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Thread: '86 635csiA stumble/misfire making me crazy. Help!

  1. #1
    Join Date
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    2001 330ci, 1986 635CSi

    '86 635csiA stumble/misfire making me crazy. Help!

    So I have a misfire or stumble or stutter while my car is at idle when warm. It also occurs at steady RPM's at cruising speed. Runs smooth when cold or warming up, but as soon as the idle settles it begins to stumble randomly. Car never stalls, doesn't hunt, always starts. It really doesn't affect the driveability whatsoever. Just annoying, more or less. It is not a rhythmic misfire like when my E46 330ci has a misfire (sounds like a Subaru), this is more sporadic and random than narrowing it down to which cylinder. Drove it like this all last year and just dealt with it, now I'd like it to be gone. Also, it doesn't occur under load or during acceleration, runs smooth as glass while accelerating (both under normal acceleration and hard acceleration).

    Video of my car idling, at operating temperature:
    https://www.youtube.com/watch?v=ME7x1FepT9c

    Things I have done/checked:
    -Valve adjustment.
    -New valve cover gasket
    -New CTS (original didn't pass the Ohm test)
    -TTS is good
    -Speed sensor good
    -Reference sensor good
    -Verified cap and rotor are like new (less than 500 mi)
    -New plugs
    -New wires
    -All new vacuum lines
    -Good thermovalve
    -AFM cleaned and verified working
    -ICV cleaned and verified working
    -No vacuum leaks at intake manifold or throttle body (via ether/brake cleaner test)
    -Throttle switch in correct adjustment
    -MEPEH rebuilt fuel injectors installed
    -Checked and cleaned all engine bay grounds
    -Coil checks out
    -Cold start injector not leaking (but haven't checked voltage, flow, or spray pattern. Lazy me.)
    -Correct voltage at fuel injectors, correct resistance on injectors

    Things I have yet to do since the car is now out of storage:
    -Fuel filter
    -Check O2 sensor condition/functionality
    -Check fuel pump(s)
    -Check FPR
    -Replace Interior air temp sensor (Mine is gone, vacuum line plugged with a screw. Yuck.)

    Previous owner (coworker) says his former mechanic claims that there is a vacuum leak at the rear main seal. In all my years of owning these cars and researching running issues, I have never heard of this. Not saying that it is impossible, but I haven't run across it in all this time. Is there a way to check this without pulling the trans? Car doesn't leak a drop of oil.

    I have followed my E28 Bentley Manual and the Hi-Performance Store Motronic guide throughout the winter, and I have checked all of the running-related topics I am capable of. Is there anything else before I throw in the towel, bite the bullet, swallow my pride and take it to a shop? I feel like I am missing something simple and when I find out what it is I'm going to kick myself. Sorry for the long post, and thanks in advance.
    Last edited by Zack01GC; 04-17-2016 at 09:03 AM.
    1986 635CSi - 2001 330ci

  2. #2
    Join Date
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    '89 635 CSi, '15 535i
    Sounds a lot like a FPR problem. It's the only thing left that really changes at idle that wouldn't be a problem under load.
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  3. #3
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    Unfortunately I haven't had much time lately to look further into this issue due to being buried at work. Still driving the car a few days a week, but haven't had the time set aside to dive in yet.

    With that said, I drove the car to work today, and took a video of the only drivability "issue" I think could possibly be related to the poor idling.

    Car tends to surge 2-300 RPM during steady cruising (not acceleration, acceleration is smooth).

    I am holding 50 MPH steady in OD going up a long, straight grade. It is under these conditions that the surge is most noticeable. I think there may be a common issue between this "cruising surge" and the poor idle. Or I could be wrong. Regardless, this is the only other issue other than the crappy idle.

    https://www.youtube.com/watch?v=W3JgIZaJEW8
    1986 635CSi - 2001 330ci

  4. #4
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    86 635,72 E3, 88M6,73 E9
    Borrow or buy a CMU and swap just to make sure. They are cheap for your car and many of us have backups just for this reason. Keep in mind that a windshield leak can cause water to damage the CMU and unless you pull it and check, you will never know.
    Rob E3

  5. #5
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    I will add that to the list. Thanks. This is the unit above the glovebox, correct?
    1986 635CSi - 2001 330ci

  6. #6
    Join Date
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    Campbell, Ca
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    1985 635CSi
    Im basically going through exactly this right now. Im about to go single in tank pump and new o2 sensor to rule them out.

  7. #7
    Join Date
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    86 635csi 98 740i 96 MB
    Quote Originally Posted by dwal95008 View Post
    Im basically going through exactly this right now. Im about to go single in tank pump and new o2 sensor to rule them out.
    I think you should rule these out by testing before replacing. If they are known to be good, there's no benefit in changing them. You could put your energy into diagnosing to find the problem
    and address the solution.

    Check out steps 5 & 13 to determine if these are questionable.

    http://www.hiperformancestore.com/motronic.htm

  8. #8
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    Just for the hell of it since I never saw this posted, there is a small cover at the level of the valves, between the engine and firewall. You cant see the thing, only feel it . Put your hand on the back of the valve cover, slide it down and over to pass side. I have no idea what this auxillary cover is for but the gasket can go and cause oil leak and vacuum issue.
    Rob E3

  9. #9
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    Hey Rob,

    How goes it? Good point about the cam cover gasket.
    A smoke test would show a leak in the system.

    See ya @ "Idiotfest"

  10. #10
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    Quote Originally Posted by 1986series6 View Post
    I think you should rule these out by testing before replacing. If they are known to be good, there's no benefit in changing them. You could put your energy into diagnosing to find the problem
    and address the solution.

    Check out steps 5 & 13 to determine if these are questionable.



    http://www.hiperformancestore.com/motronic.htm

    I had gone through both of these steps with the exception of putting a pressure gauge on the fuel line. o2 relay is getting juice and the harness side of the o2 sensor is getting power. I need an oscilloscope lol

    the pump is also getting voltage with the ignition on. I should definitely check that gasket.

  11. #11
    Join Date
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    How did you verify the AFM was working? Did you open it up and check that carbon track that wears out? Did you tweek that sweeping arm to track at a different location? It is amazing that one break in that track....one that you cannot see, can cause your issue.

    Likewise Bob..and you had to toss in that you knew the name of the thing I was talking about. I had an oil leak for years that I finally traced to that thing. Slow but steady.
    Rob E3

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