N55B30

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 weeping down the back of the block, whistling from the engine at idle, rough idle, high crankcase vacuum pulling oil seals. The crankcase ventilation diaphragm is integrated into the plastic valve cover, so the whole cover is the repair.

Check: Listen for a whistle at idle with the oil cap on, then remove the cap, a strong change in idle points to the PCV. Look for oil residue at the rear of the head.

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.

Rough running, timing codes, occasional limp mode, tick from the top of the engine. Solenoids clog with oil varnish; the Valvetronic motor wears.

Check: Scan for VANOS and Valvetronic codes including pending ones. Listen for ticking at idle once warm.