Mercedes-Benz C 200 CDI
Development of the OM611. Economical and reliable; known weakness is the crankshaft damper. Check belt tensioner.
Diesel Base Engine
85 kW CDI in the W203 — utilitarian and frugal, that's it. No fun without real power reserves.
Engine Weaknesses 6
On the OM646, all 8mm rivets in the DMF can shear. The resulting imbalance damages the clutch disc and crankshaft position sensor. Repair approximately €2,000.
Symptoms: Clutch shudder when engaging after cold start, vibrations when pulling away, loud rattling or clattering at part load
OM646 injectors wear internally through deposits; elevated return quantities cause rough running and power loss. EGR coking aggravates the problem.
Symptoms: Hard starting (especially cold), hesitation under acceleration, rough idle, dark exhaust plume
The OM646 exists in variants with and without an alternator freewheel. Belt tensioners are not interchangeable. Workshops frequently confuse the two types.
Symptoms: Belt flutter at the alternator area, squealing noises, alternator damage, belt snap
The OM646 develops rattling noises from the timing chain at higher mileage. The duplex chain is robust but the tensioner and chain guides wear out. Less common than on the OM651.
Symptoms: Quiet rattling at idle especially after cold start, noise from the rear of the engine, disappears at higher revs
The EGR valve clogs with soot deposits, especially with short-trip driving. The valve sticks open or stays closed and triggers limp mode.
Symptoms: Power loss at low rpm, rough running, engine warning light, limp mode on cold start
At higher mileages the valve cover gasket and turbocharger flange gaskets leak. Oil drips onto hot engine components and causes smoke.
Symptoms: Oil smell inside or in the engine bay, oil film on the outside of the engine, smoke, dropping oil level
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.