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Intermittent Power Loss at cruise

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    Intermittent Power Loss at cruise

    Ok so this one is starting to really bug me; I sold my friend my old '90 325is and he claimed that on the freeway cruising it would suddenly loose power and bog. Wouldnt go above 2500RPM's, it would then clear up. Also (dont know if its related) but the idle would drop to 500RPM and pick up when at a light. All this is very very random, every time I got the car for a week or so it would drive fine.

    Well he sold that car to my brother, and he says its doing it randomly also. ALSO my friend has another '90 325i he brought into my shop yesterday for the same thing. He said this time it wouldnt go over 1500RPM so he towed it, been driving for a few days and running fine. I've checked fuel pressure (good), fuel pump wave form (good), fuel pump supply (good), coil resistance (spec: .5ohm on primary, actual 1ohm ?), crank sensor (spec:125ohm, actual: 162ohm ?), smoke tested good (just some leak out the valve cover), AFM is getting 5v's and sweeper seems fine, ICV resistance in spec.

    Of course all these things are tested when the car is running fine, there might be a glitch in any one of these causing it to run the way it does. Any help would be appreciated, these cars are bugging me. Thanks.

    #2
    clogged fuel filter? injectors? faulty fpr?
    AWD > RWD

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      #3
      Fuel filter is apparently is fairly new. Would check the injectors but its a pain and they're working fine now. Fuel pressure regulator holds vacuum and no leaks...

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        #4
        An important clue is what the tach and econometer does when an event occurs. If the tach doesn't drop, but the econometer drives towards zero there is a problem with the fuel system or injector firing pulses. Tee'ing in a fuel gauge and clipping it to a windshield wiper and seeing what the rail pressure is in an event will help determine where the problem lies.

        If the tach takes a big drop as compared to the loss of road speed the cause will be something that is taking away timing data to the DME, a loss of power ground to the DME, or a fault in the DME.
        The car makes it possible, but the driver makes it happen.
        Jim Levie, Huntsville, AL

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          #5
          Originally posted by jlevie View Post
          An important clue is what the tach and econometer does when an event occurs. If the tach doesn't drop, but the econometer drives towards zero there is a problem with the fuel system or injector firing pulses. Tee'ing in a fuel gauge and clipping it to a windshield wiper and seeing what the rail pressure is in an event will help determine where the problem lies.

          If the tach takes a big drop as compared to the loss of road speed the cause will be something that is taking away timing data to the DME, a loss of power ground to the DME, or a fault in the DME.

          Thanks, I will have to ask my friend if he noticed what happens. These cars must like me cause they always run fine whenever I have them.

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            #6
            Mine just started having a similar intermintant powerloss at cruise and I noticed the tach dove well below what my RPM's should have.

            What would be the possible culprits of faulty timing data to the ECU? I swapped ECU's back in September chasing another running issue that turned out to be an ECU coolant temp sensor (which I would recommend the OP test). I might switch back to my original ECU. Just curious what other timing inputs I should start testing.
            JOY IS AN E30...

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              #7
              Possible causes of a drop in timing data would be the crank position sensor, a flaky engine harness, or a a flaky DME. An M20B25 (or SuperETA) engine could have a bad harmonic balancer.
              The car makes it possible, but the driver makes it happen.
              Jim Levie, Huntsville, AL

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                #8
                Thanks... I was thinking crank position sensor. God I hope it's not the harness. I'll start with the crank position sensor and swapping out to my other ECU.
                JOY IS AN E30...

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                  #9
                  Mine was the crank position sensor. The plastic sleeve that routes it behind the belts was missing so the previous owner zip tied the wire bundle. One of the zip ties must have came loose and the wire rubbed up against the belt which cut through the insulation. It actually tested good per the Bentley Manual procedure, but I found it after chasing the wire.



                  Thanks jlevie, hopefully this helps someone else...
                  JOY IS AN E30...

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                    #10
                    OP.
                    The resistance values for the crank position sensor don't meet spec.
                    Spec is 540 Ohms +or - 54 Ohms
                    If the recheck the same, I would consider replacing it.

                    m

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