Time to way in on this subject as I do have some 1st hand knowlege on this subject,but it was not my car that this happened to but here I go.This engine had around 20000 kms on it when it started to run rough in normal highway driving and was towed to the the dealer.It was discovered that 1 cylinder had no compression the dealer at that time determined that that the engine had been subjected to an overspeed condition when it had somewhere around 3000 kms and that BMW Canada was refusing warranty coverage because of this.
The dealer refused to tear down the engine to find the source of the problem till the customer accepted the responsiability of the labour for this.My friend went up the ladder at BMW Canada till he got hold of the person who could make the decision to tear down the motor to source the problem and 2 weeks had gone by at this point.
Once it was torn down it was determined that 1 intake valve had had contact with a piston & the valve spring was broken on that valve
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BMW at this point decided to cover this under warranty as the fault was isolated to 1 cylinder and none of the other components showed any damage caused by an overrev but they did change all 32 valve springs as a precaution..If the car had a "tune",I am quite sure BMW would not have approved any warranty coverage on this problem.My friend was was also very honest with BMW about what could have happened to cause this problem and the DME information was also sent to Germany for a complete investigation.