Join Date: Sep 2004
Location: Northern California
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Re: FAQ on Carbon Build-Up Issue (First Attempt)
The issue of carbon buildup in the entire head and cylinders compared to the issue that many people are seeing with the SES light coming on indicating a secondary air fault are similar yet two entirely different animals. There have been a couple posts with people who has had issues with the car suddenly running rough and the diagnoses being carbon buildup in the combustion chamber and exhaust ports causing the engine to run terribly - this seems to be a like issue but one that is much more inhibitive to the normal driving operation of the car. If there is excessive rich combustion during warm operation or significant oil blowby, the exhaust cylinder, piston, and exhaust ports become coated with heavy carbon creating an inefficient combustion and creating a rough running engine with predetonation issues, compression issues from a ring and valve seat contamination. Effectively the cleaning process is just as intense should you begin to have an engine that runs rough and throws codes for cylinder head temperatures, predetonation, etc.
In my case and several other cases the issue seems to be with the channels that allow the secondary air system to include additional oxygen into the exhaust ports during a cold start to provide the ability for the raw fuel to continue burning in the absence of engine heat. The SES light comes on when the computer recognizes a low pump pressure into the cylinder heads - sometimes it can be a bad pump, in my case it is the restriction created by the building of carbon over time. Permanent SES light unless the carbon is removed. The engine is going to run its most rich when it is cold and the idea I have about this issue is a flawed design that is trapping carbon during rich operation or during the ludicrous 1 ltr / 400-1000 miles that BMW claims was normal DURING the factory warranty period. I am one of those people who pushes about a litre every 1200 miles or so and it is my assumption that this, included with a poor exhaust porting design has lead to to this problem and will continue to cause issue with other M5's as they stack the miles on.
This issue has nothing to do about driving the car soft - it is bad design of the port channels / system that is particularly vulnerable to carbon contamination during cold start. I also feel that the cop-out of BMW to develop such a ludicrous standard of oil consumption normalcy has created additional carbon to buildup on the exhaust port system during hard driving (I use a heck of a lot more oil during race / high RPM driving then low RPM's.) This problem is not with driving the car soft - I am so tired of hearing that.
My extended warranty company has denied my claim for the $8K repair as they sight that buildup of carbon, varnish, etc is not a direct failure simply preventative maintenance an is not covered. There have been several people in here who have had the BMW CPO deny coverage as well which is downright crazy!
I am going to complete the Auto RX treatment for the next couple of months and hope my carbon issue does not suddenly result in an operation issue until the treatment can start dispersing the carbon. Who knows, the carbon might be so stacked up in the head it many be too late for a non-invasive procedure.