The brief
The C180 had been losing coolant slowly. The owner had been topping it up every couple of weeks, then started seeing a small puddle under the front of the car after parking and a faint sweet smell. He brought it in before a slow leak turned into an overheat.
The cooling system runs at pressure when the engine's hot, so any weak point in it eventually gives. On this engine the coolant runs through rigid plastic pipes across the front of the engine, and after years of heat cycling the plastic gets brittle and a pipe can crack or perish at a joint. Once it does, coolant escapes faster than the system can hold, the level drops, and the engine starts running hotter than it should.
The diagnosis
On the lift, with the cooling system pressurised, the leak was traced to one of the coolant pipes at the front of the engine, the plastic perished and weeping at a joint, with the green coolant residue built up around it. The other pipes, the hoses, the radiator, the water pump and the expansion tank all held and checked clean.
A perished plastic pipe isn't something you patch. It gets replaced. So the diagnosis was a clean one: a single coolant pipe, swapped as a unit.
The work
The cooling system was drained, the failed coolant pipe came off, and a new Mercedes-spec pipe went on with fresh seals at its joints.
Then the system was refilled with the correct coolant, bled the proper way so no air was left to cause hot spots, and held under pressure to confirm the new pipe and every joint were sealed with no further weep.
A run afterwards, including some slow traffic, confirmed the gauge stayed steady and nothing was dripping underneath.
The outcome
No more drips after a road test, the coolant level holds steady on the expansion tank, and there's no sweet smell.
The C180 went home with the cooling system sealed and back to spec. A weeping coolant pipe caught early, while it's still just a slow loss, is the cheap version of the story. Run it low and let the engine overheat and you're into head-gasket territory instead, which is a very different bill.
