On a Mercedes-Benz automobile, Check the Thermostat and make sure it is operating properly, if not, replace it with a new one. Second, Check the radiator and make sure it is flowing enough coolant through the core, If it is dirty, Take it to a radiator shop and have it chemically cleaned. Third, Check the upper and lower radiator hoses to see if they are worn, If they are too soft and have small cracks on the surface, replace them with new ones.After you have determined the cause and have done all the repairs, make sure the engine coolant is the proper type for your Mercedes-Benz, and that you follow the factory specifications for coolant type and mixture.