BMW N43 / N45 / N46

BMW's naturally aspirated four of the late 2000s, 1.6 and 2.0 litres, in the E87 1 Series, E90 3 Series, E60 5 Series, E83 X3, E84 X1 and E85 Z4.

Which one is fitted decides the whole diagnosis, and the numbers do not make it obvious. The N46 is port-injected and the N43 is direct-injected with a lean-burn stratified mode — a much more complicated engine that shares its number series and very little else of its fuelling.

The N43 is the one with the reputation, and it is earned. Its injectors fail, and it runs rough, misfires and triggers the engine light in a pattern that is often chased through coils and plugs first. It also carries NOx sensors for the lean-burn mode, which fail on their own and are expensive, and the stratified mode itself deposits carbon on the intake valves that the port-injected N46 does not accumulate. Nothing written about an N46's fuelling applies to an N43.

What both share is Valvetronic — throttling on valve lift rather than a plate — where the eccentric shaft sensor and the actuator motor fail and leave the engine on limp-home throttle with a marked loss of response. Both also share the timing chain, which is at the gearbox end, and a crankcase ventilation system that blocks and pushes oil past the seals.