Just because an engine comes in a premium vehicle from a well-known brand doesn't mean there can't be problems with the powerplant. As a case in point, I Do Cars tears apart the Mercedes-Benz M278 twin-turbo 4.7-liter V8 from an ML550 and finds severe issues inside the cylinders.
Things don't start off well. Turning the engine reveals some weird clunking sounds that shouldn't be present. Also, some of the spark plugs show lots of deposits on them. The intake ports present lots of carbon on the valves – even more than usual for a powerplant with a direct-injection fuel system. Also, one of the timing chain tensioners is loose, suggesting someone else worked on this mill in the past.
As more parts come off, it becomes clear that the oil in this engine is nasty. The filter is full of fine pieces of metal, hinting at the carnage to come. The problems reveal themselves as soon as the cylinders come off. There are deep scores in several of the cylinders. Eric from I Do Cars notes these scrapes aren't just deep enough to feel with a fingernail but to catch with your hand. As you'd expect, the piston skirts also show lots of wear.
Mercedes used the M278 engine in various models, including some E-, S-, CL-, CLS-, ML-, and GL-Series offerings. According to Eric, the issues with engine bore scoring are also present in the AMG variant of this powerplant that goes by the internal moniker M157. The M276 V6 isn't immune from this problem, either.
Unfortunately, Eric doesn't offer a cause for the bore-scoring issue. Folks on various Mercedes-Benz-focused forums also report this problem, but an exact reason is hard to pinpoint. Some folks say there's too much time between recommended oil changes. Others claim it's an issue with the piston rings. Meanwhile, others are having no problems.
