N47D20

Rattle from the bulkhead end of the engine on start-up and on the overrun, then chain jump and catastrophic valve damage. The timing chain sits at the rear of the block against the bulkhead. The chain and its guides wear early, and the repair requires the engine or gearbox out.

Check: Stand at the rear of the engine bay at a genuinely cold start and listen for rattle. Then demand documentation: a chain replacement with current part numbers, or walk. The repair costs a meaningful fraction of the car's value.

Later parts: Revised chain and sprockets phased in from roughly 2011; coverage was inconsistent, so verify by part number and receipt rather than by build year.

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.

Sudden loss of power on a hard pull, limp mode that clears on restart, underboost or overboost codes. Soot binds the variable vanes or the actuator linkage seizes.

Check: Do a full-throttle pull in third from 1,500 rpm to the limiter. Hesitation or a drop into limp mode is this fault. Do this twice.

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.

Often none until it happens. Rough running, then in the worst case an engine destroyed by an ingested flap. The intake manifold's swirl flaps run on spindles that wear. A detached flap can be drawn into a cylinder.

Check: Not visible without removing the manifold. Ask whether the flaps have been removed or the manifold replaced. Many well-kept examples have had this done preventively, and that's a selling point.