds are new
    -TB, WP, Radiator- all that stuff has been replaced in the past year. I've done a lot of maintenance on the engine last year, the only thing I did not touch was the head/block/crank on the engine.

    Just wondering what input everyone has for what I can check next. I never replaced my coil or CPS, not sure if they can go bad randomly. CPS is secure. As of now, whether or not it was a spark plug(s) that were bad- the car is running great as I type this.

    Just want to see what else I can investigate, because I don't like how random this issue is and not get stuck somewhere one day.

    Thanks!
    Last edited by Klug; 07-25-2015, 12:00 PM.
    555Garage - Kingston, PA

    '13 BMW e70 X5 35d
    '95 BMW e34 525i
    '92 BMW e30 325i S52 Vert (Quade the Vert)
    '92 Range Rover Classic
    '90 Range Rover Classic
    '89 BMW e30 325i Coupe (The DIRTY30)
    '81 VW MK1 Rabbit Caddy Diesel
    '76 Honda CB550K Cafe/Brat
    '67 Pontiac LeMans
    '24 Model T Depot Hack
    ....And a bunch of Motobecane, Puch, Garelli, Batavus and Honda 49cc-78cc Mopeds...


    #2
    This sounds like a problem with the CPS, engine wiring harness, or possibly the DME. I'd start with a BMW CPS. The CPS signals are run to the DME via small coax cables, which degrade in time. I have four used harnesses in my garage that all have degraded coax. So now I only use a new BMW harness, which is pricey (~$700) but certain to work.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

    Comment


      #3
      I'm going to test the Ohms of the CPS. Will report back-
      Last edited by Klug; 07-25-2015, 12:27 PM.
      555Garage - Kingston, PA

      '13 BMW e70 X5 35d
      '95 BMW e34 525i
      '92 BMW e30 325i S52 Vert (Quade the Vert)
      '92 Range Rover Classic
      '90 Range Rover Classic
      '89 BMW e30 325i Coupe (The DIRTY30)
      '81 VW MK1 Rabbit Caddy Diesel
      '76 Honda CB550K Cafe/Brat
      '67 Pontiac LeMans
      '24 Model T Depot Hack
      ....And a bunch of Motobecane, Puch, Garelli, Batavus and Honda 49cc-78cc Mopeds...

      Comment


        #4
        Ohms on the CPS is at 614. Which is higher than the threshold of 540 ohms +/- 50

        I guess a new CPS is what I will need to do!
        555Garage - Kingston, PA

        '13 BMW e70 X5 35d
        '95 BMW e34 525i
        '92 BMW e30 325i S52 Vert (Quade the Vert)
        '92 Range Rover Classic
        '90 Range Rover Classic
        '89 BMW e30 325i Coupe (The DIRTY30)
        '81 VW MK1 Rabbit Caddy Diesel
        '76 Honda CB550K Cafe/Brat
        '67 Pontiac LeMans
        '24 Model T Depot Hack
        ....And a bunch of Motobecane, Puch, Garelli, Batavus and Honda 49cc-78cc Mopeds...

        Comment


          #5
          In this case, the CPS is definitely the place to start. My preference is the OEM unit.
          The car makes it possible, but the driver makes it happen.
          Jim Levie, Huntsville, AL

          Comment


            #6
            So I just talked to a friend of mine, told me that if the sensor was warm from running, it will read different Ohms. Waited until it cooled down a bit and it was reading 580's. So maybe the CPS wasn't the issue. Just wondering if that 540 +/- 50 reading is when the sensor is cold...and if that +50 is max when it is warm- since it definately goes higher than that when it is.

            Other than that, I read that the CPS should be 1mm away from the crank wheel, turns out that was actually around 4mm. Adjusted that, still runs good....hopefully this will take care of it. I just want to be proactive with this issue so I don't drive on a big trip only for it to act up again.
            555Garage - Kingston, PA

            '13 BMW e70 X5 35d
            '95 BMW e34 525i
            '92 BMW e30 325i S52 Vert (Quade the Vert)
            '92 Range Rover Classic
            '90 Range Rover Classic
            '89 BMW e30 325i Coupe (The DIRTY30)
            '81 VW MK1 Rabbit Caddy Diesel
            '76 Honda CB550K Cafe/Brat
            '67 Pontiac LeMans
            '24 Model T Depot Hack
            ....And a bunch of Motobecane, Puch, Garelli, Batavus and Honda 49cc-78cc Mopeds...

            Comment


              #7
              A bad connection to the afm can cause similar issues (if I read your issues correctly). Try unplugging the afm and driving with the car; if it feels the same then you've found the area to work on.

              Other than that, for the issues you're describing, it sounds like some component has a bad connection. I would check (take apart, clean, grease/dielectric grease necessary bits) the rotor, coil, and afm. Since you've replaced the other components, it has to be laying in there somewhere. If none of that works, try to BORROW an ECU from someone for a quick minute.

              Comment

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