have you read the details outlined in the CPO...
warranty? Does it clearly state in the CPO warranty that carbon build up is not covered. Speak to the Service Manager and if that becomes a dead-end then tell him you want to schedule a meeting with the BMW Regional Rep. before you speak to your lawyer and have to drag this into a courtroom. If there is no specific details denying coverage written in the CPO warranty then I'd think you have a good chance at getting it fixed. Of course, if there are details in the CPO that deny the coverage then you're likely SOL.
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Re: Carbon build-up issue - how to prevent?
Is there a preventative measure a driver can do, such as regularly use Ventil Sauber or Techron?
Then ask them to ask BMW for good will repair
If I bought CPO I expect that to be warranted.
It's not spark plugs?
Carbon build-up issue is related to oil type ...
I assume that you are referring to carbon deposits on the tops of the valves. This is indeed an unfortunate side effect of direct injection. The fuel is injected directly into the cylinder, and it washes the oil down into the oil pan, which leads to oil dilution. That fact is made worse by the fact that some of the "synthetic" oils used (often BMW Castrol) is not really synthetic at all. (However, allowed to be marketed as a synthetic due to a loophole in the law). Because this oil is in fact made from Class 3 stock, it has a lower vaporization point. When vaporized, the dilution containing the oil leaves a deposit. Not just a problem for BMW...Audi and VW are having much worse problems with their DI engines. Suggestion: Change to Castrol 0-30, black container, and check to see if it says "made in Germany" on the back in small print. About $7.50 at Autozone. It is a true synthetic, made from Class 4 ingredients, (spend some time on BITOG.com for details) and it has a much higher vaporization temp. Check your used oil with someplace that will do an analysis. Much above 2% dilution and you need to change the oil. About every 2k in my 07 335i. Had my valves cleaned at 25k, now have 131k with no further problems after changing oil type. Good luck.david '07 335i Dinan S2 coupe / '94 530i / '08 Subaru Outback