Also known as: om629, 4.0 v8 cdi, 420 cdi, 400 cdi, v8 diesel mercedes, om628
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.
Cars fitted with this engine
4 generations, across 1 make. From the authored catalogue — an absent car means the catalogue has not reached it, not that the engine was never fitted.
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Posted about this engine
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about OM629.