The brief
Mr Benny brought his B180 in for a routine service, and during the inspection the team spotted a trace of coolant weeping at the expansion tank, caught before it became a real leak. He had it sorted there and then, which is exactly what the service inspection is for, catching it before it gets worse. The expansion tank is the plastic reservoir that holds the spare coolant and lets the system breathe as it heats and cools, and it sits under pressure and heat cycles its whole life. The plastic gets brittle with age and cracks, often at a seam or around the cap neck or the level sensor, and it starts to weep a little coolant. A cracked tank only splits worse, so it needs replacing.
The diagnosis
A pressure test on the cooling system confirmed it, the expansion tank was weeping from a hairline crack and starting to lose pressure, caught early. The radiator, the hoses, the water pump and the rest of the system held fine. That's a tank replacement, with a fresh cap, rather than waiting for a crack in brittle plastic to spread.
The work
The cooling system was drained enough to get at the tank, the old cracked expansion tank removed, and a new genuine Mercedes-spec tank fitted with a fresh cap and the level sensor transferred over properly. 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 and the level stayed put.
The outcome
No coolant weep, the level holding between checks, the gauge steady, and the system holding pressure as it should. The B180 went home with the leak caught and sorted. A cracked expansion tank only splits further, and the failure at the end is a sudden coolant dump and an overheat, so catching it on the service inspection and changing the tank kept it to a tidy, planned job.
