M156 head-bolt failure — the defining known weak point

The single most-discussed R63 issue is the M156 head-bolt problem, a corrosion/coolant-loss failure the community says affected all M156 engines built before early 2011 and is documented in a Mercedes TSB. It typically announces itself via a check-engine light and low-coolant lamp, and proactively replacing the bolts (or studs) before coolant loss is the accepted preventive fix. …