Gas or Diesel Engine Runs Too Hot or Too Cold

Too Cold think thermostat - too Hot think a number of possible causes

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:

Classic Sedans
W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe

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.