Mercedes-Benz C 270 CDI
Five-cylinder diesel with common rail. Unusual cylinder count, good balance of power and economy. Prone to blocked swirl flaps.
Strong CDI Diesel
125 kW CDI diesel with automatic — adequate torque for relaxed motorway cruising. No thrills, but practical.
Engine Weaknesses 6
The most notorious problem in the OM612/OM646 CDI family: failing injector sealing rings allow combustion gases to escape. A black, tar-like mass forms around the injectors.
Symptoms: Black tar visible around injectors, hissing or knocking under load, diesel smell in interior
The EGR cooler on the OM612 can develop internal leaks and introduce coolant into the intake. This leads to white smoke, coolant loss and in the worst case hydro-lock.
Symptoms: White smoke from exhaust, coolant loss, sweet smell from exhaust, coolant in air filter housing
Cylinder head cracks can form in the OM612 between two valves, only becoming visible under load.
Symptoms: Coolant loss without visible leak, white smoke, bubbling in coolant reservoir under load.
The variable turbine geometry on the single turbocharger can suffer vane breakage. Metal fragments enter the engine; catastrophic secondary damage is possible.
Symptoms: Turbo noise changes character, power loss, oil smoke, engine warning light
In the OM612, swirl flap shafts and actuating levers break due to oil carbon deposits. Complete intake manifold replacement required.
Symptoms: Power loss under rapid acceleration, soot clouds, engine limp mode, swirl flap fault codes.
The OM612 was delivered without a diesel particulate filter. EGR system, intake system and engine bay accumulate soot deposits over time.
Symptoms: Soot clouds on acceleration, visible soot deposits in engine bay, elevated particle readings at MOT
Vehicle Weaknesses 7
Rear subframe rusts from the inside out. Critical weak point as spring mounts can break away. Pre-facelift models up to 2004 particularly affected.
W203 up to the facelift of March 2004 particularly rust-prone: wheel arches, sills, door lower edges and tailgate. Rear subframe can rust through on older examples.
Rust starts on the rear wheel arches, followed by sills, door lower edges and tailgate. Unsealed flanges encourage corrosion.
MOT regularly cites anti-roll bar mountings, suspension springs and tie rod ends. Common cause of inspection failure on older examples.
Early W203 models (before March 2004) suffer from wiring breaks in electric seats and steering column stalks. SAM module failures documented.
W203 steering jolts during steering movements or is heavy. Causes: worn control arms, track rod ends or defective power steering pump.
SAM modules on the W203 control lighting, wipers and vehicle electrics. Moisture and ageing lead to sporadic complete failure of multiple systems simultaneously.