BMW 520d
First common-rail diesel with unit injector technology. Solid basic design; weaknesses are the intake manifold (swirl flaps) and injectors at high mileage.
Economy diesel
Frugal yes, fun no. The engine keeps up with traffic, but every overtake needs a run-up.
Engine Weaknesses 7
The timing chain can stretch or the chain tensioner can fail. Compared to the N47 the M47 is considered more robust, but timing chain problems occur at high mileages and with neglected maintenance.
Symptoms: Rattling on cold start from the front of the engine, easing after warm-up. With heavy stretching, engine damage through chain jump.
The turbocharger can fail through oil feed line deposits or inadequate lubrication. Especially with infrequent oil changes or engine shutdown without cooldown.
Symptoms: Whistling noises, power loss, blue exhaust clouds, oil in intercooler.
The swirl flaps in the M47D20 intake manifold are secured with 2x M3 screws that work loose through engine vibration. Detached metal parts are sucked into the combustion chamber and cause total engine failure.
Symptoms: Delayed throttle response 1–2 seconds, rough idle, sudden engine damage through foreign body in combustion chamber
The exhaust gas recirculation valve carbons up over time and no longer opens/closes correctly. The coking accelerates especially with lots of short-trip/city driving.
Symptoms: Power loss, increased fuel consumption, rough idle, fault memory entry EGR.
From about 200,000 km the common rail injectors of the M47D20 wear and cause rough running and increased fuel consumption. Remanufactured injectors are considered unreliable.
Symptoms: Hesitation and rough idle, increased fuel consumption, poor starting behaviour, uneven injection quantity detectable on diagnosis.
The EGR valve of the M47D20 carbons up through soot deposits from the exhaust gas recirculation circuit. Power drops noticeably from 150,000 km. The intake manifold also carbons up.
Symptoms: Power loss at higher revs, jerky acceleration, increased soot emissions, EGR fault code
The O-rings of the common rail injectors of the M47D20 wear and allow fuel or exhaust gas to escape. Diesel smell in the cabin and starting problems are early warning signs.
Symptoms: Strong diesel smell with engine running at idle, rough cold start, oil-diesel mixture visible at injector holders
Vehicle Weaknesses 6
Water enters through porous door seals, collects in the sills, and can't drain. Rust forms from inside; jacking points collapse. Structurally safety-critical.
Thermostat housings, coolant pipes, and the expansion tank become brittle with age. A crack on the motorway leads to overheating within minutes.
The plastic impeller detaches from the pump shaft without the pump appearing visibly faulty. Coolant stops circulating and overheating follows quickly.
Rear integral arms wear regularly. When the ball joint is worn out the rear wheel loses lateral support. Typical problem over 100,000 km.
The steering rack develops leaks on older examples almost as a rule. Power steering fluid drips from the seals; steering feels sluggish or makes noise when turning.
The butyl tape behind the door membranes ages and lets water into the cabin. Rear footwells and the Touring boot are particularly affected. Mould and electrical damage can follow.