The brief
The GLC200's gauge had been creeping above its usual position on longer motorway runs, the owner could see fresh coolant traces around the thermostat housing on a quick bonnet check, and the expansion tank was dropping faster than evaporation would explain. Three pointers, all at the same area.
The thermostat controls when coolant flows through the radiator, and on this engine it sits inside a plastic housing that also carries some of the coolant pipework. The plastic gets brittle with age and heat, and a crack can open up along the moulding line, letting coolant weep out, which is the traces and the dropping level. And if the thermostat inside has lost some travel and is slow to fully open, the engine sheds heat too slowly in traffic, which is the gauge creeping up.
The diagnosis
A pressure test confirmed the leak: a hairline crack in the plastic thermostat housing, along its moulding line, the common ageing failure on this engine. The pink coolant residue was crusted around it.
And a check of the thermostat itself found it had started losing some of its travel, slow to come fully open, which was the running-hot half of the story. So it wasn't just a seal, and it wasn't just the thermostat. The right fix is the complete housing, which comes with the thermostat built in, and fresh seals.
The work
The cooling system was drained, the cracked thermostat housing came off, and a new Mercedes-spec housing went on, the thermostat integrated and fresh seals all round.
The system was refilled with the correct coolant and bled the proper way so no air was left to cause hot spots, then held under pressure to confirm the new housing and its connections were sealed.
A run afterwards, including a stretch of faster road, confirmed the gauge held steady and there were no coolant traces around the housing.
The outcome
The gauge holds steady on the motorway, there are no coolant traces around the housing, and the level holds.
The GLC200 went home with the cooling system both sealed and regulating properly. Doing it as the complete housing means the brittle plastic and the tired thermostat are both replaced in one go, so it's not a leak fixed today and a thermostat job booked for later.