Mercedes-Benz OM612 / OM648

Mercedes-Benz's common-rail diesels of the early 2000s — the OM612 2.7 five-cylinder and the OM648 3.2 straight-six — in the W203 C-Class, W211 E-Class, CLS and, under licence, the SsangYong Rodius.

The fault every one of these is known for is the injector seals, and its nickname tells you what it looks like. The copper washer under an injector fails, combustion gases and soot escape past it, and the black deposit builds up around the injector until it is effectively welded into the head — black death. A sooty crust at the base of an injector is not dirt, it is the diagnosis, and the longer it runs the more likely the injector has to be cut out rather than pulled.

It is why injector seals are a service item on these engines rather than a repair, and why a car with no record of them having been touched is the one to look at hardest.

The swirl flaps in the intake manifold are the other recurring item. Their linkage and actuator wear, the flaps stick, and the engine loses low-end response and sets an air-management code — and on a neglected engine a flap can be drawn in. These are otherwise very long-lived engines, so a running fault is far more often one of these two than anything internal.