Mercedes-Benz W203: coolant loss
Mercedes-Benz W203: what usually causes coolant loss?
Common leak points are the water pump, the heater core, the thermostat housing, the auxiliary heater gasket and the heating coil of the windscreen washer reservoir. The water pump can at first leak a little without any noise, and the coolant disappears gradually.
Mercedes-Benz W203: how do you locate a coolant leak?
The leak is found with a pressure test: the cooling system is pressurised and the leak point is looked for. First check the area around the water pump and the hoses from underneath and in the engine bay. A heater core leak can show under the heater housing.
Mercedes-Benz W203: can you add a leak sealer to the cooling system, or is air the reason the coolant disappears?
A leak sealer is not worth using, because the water passages of the heater core are thin. After a coolant change or a part replacement, air can remain in the system, in which case coolant has to be topped up until the air has been removed.