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Thread: Need help: weird idle/throttle issues with the M6

  1. #1
    Join Date
    Oct 2003
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    Need help: weird idle/throttle issues with the M6

    So here's a sorta weird one. My M6 has been acting up recently and neither of the two mechanics that I've brought her to can sort it out. The issue was manifesting itself as a hesitation when I get on the throttle in first--I'd ease the clutch pedal out, give her some gas and see that she was struggling to get the engine revs up. This weird stumble would happen every now and then and I could never get her to replicate it on demand--there never seemed to be any predictability as to when it would crop up.

    Both times, that I've brought her in, I've been discouraged from going on a fishing expedition to discover the problem, because the cost in labor and time would be prohibitive. Well, I just got her back from having her cat replaced and having a valve adjustment done. In general, she's driving better, but I've now discovered that the issue is worse. For the first 10 or 15 minutes of driving around the city, she's fine. But two days in a row now, once she's warmed up, the hesitation/stumble shows up. Tonight, I was waiting at a light when the idle started to fall. She usually idles at around 700 rpm or 800 rpm when warm. I watched as the needle fell suddenly to around 500 rpm or just a hair under. The light changed then, and I had to get going, at which point, she struggled to get up to 1200 rpm or so, and then took off. She continued to be a bit sluggish off the line for the rest of the ride home. Things got worse when I was waiting for a parking spot on my street, when the idle actually fell to a point when the engine cut out. She started up again just fine, but it's a really annoying issue to have. I also noticed when I got out of the car that she was smelling hot.

    At this point, I'm suspecting maybe the idle control valve might be at fault? Or maybe the throttle position sensor? At this point, I just need some ideas for prime suspects to go after.

  2. #2
    Join Date
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    1972 3.0 CSi-1988 M6
    From your descriptions of the symptoms, I suspect you may have a bad ecu. About 10 years ago, I expericenced similar problems with my car and with high frequency, the car would die when it came to a stop. It was quickly traced to the dreaded bad ecu and was replaced accordingly.

    As you're describing running problems other than idling, I would doubt it's the idle control valve. A bad or misadjusted throttle position sensor would cause severe backfiring from your exhaust, so that's probably not your problem either. You may also want check for a severely clogged air filter as little air is being sucked in during low speed operations.

    I'm surprised the repair shop gave up so easily in diagnosing your problems. Was the car service at Phaedrus or at another shop? I would suggest you have the shop locate and fix the problems as soon as possible as these running issues will probably get progressively worse and will leave your car in some precarious sitiuations. You do not want your car to die in the most inoportune time, such as rush hour traffic on the freeway.

    I hope you can resolve your M6 running problems and have the car back on the road, purring and roaring again. Good luck.
    Bert
    91 850i-A
    1972 3.5 CSi (L-jet, Getrag 265 5 speed)
    1988 ///M6
    2021 X3 M40i

    The back to back BIG COUPE” duo

  3. #3
    Join Date
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    Sounds like it is running too lean at warm idle.

    Do check the TPS just to be sure.

    Check the Coolant temperature sensor plug for cracks and looseness, check the sensor with a ohmmeter.

    Clean the Idle control valve.

    It could be a clogged fuel injector that shows up as the ECU leans the mixture as the engine warms. Run some Techron? through it.

    Could be a faulty O2 sensor, was it replaced with the new Cat? Check the connections and you can check the operation with a voltmeter, maybe they got "anti-seize" compound on the O2 probe which can screw it up.

  4. #4
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    Quote Originally Posted by M6csi View Post
    I'm surprised the repair shop gave up so easily in diagnosing your problems. Was the car service at Phaedrus or at another shop? I would suggest you have the shop locate and fix the problems as soon as possible as these running issues will probably get progressively worse and will leave your car in some precarious sitiuations. You do not want your car to die in the most inoportune time, such as rush hour traffic on the freeway.
    The first shop I took her to was indeed Phaedrus. At the time, the problem had just shown up and Dale took her for a day to diagnose it. He couldn't replicate the problem, but I suspect now it's because the car exhibits normal behavior until she's been in stop/go traffic in the city for 15 minutes or so. For example, when it happened last night, I'd driven from the Embarcadero end of Market to Dolores and 18th. The near stall happened on the drive back home at Franklin and Market. The actual stall happened soon afterwards when I got home.

    Thanks for the suggestions on what to look for. I hope I can get this sorted out soon, even though the M6 is not a daily driver.

  5. #5
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    Ok, so I posted this on the roadfly E24 board as well and one of the posters there has turned up a very interesting possibility:

    http://bimmer.roadfly.com/bmw/forums/e24/8610186-1.html

    That post makes the most sense to me--especially the part about the poor fuel economy, which I've previously discounted in my car (I'd always thought I had a flaky fuel gauge.) I'm having the coolant temp sensor replaced at Phaedrus on Thursday and we'll see if that solves the issue. I'll keep this thread updated on any progress I make.

  6. #6
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    Good place to start, if you notice it's the first thing on my list after the TPS which you check out of hand, but be aware that the problem can be in the plug and not the sensor it's self (although I don't believe your car has the resistors he talks about). FYI, it's always a good idea to include things that will indicate if the car is running rich or lean, it helps narrow down the list.

  7. #7
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    Quick update:

    The CTS was replaced and while they were doing that, my mechanics discovered that the plug was also broken, so they swapped that as well. I drove her a little over the weekend and it seems to have fixed the problem! Fingers crossed. On a related note, the "hot smell" turned out to be a plastic bag in my muffler. I have no idea how it got in there, but it's gone now. Whew!

  8. #8
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    I've noticed my car runs much better since my valve adjustment, too...

  9. #9
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    Quote Originally Posted by e24mpwr View Post
    I've noticed my car runs much better since my valve adjustment, too...
    Definitely. I would encourage anyone who hasn't done this to do so as well.

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