M57D30TU2 / M57D30TOP

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.

Oil pooling on the subframe, oil on the serpentine belt, drips on the driveway. The gasket between the filter housing and the block fails. Oil then lands on the belt, and a belt that shreds can be drawn into the front crank seal.

Check: Put the car on a lift. Check the belt itself for oil contamination. An oily belt is the version of this fault that ends badly.

Ticking that grows louder, black soot deposits around the injector bases, diesel smell in the cabin. Copper sealing washers fail and combustion gases track past the injector, eventually welding it into the head.

Check: Remove the engine cover and look at the base of each injector for black crusting. Caught early this is cheap; ignored it's a cylinder head job.

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.