The brief
The C180 was leaving a small coolant puddle under the front of the car, the expansion tank was dropping over the week, and the temperature gauge crept high in heavy traffic. That pattern points at the radiator, particularly its plastic side tank.
The radiator is where the coolant offloads its heat, hot coolant running through a fine aluminium core in the airflow. Its end tanks are plastic, though, and over years of pressure and heat cycles they can crack along a moulding seam. The slow drop is the leak, the puddle is what reaches the floor, and the running-hot is the system no longer having quite enough coolant to keep up when airflow is low, like in stop-start traffic.
The diagnosis
A pressure test pinpointed the leak: a hairline crack at the seam between the plastic side tank and the radiator core, marked out by the crusty residue the coolant had left as it dried. The plastic had aged out and was not reliable to repair.
The right fix was a new radiator, with a full flush of the cooling circuit while it was open, since the old coolant had carried debris through and the new radiator should start clean.
The work
System pressure was released, the coolant drained, and the front trim taken off for radiator access. The failed radiator came out, a new Mercedes-spec radiator went in with fresh hose clamps, the cooling circuit was flushed clean, and the system refilled with the correct coolant mix and run through its bleed cycle on the scan tool.
Then pressure was held on the system to confirm it was sealed before the car was road-tested.
The outcome
No puddle under the car, the expansion tank holding level over the test cycle, and the temperature gauge steady even with the air-con on at idle.
The C180 went home with the cooling system sealed. For the owner that is the end of the puddle and the topping up, and replacing the radiator before it failed outright meant no overheating event, the difference between a planned cooling-system job and a much bigger engine repair.