Case Study
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Thread: Case Study

  1. #1
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    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
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  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
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    "Any day above ground is a good day"

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  4. #4
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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?
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  5. #5
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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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    Re: Case Study

    PIP capture, looks good.Name:  PIP capture.jpg
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    PIP, and SPOUT capture at idle. Doesn't look so good. Name:  PIP capture2.jpg
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    PIP and SPOUT at approx. 3000rpm. Name:  PIP capture3.jpg
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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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    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/

  9. #9
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    Re: Case Study

    If it doesn't look like a duck, it probably ain't a duck

    Scope wasn't hooked up right, and got someweirdwaveforms

    Put up the right crap later on in the day.
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  10. #10
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    Re: Case Study

    Quote Originally Posted by farmersamm View Post
    If it doesn't look like a duck, it probably ain't a duck
    Scope wasn't hooked up right, and got someweirdwaveforms
    Put up the right crap later on in the day.
    In the computer industry we call it GIGO!
    Be wary of The Numbers: Figures don't lie,. but liars can figure.
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  11. #11
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    Re: Case Study

    Got it all squared away

    Good to know the circuit before you test it.

    Name:  signal flow tfi.jpg
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    You can follow the signal from the PIP output on the distributor, to the PCM, and to the ICM. This truck will run on PIP signal only, in limp mode, or when push started I believe.
    And, you can follow the SPOUT signal from PCM to ICM. This all gives me an idea as to where to run some tests.



    All testing was done at the ICM. If it's good there, it's good all back up the line.

    You might note that no problems were seen in the Primary Waveforms. This flies in the face of a lot of folks that say that the Primary mirrors the Secondary. A problem in the secondary should show up in the primary,,,,so they say. Tain't so, at least here.

    I feel that the spark is outside the cylinder. I can hear it pop, and see it on the scope. But even pulling the plug wire, and using a spark tester doesn't give me a solution. The spark tester shows all secondary wiring up to the plug is good.

    Name:  spark tester.jpg
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    So...........my next thought is a bad insulator on the plug itself. A crack allowing spark to go to ground, instead of going thru the electrode to fire the cylinder.

    Pulled the plug, and looked at it. No carbon tracking, and a pretty clean electrode. Being as it's a brand new plug, it's normal to think it's ok.....but maybe it isn't. Need to substitute another plug sometime today.
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  12. #12
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    Re: Case Study

    I'm thinking............why waste the bucks on a horse, I got two good donkeys right here on the place. Bit slow, but they'll get ya there eventually
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  13. #13
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    Re: Case Study

    Oops.

    <>
    Be wary of The Numbers: Figures don't lie,. but liars can figure.
    Welders:
    2008 Lincoln 140 GMAW&FCAW
    2012 HF 165 'toy' GTAW&SMAW
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  14. #14
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    Re: Case Study

    So is this fixed yet. All this reading....and no hard diagnosis or fix.
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  15. #15
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    Re: Case Study

    I would swap plugs between cylinders to rule it out. Hard to say if somewhere someone dropped the box of plugs. Hell I chased a bad electric fuel pump for a month because it was only a couple months old.
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  16. #16
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    Re: Case Study

    Good to go, replaced "new" plug with one of the old ones. So, I guess I was right to call a bad plug Feels good to be right ONCE in a while. The miss is history.



    Can't trace the ticking sound to anywhere on the left side of the engine (ignition side), but it's driving me nuts Put a stethoscope on the plugs, distributor, coil, all secondary wires, and anything close to the ignition.......including near where the plug wires connected to plugs/rotor.............NOTHING

    Buddy of mine suggested firing it up at night, and looking for a light show. Gonna do that tonite I guess. Worth a try, but no opens, or shorts to ground, showed up on any waveforms............ And it IS running nice again............

    Only thing I can think is that some injector noise is telegraphing underneath where the plenum goes over the top of the engine
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  17. #17
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    Re: Case Study

    So.......like I go out there in the dead of night........tripped over some stuff in the yard........get to the truck.

    Fire it up......pop the hood.......and shove my beak right in there (I have a pretty good idea where the radiator fan is, so all's good)

    Nothing, nada, zilch, zippo, squat.......... No sparking to ground from anything.

    Runs like a top. EXCEPT FOR THE TICKING NOISE

    I dunno what a leaky hyd. lifter sounds like on one of these. I do know what it sounds like on a big block Chrysler, but it doesn't sound this scrawny.

    Anyways...did take it out on the blacktop today, after replacing the bad plug. Gotta big puddle in my driveway that gets the tires wet before you hit the pavement. She'll break the rear tires loose when wet Hasn't been able to do that in a month of Sundays, so......we be good to go!! Not bad for an old mill with 265K on it. The old inline 300 was one heck of an engine in its day.
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  18. #18
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    Re: Case Study

    BTW......no beer was harmed in the making of the video
    "Any day above ground is a good day"

    http://www.farmersamm.com/

  19. #19
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    Re: Case Study

    If you can run the engine with the valve cover off, you can stick your finger between the rocker and valve retainer. Use the knuckle just below the nail. If everything is good your finger will go up and down with the valve and rocker. If the lifter is bad it will try to pinch your finger as the lash opens and closes.

  20. #20
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    Re: Case Study

    Quote Originally Posted by 76GMC1500 View Post
    If you can run the engine with the valve cover off, you can stick your finger between the rocker and valve retainer. Use the knuckle just below the nail. If everything is good your finger will go up and down with the valve and rocker. If the lifter is bad it will try to pinch your finger as the lash opens and closes.
    Hold a screwdriver against the end of the rocker arm. It will make noise if it's got lash in it, and it's easier on the fingers.
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