Mercedes-Benz M276 (3.0 and 3.5 V6)

The 60° V6 that replaced the M272, as a 3.5 naturally aspirated and a 3.0 biturbo, and was itself replaced by the straight-six M256. Direct injection and multi-spark ignition, and about a quarter less fuel than the engine before it.

Its known complaint is a rattle on cold start, and the cause is oil that has drained away rather than a chain that has stretched. On early engines the oil leaves the front timing covers when the car is parked, so the chain tensioners are empty for the first seconds of running; Mercedes' answer was check valves in the tensioner supply openings on both cylinder heads. A rattle that stops within a second or two of starting is this, and it is a repair rather than a warning of failure — but ignoring it lets the chain and the plastic balance-shaft sprocket wear for real.

Do not confuse it with the M272's balance-shaft failure, which is a different engine and a different fault: that one was a gear that wore out in normal service and was tracked by VIN. Here the sprocket wears when oil changes are stretched past about ten thousand miles or thin oil is used, which makes it a maintenance-history question first.

The oil separator and carbon on the intake valves account for most of the rest — the latter being the tax every direct-injection petrol engine pays, since no fuel washes the back of the valve.

⚑ It is sold at two sizes. Our rows carry 3.0 and 3.5 litres across eight models, and the 3.0 is twin-turbocharged where the 3.5 is not — so the two behave differently and fail differently, and a symptom read across them is read across two engines.