Mercedes-Benz C 250 CDI
Widely used Mercedes diesel with aluminium block and steel liner. High mileages possible but known for injector seal issues, timing chain stretch and EGR problems.
Strong CDI 250
150 kW CDI in the W204 — the most comfortable diesel variant. Adequate power, solid economy, pleasant touring machine.
Engine Weaknesses 6
Over 100,000 vehicles recalled: the chain tensioner seal leaks, allowing engine oil to drip onto the exhaust system and potentially ignite.
Symptoms: Dropping oil level, oil smell from engine bay, oily deposits under the vehicle, possible oil fire
The OM651 uses a simplex timing chain. With short-trip driving and overdue oil change intervals the chain stretches.
Symptoms: Light rattling noise on cold start from the rear of the engine, no fault code stored
Same Delphi piezo injector problems as in the DE18: 220 CDI and 250 CDI variants affected. Mercedes replaced them with solenoid injectors as part of a service action.
Symptoms: Vibrations and rough running especially under acceleration, elevated fuel consumption
The EGR cooler of the OM651 develops hairline cracks at the connection stub and loses coolant. Green staining next to the fuel filter under the engine cover is typical.
Symptoms: Dropping coolant level without visible leak, intermittent coolant warning, green spots under the vehicle, heater output diminishing
The DMF on the OM651 suffers under the high torque output. Documented case: all 8mm rivets sheared at 149,000 km, with consequential damage to the clutch and crankshaft speed sensor.
Symptoms: Rattling and juddering at warm idle, vibrations when pulling away, shuddering at low RPM, noise increases with warm engine
From around 150,000–180,000 km the OM651 turbocharger leaks oil through worn bearings, which drips onto the EKAS intake port deactivation actuator and destroys it. Double damage.
Symptoms: Black smoke under acceleration, oil residue in charge air hose, power loss, fault code P0299 boost pressure underrun
Vehicle Weaknesses 7
Rear subframe rusts from the inside out. Problem not immediately visible externally. In severe cases the rear differential can shift.
Brake lines corrode relatively quickly and are flagged at MOT. Affected two to several times more frequently than the vehicle average.
W204 rear subframes systematically rust through from the inside. Powder coat cracks and moisture penetrates. Mercedes replaces under goodwill (15 years from first registration). In extreme cases the axle breaks.
Brake lines above the rear axle rust through after approximately 8–10 years. Brake fluid loss and brake failure possible. Mercedes goodwill arrangement known up to 8 years.
Pre-facelift models (up to 2011) show clunking front axles from defective shock absorbers, strut top mounts and wishbones. Improved after facelift.
Xenon headlight levelling actuators fail frequently. Error message and dazzling of oncoming traffic.
Shock absorbers and strut mounts of the front axle wear prematurely on the W204. Rumbling and wallowing typical from 100,000 km.