Mercedes-Benz OM629

Mercedes-Benz's 4.0 V8 diesel — the S 420 CDI, ML 420 CDI and GL 420 CDI — and the largest diesel the company put in a passenger car.

Its faults are the CDI family's, concentrated by the vee. The injector seals fail exactly as they do on the OM612 / OM648: the copper washer lets combustion gases past, soot builds into a hard crust around the injector, and the injector is eventually welded into the head. A sooty ring at the base of an injector is the diagnosis rather than dirt, and on a V8 there are eight of them in the least accessible place on the engine.

The oil cooler and its seals sit in the vee and leak, which on this engine means oil appearing at the back of the block with no obvious source, and reaching them means the intake comes off.

Very few were built, so the long-term pattern is thinly documented compared with the five- and six-cylinder CDIs — which is worth stating rather than borrowing from them. What transfers reliably is the injector-seal discipline: treat it as a service item rather than a repair.