If your engine is not coming up to temperature (running too cold) it is almost always the thermostat. If replacing the thermostat does not fix the problem then look for a possible sending unit, wire or gauge problem.
Problem & Solution
Common among these chassis:
If your engine has a tendency to overheat finding the problem can be a little more challenging. Engine overheating is a fairly common problem on older cars. On some engines a number of factors can come into play to make the coolant run hotter than normal. Here is a list of causes listed in the order of most to least common:
Failed thermostat (actually stuck closed or not opening all the way)
Blocked radiator
Corroded passages inside block and cylinder head
Poorly sealing radiator cap (causing fluid loss)
Cracked cylinder head
Head gasket leak
Water pump belt slipping
Water pump impeller malfunction
Inaccurate temperature gauge
To find the problem quickly I would recommend you take your car to a competent radiator repair shop. They do not need to be Mercedes specialists. They should be able to do some inexpensive tests that will isolate the problem. Once you find out what the problem is then you can proceed to repair it yourself or find a reputable shop that specializes in the problem.
Some models such as the 190 2.6 tend to run hot. If you operated in a hot climate where you have to run the air conditioning a lot, I would recommend that you use Water Wetter to help with your cooling issues. This product helps increase the efficiency of your coolant.
