Mercedes-Benz OM646

The four-cylinder diesel that preceded the OM651 — fitted to the W203 and W204 C-Class, the W211 E-Class, the Sprinter and the Vito, and generally regarded as more robust than the engine that replaced it.

The distinction matters. The OM651's rear timing chain and its expensive failure dominate any search for a Mercedes four-cylinder diesel problem, and none of that applies here — the OM646's chain arrangement is not the same and its reputation is markedly better. Confirm which engine is fitted before repeating any of it.

What the OM646 does bring is injector wear, which presents as a rough cold idle and a knocking sound, and — on higher-mileage examples — the crankshaft position sensor and the turbocharger's variable-geometry mechanism sticking with carbon. On the Sprinter and Vito in commercial use, the duty cycle rather than the mileage is what determines condition.