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Thread: SES Light and Carbon Buildup in Cylinder Head Ports

  1. #1
    Z8 Novice
    Join Date
    Feb 2014

    SES Light and Carbon Buildup in Cylinder Head Ports

    Hello everyone. I am Marc--haven't posted here other than in the registry--I have 61979, an 03 Stratus/Crema Z8. Fabulous car; I've had a ton of fun with it--now has about 105k miles on it and the engine still has that edgy, crisp quality about it, but now I'm worried.

    For about a year I've been having 'service engine soon' light issues. The BMW dealer has addressed it, said it was "a valve", charged me a couple hundred, and the light came on again soon after. I flirted with a local independent garage; someone told me they were capable. When I asked them if they were comfortable working on the car, they said "sure, we sent our technician to train in Germany". Frankly sounded no worse than what you really get at the authorized dealers--from what I can tell, one guy who who went to Munich and 20 screwdriver mechanics who can change parts. They kept the car a couple days, and when I came back it was on a half hoist with a sheet of paper downloaded from the internet under a wiper on the windshield. They said it was either a $50 valve, or else a condition of carbon buildup (I think in the intake ports of one of the cylinder banks). If the latter, would require R&R of the engine.

    I passed on the work, and have simply enjoyed the car for a full year, about 6k miles. In that time, the car has always run fine. The SES light, which they turned off at the independent garage, came on a few weeks later. The light stays on about a month, then goes off for about two weeks, then comes on again and repeats the cycle. Actually, the light has now been off for about two months. The car also had no problem passing its CA smog test just a couple months ago.

    Now due for routine service, so I had my BMW dealer query the codes again. They are telling me that it's the carbon buildup thing and they need to pull both heads, send them out for de-clogging, and put em back on. $15,000! Also one of the catalytic converters needs replacing, they say--$3600 for that.

    Now, there's just something wrong about spending $20k to repair a car that runs fine. Also, I am loathe to recondition the top end of an engine with over 100k miles on it. As I said above, the engine is still crisp and edgy. While I really like my service manager at the shop, in the past, I've had to scream at him because the technicians did sloppy work, and I can assure everyone that this is not some discount shop--authorized BMW dealer charging serious money. Given my past experience, I'm fine with these techs changing external parts like the starter motor ($1500 for a rebuilt one), AC compressor (same), etc. But to pull the heads?

    Is this a common problem on Z8s/contemporary M5s? Any ideas?


  2. #2
    Hi Marc,

    Do some research on, specifically the E39 M5/Z8 forum. Carbon build-up is a known problem within the E39 M5 community, where higher mileage cars (100k miles up to 300k miles) are commonplace. Apparently the carbon buildup issue does not affect performance, and the SES light can be shut off through the use of a software fix which is available for a few hundred bucks, and cars will still pass emissions tests. However, I don't know if the fix is specific to the M5 or whether it can be used on the Z8 as well.

    Good luck, and let us know how things work out.


  3. #3
    Team Z8
    Join Date
    Jul 2003
    Hmmm...sounds a lot like the diagnosis I'm getting after my on again-off again "service engine soon" and codes were downloaded. Without a date stamp on the codes the garage here in the South Bay (LA) cleared the codes from the history and I'm driving the car a couple of weeks to see what comes back...still no check engine light and the car continues to run great with 44k miles.

  4. #4
    Z8 Novice
    Join Date
    Feb 2014
    Bill and Ron, thanks so much for the comments. I'd like to say, boy, those M5 guys are obsessive, but instead I'll say they're admirably detail-oriented. There's a ton of commentary about this issue on the forum Bill referenced, but Bill's comments sum it up. As the story goes, the issue does't affect performance because it's in the secondary airflow system, a cold-start emissions system loop which closes off at operating temp normally. The fault code is 170 "secondary air quantity low". Seems like it's usually in the high mileage cars, but has occurred in cars (M5s) with only 10k too. Also, some report that, even after spending the money (on the M5 Board they say the cost is around $8000; my quote at Long Beach BMW was $14,886.80) the problem may recur. Whether or not the problem affects smog certification is somewhat iffy--most comments say the cars still pass (as mine did), but I wonder if that is because I had it done during one of the "off-again" intervals for the light.

    I do get a kick out of the fix that essentially does nothing but block the light coming on for this problem--there's some amusing commentary on the M5 Board "wait, does the fix prevent the actual carbon buildup (no) or just prevent the light coming on (yes). Also some said that using that fix itself may jeopardize emissions testing in some states.

    Anyway, I'm much less panicked than I was yesterday--for a while I actually contemplated letting the car go, but after the research, I feel comfortable continuing to enjoy one of the world's great motoring experiences. I have an Audi R8 Spyder too--awesome car in its own right, but more video game/Boy Racer/Millenial kind of vibe. The Z8 is for discriminating adults, and much easer to get in and out of!

    I'll comment again in a few months (earlier if there's a related problem). Thanks again!


  5. #5
    DSC Off JohnAnthony's Avatar
    Join Date
    Jun 2011
    Hey Marc,

    yes, the topic has been covered on M5Board extensively and most people choose to eliminate the cold-start cycle and the secondary air pump completely even before they experience an SES light as a result of the Secondary Air System Carbon Build up. The issue as you've read is an emissions only issue and will NOT affect any other aspect of the engine. By "emissions only" I'm referring to your specific car's attempt to reduce cold start emissions by warming up the catalytic converter through the Secondary Air System, not a state Emission test/facilty issue. There are tuners out there that offer different levels of support and should be able to provide you simple Cold-Start Delete tuning (if you do not wish to modify your factory tuning). This type of tuning will PASS emissions even in California, because when you bring your car for SMOG check you've already warmed it up and the Cold Start will be irrelevant. The tuning originally told the car to ignore the condition for restricted secondary air flow and thus the SES would not be triggered. Today, depending on who you choose to get your tuning from, they will eliminate the sensor for the secondary air system and adjust your cold start fuel map to exclude the cold-start cycle which is a much more elegant and torough solution compared to the old way.

    Now everyone should note there are 2 types of Carbon Build UP
    1) Secondary Air System Carbon Build Up (found in the Secondary Air System passages which is inconsequential)
    2) Combustion Chamber Carbon Build up (found on your pistons and valves)

    with the exception of a few cars (likely tuned or driven very aggressively for a majority of the time) will ALL experience BOTH types of Carbon Build Up. It is a result of factory tuning, driving style and engineering drawbacks.

    Marc while the clogged secondary air system does not affect performance, remeber that your car still attempts to run the "cold-start" cycle everytime the car is started from cold, it does not compensate for the blocked passages. So the ignition timing and fuel ratios are those of a car with free-flowing passages. I suspect it will take some time, but the enriched fuel:air mixture could eventually hurt your catalytic converters over the next 100,000miles. This is dependant on how many cold starts, ambient temp, driving habits, etc.

    My personal take on it, if you care for it, is to simply delete the system once I get my first SES for Secondary Air System and purchase the cold-start delete tuning.

    Secondary Air System Pump for Z8

  6. #6
    Hi JohnAnthony,

    I think every internal combustion engine is subject to some level of combustion chamber "carbon buildup" depending on a variety of variables such as engine design, engineering tolerances, fuel quality, driving style, maintenance, etc. Some early examples of the S62 motor as used in the E39 M5 and the Z8 (particularly pre-2001) were known to consume copious amounts of oil, which might have in some cases caused problematic combustion chamber carbon buildup, but at the risk of misinterpreting your comments I don't think it would be accurate to say that ALL S62 motors are subject to problematic carbon buildup. The S62 has its known issues (VANOS being the biggest), but there are many, many examples of these motors going 200,000+ miles without major mechanical issues. Carbon buildup in the combustion chamber is not, to my awareness, a known issue in the M5 community, and the secondary air pump issue has been addressed in a manner that makes it pretty much a minor concern. Perhaps I am getting tangled up in trying to parse what is normal and acceptable versus problematic?


  7. #7
    Team Z8 Satch's Avatar
    Join Date
    Sep 2014
    Two words: Tech Ron

    Okay, it's really one word, but it's one of the few snake-oil additives that truly works. If you don't use Chevron, buy Techron anyway. Unless you have a MODERN BMW with direct injection, in which case the Techron won't reach the intake valves!

  8. #8
    Team Z8
    Join Date
    Jul 2003
    Funny coincidence...I just added that when filling up yesterday...100 miles now and no SES light.

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