Case Study
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Thread: Case Study

  1. #1
    Join Date
    Aug 2008
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    10,545

    Case Study

    94 F-150, 4.9L, EEC-IV engine control system. Misfire.



    Visual Inspection.......Cap/rotor don't show any excessive burning. Coil had a burned secondary tower connector, half eaten away. Plugs had worn to .080 gap. Replaced coil, and plugs. Wires tested ok.

    Misfire isolated to #3 cylinder. Least that's a starting point.

    This is a distributor type ignition, with a Hall Effect sensor.

    The complete lack of spark leads me to think the problem is in the primary. The GMM (similar to the KV Module on the older Snap On Vantage) shows intermittent 0 spark voltage. The scope also shows intermittent lack of secondary voltage (Any voltage!).

    I'll get around to checking the primary waveform on the scope. I think I can trigger the scope on #1 cyl to observe a primary parade pattern. If the parade pattern is unstable, this will hopefully confirm that the sensor isn't being triggered by the windows properly. A fault inside the distributor, or related to the distributor.

    I might note.......plug wires were switched to verify it isn't a bad wire, and the cap/rotor were switched to verify that the cap/rotor isn't the problem (old parts off my donor parts truck). I felt this completely eliminated any thought directed at the secondary.

    So, now I'm at the fork in the road, and it seems to be leading inside the distributor
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  2. #2
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    Re: Case Study

    Since that is a TFI system I would suspect the module, they were not bullet proof like the Duraspark.
    Possibly worn bushings in the distributor but they would have to be really bad.

  3. #3
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    Re: Case Study

    Quote Originally Posted by rexcormack View Post
    Since that is a TFI system I would suspect the module, they were not bullet proof like the Duraspark.
    Possibly worn bushings in the distributor but they would have to be really bad.
    I think we're pointed in that general direction at this point, for sure.

    This particular truck sort of throws a wrench in the works though. It has a remote, fender mounted, ICM, with a PIP sensor mounted on the distributor (the Hall Effect sensor).

    PIP sensor is simply a Crankshaft Position Sensor which tells the PCM what cylinder is at TDC. PCM then monitors the voltage change, and tells the ICM when to interrupt the primary magnetic field in the coil. The PIP is basically an on/off switch.

    So.......I'm thinking I'll start at the source, so to speak. We have no secondary voltage at #3, with known good secondary wiring/circuit. The next link in the chain would be the rotating vane in the distributor, which has windows(slots) that interrupt the magnetic field at the PIP sensor. Being as something is going on with one cylinder only, I'm thinking about the signal to the ICM, rather than the ICM itself. The other cylinders work ok, which would lead one to think that the ICM is doing its job on the other 5 cylinders.

    I have to figure out why the PCM isn't getting a signal on #3.

    .................................................. .................................................. ........................................

    I like your thinking on the possibility of bad distributor bushings.

    Back in the day, when we had points, bad bushings would show up when you lost power, and a dwell meter (remember those?) would show a fluctuating dwell. Bingo....you had bad bushings.

    If I had bad bushings, I'd assume there would be an effect on all cylinders, not just one in particular. Dunno, but that would be my guess. I didn't notice any play when I checked the rotor on the visual.

    When the weather clears today, I'll do a quick test on the PIP sensor when I do a check on the general primary parade. Something is causing a possible loss of signal to the PCM.

    .................................................. .................................................. .................................................. .......................

    It's been a real stumper. I have mixed feelings about leaving the secondary behind, and pursuing the primary.

    An "open" in the #3 secondary would show up on the scope on the firing line (the tall initial spike). Spike would be high due to the high resistance when trying to jump the gap. And, the spark line would be extremely short, or nonexistent.

    A short to ground would show up as a short spike on the firing line, and the firing line would drop to ground. Although it's almost impossible to have a true short to ground in a distributor system because there's still a gap between rotor and cap.

    Total lack of an "event" means either that the voltage isn't getting to the secondary (problems in the cap....crossfire, or huge open) There are no signs of carbon tracking in the cap, or any kind of excessive burning inside the cap at the #3 position.

    For those unfamiliar with all this talk about firing lines, spark lines, etc...........................

    Name:  secondary waveform.gif
Views: 182
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    "Any day above ground is a good day"

    http://www.farmersamm.com/

  4. #4
    Join Date
    Sep 2013
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    Re: Case Study

    If it's just the #3 cyl and all others are firing reasonably, then *logically* you're dealing with just something associated with that cylinder.

    Obvious, silly questions first:
    - New or swapped around ignition wires and spark plug to confirm it's #3?
    - Compression test to see if there is a mechanical issue?

    Or did I miss that the whole engine isn't running and the only code the computer is throwing indicates a #3 misfire?
    Be wary of The Numbers: Figures don't lie,. but liars can figure.
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  5. #5
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    Oct 2008
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    New Jersey (yeah it sucks)
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    Re: Case Study

    Condenser? An open condenser can cause a weak spark and very short dwell; your oscilloscope may not have enough bandwidth to detect this problem. Is it a solid misfire on #3, or is it intermittent.
    I had a fwaking misfire on an old slant six, turned out to be a leaking intake manifold gasket.

  6. #6
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    Re: Case Study

    What about an intermittant open on the coil module tiggering (ie corroded terminal) for cyl 3?
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  7. #7
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    Re: Case Study

    These years of the F series have that somewhat common problem with the ECM getting soaked and corrosion then sets in..
    I chased a misfire on a truck that was a 5.8 but basically the same engine management system
    The ECM would intermittently not read the pip signal from the ignition module

  8. #8
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    Aug 2008
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    Re: Case Study

    PIP capture, looks good.Name:  PIP capture.jpg
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Size:  53.7 KB

    PIP, and SPOUT capture at idle. Doesn't look so good. Name:  PIP capture2.jpg
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Size:  45.9 KB

    PIP and SPOUT at approx. 3000rpm. Name:  PIP capture3.jpg
Views: 31
Size:  53.3 KB Doesn't look so hot either.

    This is a good capture I found on the net. Name:  Image5.gif
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Size:  18.6 KB

    Before I accept any of this as good evidence, I have to put the scope on a known good system, which I do have in another truck. I'm in uncharted territory here, and I'm really not sure as to what I'm seeing.

    Almost makes ya want to buy a horse

    Anyways, gotta do some research on the actual sequence of events in this circuit to better understand it. Should know something by tomorrow.
    "Any day above ground is a good day"

    http://www.farmersamm.com/

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