N20B20 2.0 turbo

A whine or rattle from the front of the engine, worst at a cold start, and eventually a timing fault code or a sudden loss of power. Early N20 engines used chain guides made of a plastic that could become brittle and crack. Once a guide breaks the chain loses control, and if it jumps or snaps the valves meet the pistons. BMW revised the guides, so the parts fitted decide the risk.

Check: Start the car from stone cold and listen for a whine or rattle from the front of the engine. Ask for receipts showing the chain and guides replaced with the revised parts, and treat an early car with no paperwork as carrying the full cost.

Later parts: Revised chain guides from later production, and fitted as a retrofit, so it applies to cars built up to 2015.

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.