Mercedes-Benz R 350 CDI

The best-known problem of the OM642: the oil cooler gaskets in the V-section of the engine fail. Oil leaks into the coolant or externally. Particularly prone on vehicles built up to 2009.

Symptoms: Oil loss warning, oil spots under the left side of the vehicle, oil streaks visible on the belt tensioner

The plastic linkages of the swirl flaps in the OM642 intake manifold break. Aluminium repair kits are available and more durable. In the event of complete failure there is a risk of debris entering the engine.

Symptoms: Engine warning light, fault codes for intake flap/swirl flap, power loss particularly at low rpm

Piezo injectors seal poorly from heat, diesel enters the engine oil. Oil loses lubrication properties, tar-like residue around injectors is the typical telltale sign.

Symptoms: Rough idle, increased fuel consumption, knocking noises, oil level rising due to diesel contamination, engine oil smells of diesel

Variable turbine geometry actuator linkage seizes due to soot deposits. Engine enters limp mode. Fault codes P0404, P2598, P2385. If runner damage occurs, turbo costs from €1,300.

Symptoms: Sudden power loss on kickdown, limp mode, black smoke, engine oil in intercooler area

The standard oil pump delivers insufficient pressure at high revs and temperatures. No warning in the cockpit. Connecting rod bearings wear unnoticed between 150,000–200,000 km, total engine failure looms.

Symptoms: Cold knocking at idle, metallic rattling during load changes, fine metal particles in oil filter, oil pressure drop only measurable late

With frequent short-trip driving the EGR system on the OM642 becomes heavily coked. Soot deposits in the intake manifold significantly reduce airflow.

Symptoms: Power loss particularly under load, poor throttle response, soot clouds, elevated fuel consumption

Thin-walled glow plugs corrode in the cylinder head and break during removal. Broken heating element requires time-consuming drilling out. Replace all 6 plugs preventively.

Symptoms: Difficult cold starting, extended pre-glow time required, glow indicator flashes, white smoke at cold start in winter

The Airmatic air spring bellows develop leaks and cause the vehicle to sink. The vehicle sits lower on one side when parked. Arnott replacement bellows approximately €200; workshop costs up to €1,900.

The 7G-Tronic gearbox does not pair well with diesel models; gearbox calibration is rough. At higher mileages torque converter defects occur; 3rd and 4th gear clutches burn out. Gearbox oil flush every 60,000 km recommended.

The Airmatic air suspension compressor wears at high mileage, particularly if the air spring bellows were already leaking and the compressor had to run continuously. Replacement compressor €400–700; expensive to fit.

Design-related water ingress into the boot or footwell damages connectors. Rear SAM control unit costs approximately €1,100 and fails when wet.

Rust appears on doors and black-painted chassis components. First two model years (2005–2006) particularly prone. Improved from 2008/2009 onwards. Avoid early vehicles if possible.

Doors and tailgate show rust perforation even on younger examples. Water ingress via defective seals promotes corrosion from the inside. Body shops quote €1,500–3,200.