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M42 Stumble and Spark Plug Wire Resistance

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    #1

    M42 Stumble and Spark Plug Wire Resistance

    Hi everyone,

    I'm trying to diagnose an intermittent stumble when the engine is under load, ex. at 50% or more throttle at low RPMs or when pulling away from a stop in first gear. Already replaced all of the vacuum hoses and intake boot so I'm fairly sure it's not a vacuum leak.

    I had read that some other people with a similar issue solved it by replacing the spark plug wires. But before I spend $150 on new wires I want to make sure mine are bad. Does anyone know what the resistance spec should be on these wires? All four of mine measure about 5.5-5.8 k-Ohms. They look to be in OK shape, but I noticed the boots say "made in W. Germany" on them, so it's possible they are the factory wires.

    Thanks!
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    #2
    Definitely stock wires if it's got the W. Germany stamp but I would be more concerned about the Ignition Coils than the wires first. How would you describe the stumble?

    I remember thinking something similar until I realized that is was caused by the open diff hitting a section in the road but this is also my first car so not the brightest over here.

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      #3
      It just feels like a near-total loss of power with a corresponding change in the exhaust note. Backing out of the throttle and sort of feathering in a really small amount of throttle can sometimes cause the issue to go away. I'll try to get a video of it and post it up. It's definitely not the diff as this can happen on smooth pavement pulling away from a stop.

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        #4
        If the leads are all the same then i would think they are "fine" as surely its unlikely for all of them to fail at once.

        how do the sparking plugs look? if a bit shit give them a go with a wire brush and reset the gap. if this solves it then some more sparking plugs are in order. Coils are a bit harder to test, but you can measure the resistance of them too (primary and secondary coils on each) and see if any of them are different to the others.

        Hows the fuel filter? changed recently? any history on the injectors? unlikely but they can cause issues if they are clogged etc.

        any ecu codes?


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          #5
          My M42 was doing the same things and it was my fuel pump going out. Check your fuel pressure.

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            #6
            Originally posted by e30davie View Post
            If the leads are all the same then i would think they are "fine" as surely its unlikely for all of them to fail at once.

            how do the sparking plugs look? if a bit shit give them a go with a wire brush and reset the gap. if this solves it then some more sparking plugs are in order. Coils are a bit harder to test, but you can measure the resistance of them too (primary and secondary coils on each) and see if any of them are different to the others.

            Hows the fuel filter? changed recently? any history on the injectors? unlikely but they can cause issues if they are clogged etc.

            any ecu codes?

            Plugs look OK to my eye, see below for a pic. How do I measure the coils? Been trying to find that procedure too.

            Fuel filter was replaced May 2019 (~5k miles ago), so it's probably OK. Injectors were rebuilt earlier this year when I replaced the vacuum lines. I assume no codes since there is no CEL. Any way to verify this?

            Thanks!

            Comment

              #7
              Originally posted by Victell View Post
              My M42 was doing the same things and it was my fuel pump going out. Check your fuel pressure.
              This was also my thought but a buddy somehow convinced me that it couldn't be the problem. I'll add this back to the list of things to check.

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                #8
                Looks like I may have accidentally fixed this... I reinstalled the plugs and wires with some fresh dielectric grease on the wire connections and the stumble is completely gone. Car pulls strong (for an M42, lol) and I can't make the engine stumble in any of the situations where it would do it every time before.

                I'm guessing I must have had a poor connection on one of the wires?

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                  #9
                  You described the exact same issue I have with the stumble...where did you apply the dielectric? I thought dielectric was an insulator and would inhibit a connection

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                    #10
                    I just wiped a bit on the inside of each boot on the plug side (don't think I did the coil side). The grease is just there to keep moisture out, etc. It was more likely just the act of disconnecting and re-seating all the connections that solved it for me.

                    If you're having the same problem, I would suggest pulling the wires and inspecting for burns, tears in the insulation, corrosion on the connectors, etc. Wouldn't hurt to check the resistance on all of them and see if it's in the same range as mine. If you haven't looked at your plugs in a while, I would do the same for them. Then assuming everything looks OK, re-assemble making sure you get all the connections seated properly. Then go for a drive and see if that fixes it!

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