OM651 2.1 CDI

Hard starting, rattle at idle, smoke, poor running and injector codes; sometimes fuel dilution in the oil. First-generation piezo injectors fail and can leak into the cylinder. Later production moved to a revised solenoid injector.

Check: Cold start and listen for an uneven rattle; scan for injector correction values, which a specialist can read and interpret. Ask which injectors are fitted. A receipt naming the revised part is worth real money on these cars.

Later parts: Revised injectors on later production

A rattle or whirr from the front of the engine for two or three seconds on cold start; at worst the chain jumps teeth and bends valves. The early oil-fed tensioner and camshaft sprocket design on pre-2012 OM651 engines wears; Mercedes revised the sprockets, tensioner and cover for later cars. Extended oil intervals and low oil level make it worse.

Check: Listen at a genuinely cold start on any pre-2012 car for a front-of-engine rattle, and favour a car with the revised sprockets or documented chain work.

Later parts: Revised sprockets, tensioner and cover from around 2012

Warning light, limp mode, oil level rising above the maximum mark from fuel dilution. The filter needs sustained hot running to regenerate. A car used for short urban trips never completes a regeneration cycle.

Check: Read the DPF soot load and the distance since the last regeneration on a scan tool, and check the engine oil level and smell it for diesel. Ask what the car was used for. A short-trip city diesel is the wrong car regardless of mileage.

Coolant loss with no visible external leak, white smoke, sweet smell. The EGR cooler cracks internally and leaks coolant into the intake. BMW recalled affected diesels over a fire risk from glycol deposits igniting.

Check: Check coolant level and look for unexplained top-ups in the service history. Verify the recall was carried out. It's free and searchable by VIN.