Mercedes-Benz OM651

Mercedes-Benz's four-cylinder common-rail diesel family, introduced in 2008 to replace the OM646 and fitted from the C-Class through to the Sprinter van. It spans two displacements under one designation — roughly 1.8 and 2.1 litres, badged variously as 200, 220 and 250 CDI — so the badge on the boot does not identify the engine and no single capacity is recorded here.

The well-known theme is the timing chain and its sprockets, where wear produces a rattle and eventually cam-to-crank correlation faults; the chain runs at the rear of the engine on this family, which makes the repair far more involved than the part cost suggests and is the same trap the BMW N47 sets.

Early examples used piezo injectors that were superseded by solenoid units, and injector faults across the family are common enough that a rough idle or a starting complaint is worth testing at the return-flow stage early. Delete/EGR-related aftermarket modification is widespread on these engines, so establish whether the emissions hardware is as built before diagnosing an emissions complaint — a missing component reads as a sensor fault.