M156 head-bolt issue on early cars

The single most-discussed reliability concern is the early M156 head-bolt problem, which the community consistently pins to roughly 2008 through early/mid-2011 engines and considers resolved on 2012-2015 facelift cars. Owners describe failure as a slow, low-single-digit-percentage event that starts as coolant seepage into the oil, monitored via oil analysis and fixed for around \$2,000 by swapping to later bolts. …