Mercedes-Benz OM642

Mercedes-Benz's V6 diesel, badged 320 CDI and later 350 CDI, and fitted to almost everything the company sold with a diesel option — saloons, SUVs, the Sprinter and the Vito. Common-rail, 72° V6, all-aluminium.

The fault it is known for is the oil cooler seals, which sit in the valley between the cylinder banks under the intake. They harden and leak, oil appears at the back of the engine, and because the leak is above the bellhousing it is routinely misread as a rear main seal or a gearbox problem. The repair is labour-heavy because the intake assembly has to come off; the part itself is trivial.

The second theme is the intake tract: swirl flaps and their linkage, and heavy carbon build-up in the manifold on short-journey cars. Both produce airflow and performance complaints with no single obvious component at fault. On higher-mileage examples the turbocharger's variable-geometry mechanism sticks, which reads as a boost fault and often frees with cleaning rather than replacement.