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  1. #1
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    Engine head carbon buildup?

    About a year ago the check engine light came on in my 2001 M Coupe. I checked and the code was P0492 (secondary air bank 2). As it seemed fairly minor, I disabled the warning light to see if it would repeat. A few weeks later it came back on so I took the car in to the dealer. They weren't able to reproduce the problem, but guessed it was the Euro Valve and replaced that.
    A month later the light came back on again. When I took it in this time they claimed to have actually found the air pump to be failing and replaced it.
    Again, the light came back on. The third time they claimed to have evidence that one of the O2 sensors was failing and tried that.
    Once again the warning light came on so I took the car in for more investigation. After about a week of work they suspect that carbon buildup in the engine head. They want to remove the head and re-machine it to clean out deposits, and expect 20 hours of labor and about $5,000 to fix it.

    So the big question is, does the dealer's explanation make sense, and is this something I should have done?

    I've had the car for 4 years, and only driven it about 24K miles. Although I don't drive many miles, I rarely drive trips of less than 5 miles. The dealer claims that the low miles contributed to the problem, which doesn't make much sense to me. I have been changing the oil every year regardless of how many miles I've driven.

    The warning light would always come on shortly after starting the car, and happened more often during colder weather.

    I haven't noticed any difference in gas milage or power, although I probably rarely use anything near full power.


    Again, does the dealer's diagnosis make sense, or should I get a second opinion? If carbon buildup really is a problem, is it in danger of causing more damage if I don't fix it immediately, or is this something I can ignore?

    Thanks for any advice.


  2. #2
    Steve D (AL)
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    get a 2nd opinion

    Sounds like they are taking random guesses at your expense. I think a second opinion is worthwhile. If it were my $5000, I'd want a very detailed, logical explanation of what codes they are reading, how that relates to carbon buildup and what is a likely cause. I don't think that "carbon buildup" with no noticeable driveability problem is an immediate danger to your engine. But it is probably some other problem that they have not correctly diagnosed. Probably an emissions-related issue, since that is the class of problem that triggers the check engine light.

  3. #3
    FloodCityM
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    Re: Engine head carbon buildup?

    The carbon build-up story sounds pretty lame.... they're just shooting in the dark at your expense. It is almost certainly a sensor issue and, unfortunately, the only way to pin it down is trial and error. Since the dealer seems to be saddling you with the expense of his errors, it might be time to either try some sensor replacement yourself or find a good indy shop that knows BMWs. In the mean time, since you are having no driveability or operational issues, I think you are safe to just keep driving it.

  4. #4
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    Re: Engine head carbon buildup?

    Thanks for the suggestions.
    After about 2 weeks they finished all their tests and stick by their diagnosis. Supposedly their top mechanics have been working on it. But they also said that not addressing the issue shouldn't cause any damage.
    Since I haven't noticed any decreased performance or gas milage I'm just going to leave it be for now and keep an eye on how often the warning light comes back on.

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