The brief
The GLA180 had a sweet smell coming off the engine bay after a drive, the temperature gauge had started climbing in slow traffic, and the owner could hear a hiss after switching the engine off. That hiss after shutdown is the classic one: pressurised coolant escaping somewhere it shouldn't, the system bleeding itself down through a crack.
The expansion tank is the plastic reservoir that holds spare coolant and lets the system breathe as it heats and cools. It's under pressure when the engine's hot, and the plastic gets brittle with age and heat, so a hairline crack at a stress point lets coolant and pressure out. The hoses feeding it harden and split for the same reason. Either way the system can't hold pressure, and the engine runs hot.
The diagnosis
A pressure test put numbers on it: the system wouldn't hold pressure, and there were two fault points. The plastic expansion tank had a hairline crack near one of its mounting points, and a coolant hose feeding the tank was starting to split along a fold where it flexes.
Replacing only the tank and leaving an old, splitting hose, or the other way round, would just mean the car back here in a few weeks. So it went down as a combined job: the tank, its cap, and the failing hose, all together.
The work
The cooling system was drained, the cracked expansion tank and the failing hose came off, and a new Mercedes-spec tank went on with a fresh cap, plus a new hose on proper clamps.
Then it was refilled with the correct coolant, bled the proper way so no air pockets were left, and held under pressure to confirm the new tank, cap and hose were all sealed.
A run afterwards, including some slow traffic, confirmed the gauge stayed steady, there was no hiss after shutdown, and nothing was weeping.
The outcome
No more sweet smell off the bay, no hiss when you switch off, the gauge sits steady in traffic, and the coolant level holds.
The GLA180 went home with the cooling system tight again. Doing the tank, cap and hose as a set means the brittle, age-related parts of that circuit have all been refreshed at once, not picked off one failure at a time.