Mercedes-Benz OM639 (with the OM660, 0.8 and 1.5)

The 1.5 three-cylinder diesel of the Mitsubishi Colt and the first Smart Forfour — a Mercedes designation on an engine from the DaimlerChrysler–Mitsubishi joint platform, which is why the same engine wears OM639 in one catalogue and DI-D in another.

Establishing that is most of the value of knowing its name. The Colt and the Forfour are the same car underneath, built in the same factory, and a fault documented on one describes the other exactly — which almost nothing published about either says, because the two were sold as unrelated products by unrelated dealers.

It is a small, hard-worked diesel with the ordinary consequences: the injectors and the glow plugs are the recurring items, and the exhaust-gas recirculation valve clogs. On short journeys the emissions equipment cannot complete a regeneration, which is the duty cycle most of these cars actually saw.

Very few were built and the long-term pattern is thinly documented, which is worth stating rather than borrowing from Mercedes' four- and five-cylinder CDIs. It is not one of them.

⚑ This page declares no displacement, because it holds two. The OM660 is 799 cc and the OM639 is 1,493 cc — nearly double — and a figure, a part or a service quantity quoted for the three-cylinder Smart diesel is worth nothing until somebody says which.