N53B30

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.

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.

Engine warning light, noticeably worse fuel consumption, loss of lean-burn mode. The NOx sensor in the exhaust degrades. The engine then runs permanently rich, which is drivable but expensive.

Check: Scan for NOx sensor and lean-mode codes. Ask the seller what the car returns on a run. A lean-burn four that drinks like a six has a dead sensor.

Rough idle, shaking at standstill, misfire warnings, poor cold running, sometimes limp mode. The stratified-charge petrol engines use piezo injectors that are sensitive to fuel quality and wear out. They typically fail one at a time over several years.

Check: Scan for per-cylinder misfires and check how many injectors have already been replaced. A car with all six done recently is worth a real premium over one with none.

Extended cranking, hesitation under load, limp mode. The same class of failure as other BMW direct-injection pumps of the period, at a lower rate than the N54.

Check: Cold start, then scan for fuel-pressure faults.