On my 190 the engine driven fan clutch tightens at about 100C and the two auxillary electric fans engage at high speed at about 105C, so temps up to 105C are normal - very hot weather and low speed like traffic congestion.

If you are boiling over before 125C the pressure cap may be defective. It should hold at least 15 psi. Any auto parts store should have a tester to test its pressure holding capacity.

If you have been using green antifreeze it's possible that the radiator is clogged up and its heat transfer efficiency has gone to the dogs.

The best antifreeze to use is Zerex G-05, which is the same as MB antifreeze. Dexcool is a good alternative. Green stuff can ruin your radiator in less than ten years due to the salts and silicates preciptating out and clogging the tubes.

If it's overheating on the freeway at cruise speed the thermostat may be defective.

One test you can do is to let the car idle with the AC on max and sunroof/windows open. When it gets up to 100 and you rev the engine you should hear the engine driven fan roar. At about 105 the electric fan should come on. Also, monitoring the coolant outlet with an IR gun and comparing to the gage temp reading will tell you if the sending unit is defective.

Duke