Thermostat is necessary
The thermostat in MBZ does 2 things at once--opens the passage to the radiator, and CLOSES the recirculation passage.
Without the proper thermostat, coolant will just recirculate back to the water pump without ever going through the radiator.
Pure distilled water and Redline Water Wetter has been well recomended.
Also, run a lower concentration antifreeze mix 30% max. It cools better.