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  • S14 - strange miss

    I am a new member and am hoping someone has an idea as to what is causing my engine to miss at higher RPM's on my M3 race engine,

    Symptom - Under heavy load in 4th or 5th gear engine misses at approx. 6500, 7200, 7400 RPM. When miss occurs RPM gauge drops approx 500-700 and comes back up. If I accellerrate more slowly the miss is more frequent and moves lower in the RPM band, however the symtom is intermitient. The only other partern that I have noted is that the problem worsens at higher temperatures.

    Here is what has been replaced:

    Fuel Pump
    Fuel Filter
    Fuel Press Regulator
    Injectors
    Cap
    Coil
    Wires
    Rotor
    Plugs
    O2 Sensor
    DME / Chip
    Inspected and Swaped Sensors on Tranny Bell housing

    Installed fuel Press gauge and have found no change pressure when miss occurs. It looks electrical but I do not know what else to change?

    Does anyone have any ideas?
    sigpic

  • #2
    sounds wierd but... are you turning hard when this happens? does it happen even when you have a full tank of gas?
    1990 M3/1998 M3 Sedan

    Comment


    • #3
      The AFM may suffer from a spike in the output voltage. At that moment the DME doesn't get the signal to put a load of fuel into the cylinders and the car developes a hesitation. The recommended method of testing is to measure the voltage output from the connected AFM while slowly pushing the vane open. You should see a steadily, smoothly increasing voltage (from infinite to about 4.75-5 Volts).

      Also the blue watertemp sensor for the DME in the coolant tube above the exhaust header may be shot. This can lead to strange stuff happening...

      From the SIG:

      ================================================== ======
      >Gruppe:
      >I can't seem to find the specs for the coolant temp sensor. Somebody
      >posted that the resistance of the sensor would increase with
      >temperature.

      No. The sensor is an NTC (Negative Temperature Coefficient), that is,
      a resistor whose resistance decreases as the temperature rises.

      I think that if a bad connector or open circuit mimics a
      >cold engine, then the resistance must be high when cold, as an open
      >circuit has infinite resistance.

      Correct. The resistance should be around 2.5 kohms at 20C/68F.
      It's important to note that the NTC has an exponential behavior.


      As the engine warms, resistance would
      >drop. The two leads for the sensor are for the engine computer and a
      >path to ground. If a bad sensor is suspected for poor emissions or
      >fouled plugs, would shorting out the leads to the sensor (no resistance)
      >correct the problem for a warm engine? Or is the reading for a fully
      >warm sensor higher than zero?

      The resistance is aprox. 200 ohms at the operating temperature.
      (temp. gauge at 1/2). If you want to simulate this condition, connect
      a 200 ohms resistor to the blue plug, it's not necessary to short
      anything.

      >Also, since this component is so critical for emissions and
      >driveability, the leads also should be examined. Where would you place
      >a continuity checker to check the wires that go to this sensor?

      Path to ground: between the female pin of the blue plug and the
      bolt/nut that is located to the left of the DME. You will see a lot of
      brown wires leading to that bolt.

      Path to the DME: between the other female pin of the blue plug
      and the pin #13 of the DME connector.

      It would be a good idea to wiggle the wires of the sensor while you
      are checking the continuity.

      Jose Santana
      '83 316
      '88 ///M3 Macaoblau 2191116
      Canary Islands, Spain.

      ================================================== ====

      Adwo

      Comment


      • #4
        does the tach *drop* to lower rpms or does it actually *SLAM* to zero then *SLAM* back up to the correct rpm?

        If the former, I'd Suspect a fueling/mixture problem. I wouln't play around with this one as you'll lose the motor very quickly...It sound like it is high load lean (it is detonating).

        If the latter, I'd suspect an ignition failure (secondary).
        m.

        Comment


        • #5
          Other then the AFM.

          Are you running the stock plug wires in the plastic loom? If so get some new plug wires and do not use the loom.

          The loom cuts the wires and can cause a loss of spark, also causing what you have minus the tach problem.

          How are the 2 ground straps on the engine? You might want to remove, clean, and retorque.

          Do not click
          At least it's German

          Comment


          • #6
            Thanks for the suggestions.

            The only thing I forgot to mention is that I am running a Cyntex Alpha N Set up. (No Airflow meter)

            As for the wire idea, been there done that.

            I will investigate the Lean issue further

            Thanks agian,

            Mike
            sigpic

            Comment


            • #7
              The Cyntex never worked really well if you want to believe Jim Conforti... Do you have AFR readings from your engine?

              If you have a lean running engine you will have misses as you described. Another option is that the temperature correction used is not working very well (ambient temperature)...

              HTH,

              Adwo

              Comment


              • #8
                Originally posted by Adwo
                The AFM may suffer from a spike in the output voltage. At that moment the DME doesn't get the signal to put a load of fuel into the cylinders and the car developes a hesitation. The recommended method of testing is to measure the voltage output from the connected AFM while slowly pushing the vane open. You should see a steadily, smoothly increasing voltage (from infinite to about 4.75-5 Volts).

                Adwo
                For what it's worth, even though he said he's AFM-less, I'd just like to add that my AFM recently did this; but it wouldn't 'misfire' as such; it would just run out of power suddenly, as if I took my foot out of it. No detonation or sputtering of any kind, still ran perfectly smooth, just with no power after about 6k.

                Comment

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