Mercedes-Benz C 200 CDI
Development of the OM611. Economical and reliable; known weakness is the crankshaft damper. Check belt tensioner.
Older Diesel, Too Weak
90 kW OM646 in the W204 — no longer fit for purpose. The car deserves a stronger engine.
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. 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.