Mercedes-Benz M104

Mercedes' 2.8 and 3.2 petrol straight-six — the M104 — in the W124, W140 and W202, and licence-built into the SsangYong Rexton, Kyron and Rodius.

Its SsangYong life is the reason it is worth a page here. Long after Mercedes stopped fitting it, SsangYong went on building it, so a 2000s Korean SUV can carry a 1990s German straight-six — and Mercedes parts and Mercedes diagnostic material apply directly, which almost nothing else about those vehicles does.

⚑ Its notorious fault is the wiring harness. Mercedes used biodegradable insulation on engines of this era; it becomes brittle and crumbles off the loom, producing shorts, phantom sensor faults and misfires that chase from cylinder to cylinder. A harness in that condition cannot be repaired by chasing individual faults — it is replaced, and diagnosing around it wastes days.

The head gasket and the timing chain guides are the other known items on high-mileage engines, and the chain rattle on a cold start is the usual warning.

Otherwise it is a durable, unstressed engine, which is exactly why it stayed in production for so long after the cars it was designed for.