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Thread: Need help diagnosing misfire

  1. #1
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    Need help diagnosing misfire

    here are the initial symptoms on my friends 1995 3.0L US M3.

    driving to work his car suddenly lost power and vibrating pretty badly, he was close enough that he was able to limp it the rest of the way in. During lunch I went out and looked at it with him, and it sounded like one of the cylinders was not firing. The car still idles, but just barely. The check engine light was not on. And no fluids come out the tailpipe. Water was not mixed with oil and vise-verse.

    We put it on the trailer and took it to my shop after work. Here is the list of mods he has in his car. Shrick cams, Euro HFM, 24 lb. injectors, Jim C. intake, and Jim C chip for the above mods.

    We suspected the car had bad gas or water in the gas tank, so we added some of that stuff you can buy at the parts store (already a full tank of gas). Then we went to work on trouble shooting the problem. Ran the Peake fault code reader on it, no codes came up. Next we tested the coil packs for resistance, all came up within spec. Then we pulled the spark plugs. All looked good, except #5 cyl plug was extremely wet with fuel.

    We turned the engine over with all the plugs out of it and listened for any mechanical anomalies, and it all sounded perfect, no valves banging or anything like that.

    Cleaned the fouled plug really well, and put the plugs back in the head in no particular order. Next we swapped the #5 and #1 coil packs, and started it up again. Still misfiring, but we were hoping it was cylinder #1 now. Took the #1 plug out, but it looked good, then took out #5 plug, and it was covered in fuel! And that is about where we are at right now.

    we checked the injectors staticly for voltage, and pulse voltage while running, and they all look fine.

    Here are the things that we know so far.

    1: something weird is going on in cylinder #5 for sure
    2: could be a stuck open injector?
    3: faulty wiring leading to the coil pack?
    4: faulty DME?
    5: compression on cyl #5? (that would suck)

    any ideas? any help would be greatly appreciated!

    Thanks
    -Mel

  2. #2
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    Cleaned the fouled plug really well, and put the plugs back in the head in no particular order. Next we swapped the #5 and #1 coil packs, and started it up again. Still misfiring, but we were hoping it was cylinder #1 now. Took the #1 plug out, but it looked good, then took out #5 plug, and it was covered in fuel! And that is about where we are at right now.
    You said you put back the plugs in no particular order but did you inspect spark plug #5 for any cracks or damage?

    Try to swap injectors as well and see if the wet spark plug jumps.

    Next step I would take is compression and leakdown tests and do it on all cylinders just to make sure.

  3. #3
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    Originally posted by Jon Caldito


    You said you put back the plugs in no particular order but did you inspect spark plug #5 for any cracks or damage?

    Try to swap injectors as well and see if the wet spark plug jumps.

    Next step I would take is compression and leakdown tests and do it on all cylinders just to make sure.
    Yep, inspected the plugs really well (only 3k miles old) and they looked good.

    ,Next on the adjenda for tomorrow is swapping the injectors around, compression test, and mabye a leakdown test.

  4. #4
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    How long has the JIM C stuff been in it?

  5. #5
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    Figured it out now...

    OK, ran the compression test today, found that cyl #5 was getting 0 psi compression, and the other ones were all pretty good.

    took off the valve cover, everything looked OK upon initial inspection. Then took off the intake manifold, and found that one of the #5 intake valves was dropped slightly, and through a more 'tactile' inspection we found that the valve spring was broken.

    Does not appear to be any more damage to the engine (piston) because the secondary valve spring kept the valve from dropping all the way (racing springs). Next decission is whether or not to replace the spring ourselves, and let the tech degree the cam, or just let the tech do it all....

    Thanks for the help guys, Ill try to return the favor sometime soon.

    -Mel

  6. #6
    Bob ///M3's Avatar
    Bob ///M3 is offline
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    Didn't the '95 model M3s have the valve retainers that were known to be weak?

    Bob ///M3

  7. #7
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    I've been told its only the early build dates that had the weaker retainers. There is some debate over when the redesigned units came out, although the consenus seems to be 11/94. I'll see if I can find a link that backs up my writing.

    Oh by the way, my M3 was built in 10/94. Damn.
    1995 Cosmos M3

    Wanted (New or Used):
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    Many E36 M3 Items for Sale

  8. #8
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    Originally posted by Bob ///M3
    Didn't the '95 model M3s have the valve retainers that were known to be weak?

    Bob ///M3
    Forgot to mention that the car also has racing valve springs, Titanium retainters, milled head, and port and polish. Looks like the spring was defective I guess.

    -Mel

  9. #9
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    busted spring

    Be sure to check your installed height and your fully compressed height on the new spring. If those numbers are within spec, and the springs are being used in an "approved" application, then you have to worry about the quality of the springs. Better keep an eye on all of them...

    BTW, are you going to swap the spring with the head on? I've done that with a pushrod engine before. Never tried it with overhead cams, but it should still work. Write back if you want a description...

    Gary

  10. #10
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    Hey Mel,

    As mentioned right before this post, make sure to check the free length and installed length of the springs.

    Also, you should really pull one of the good springs too and have the rate checked. You are really supposed to check the rates once a year on race springs... making sure they are within tolerance.

    On the Mustang (which is still dead), the spring rates were incorrect which lead to 4 broken springs. Of course, with my luck, the pieces were injested which lead to trashed crank, bearings, cam and lifters. Your friend got lucky!

    Any machine shop should be able to drop the spring on the press and check the rate.

    Swapping out the springs is easy. Getting the cams timed again is the bitch.

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