N54B30

Rough idle, cold-start stumble, misfire counts on one or two cylinders, occasional fuel smell. Piezo injectors leak or lose spray pattern. BMW revised the design repeatedly. The index number on the injector tells you which generation is fitted.

Check: Scan for per-cylinder misfire counts on a warm engine. It is the only honest check. Ask which injector index is fitted. Assume a car with no injector history will need some.

Metallic rattle on light throttle at low revs, later boost faults, underboost codes and power loss. Wastegate arm bushings wear and the flapper rattles in its seat. The turbos themselves are otherwise reasonably durable.

Check: Listen at cold idle and at light part throttle from 1,500–2,500 rpm. Rattle alone isn't fatal, cars run for years with it, but it should price the car.

Rough cold idle, slow throttle response, misfires, gradual loss of low-end response. Direct injection puts no fuel over the intake valves, so crankcase vapour deposits bake on undisturbed.

Check: Not visible without a borescope. Treat it as a service item that's due unless there's a receipt. It's not a fault, it's an unbudgeted maintenance cost.

Long crank, extended cranking when warm, limp mode, misfire and hesitation under load. The direct-injection high-pressure pump loses delivery pressure and eventually fails outright. Early pumps went through several revisions.

Check: Insist on a cold start. Scan for fuel-pressure and misfire codes, and ask for a pump receipt with the part number. Revised part numbers appeared from 2010, so check the number rather than the build year.

Later parts: Revised pump part numbers from 2010 onward; not all cars received one.

Overheat warning with little notice, coolant temperature swinging, sometimes an immediate limp home. The electric pump's controller fails. It's a wear item with a service life, not a fault.

Check: Age and receipts matter more than symptoms, it usually gives no warning. Treat a car past 120,000 km with no pump history as owing €600.

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.

Sudden loss of boost, loud air rush under load, limp mode. The plastic charge pipe from the intercooler splits, usually at the coupler.

Check: Look for an aftermarket aluminium pipe already fitted. That's a good sign, not a bad one. Otherwise treat it as pending.