I have the C200 version which is the same block with slightly different peripherals. A common weakness is the thermostat which is easily replaced. The temperature should get up to nearly 90 degrees within five miles and stay there. If it rises when in traffic and drops on the open road, change the thermostat. Sealing rings on the low pressure fuel pipes can leak. Again, an easy job to fix. A more serious problem is known as 'Black Death', although the name is the scariest bit. The seal between a fuel injector and the cylinder head can leak, allowing combustion gases to eventually form a nasty black deposit around the injector. Listen for a chuffing or ticking noise, and you may get diesel smells in the cabin. Any decent independent MB specialist is very familiar with the repair, which amounts to a good clean, possibly recutting the seat and refitting with new bolts and washers. Don't let this one go on for too long, as the black goo will harden into something resembling industrial diamond, making it a tedious job to chip off. If a glow plug fails, it may stick in the head or even snap off leaving the business end behind when removing it. Best let a good MB independent with the tools and experience do this one. In our climate, they'll easily start without glow plugs anyway. The exhaust gas recirculation (EGR) plumbing can get gunked up, a good cleaning usually sorts it. The cam chains are not known for failing, just keep to the recommended oil changes intervals and you should be fine. All the above sounds terrible when you see it written down, in practice these are some of MB's better engines and are extremely reliable. None of these faults will put the car off the road.
