The brief
The owner of this C220 had a coolant level that kept dropping with nothing obvious to show for it, and brought it in for us to find the leak. Coolant that disappears is going somewhere, and a slow leak becomes an overheat the day it lets go, which is an expensive way to find out, so it's worth pinning down early. A common coolant leak on these is the thermostat housing. The thermostat sits in a moulded plastic housing on the engine, and that plastic spends years under coolant pressure and heat cycles until it gets brittle and cracks, usually around the housing seam or a hose neck. It weeps a little, the engine heat dries some of it off so there's no big puddle, and the level keeps creeping down. A cracked plastic housing doesn't reseal, so it gets replaced, and since the thermostat lives inside it, the two come as one module.
The diagnosis
A pressure test on the cooling system pinpointed it, the plastic thermostat housing was weeping from a hairline crack and bleeding pressure slowly, which is the disappearing coolant. The radiator, the hoses, the water pump and the rest of the system held fine. That's a thermostat housing replacement, the housing and thermostat as one module with a fresh seal, rather than chasing a crack in brittle plastic that's only going to spread.
The work
The cooling system was drained enough to get at it, the old cracked thermostat housing removed, and a new genuine Mercedes-spec housing and thermostat module fitted with a fresh seal and the hose clamps renewed. The system was refilled with the correct Mercedes coolant, bled the proper way so no air pockets were left, and pressure tested again to confirm it held with no weep. A road test confirmed the gauge sat steady, warmed up at the right rate, and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, the engine warming up at the right rate, and the system holding pressure. The C220 went home with the leak resolved. A cracked plastic housing only splits further, and the failure at the end is a sudden coolant dump and an overheat, so changing the module kept it to a tidy, planned job.
