The brief
The 216d was losing coolant slowly. A faint whining was coming from the front of the engine. And the owner had topped the expansion tank up twice in a fortnight.
Three signs pointing at the same component. The water pump on this engine has a shaft seal that holds coolant back while the pump turns, and a bearing that supports the shaft. When the seal weeps, coolant escapes and the level drops, hence the top-ups. When the bearing wears, it whines. Two failures in one part, and both were showing.
The diagnosis
We pressure-tested the cooling system. The pump shaft seal was weeping, with white crystalline deposits on the housing where coolant had dried onto it over time, which is a clear sign it has been going for a while. And the whining noise matched bearing roughness when the pump was checked by hand.
So both the seal and the bearing were on the way out. When a pump is failing on two fronts, you replace it. And since getting at the pump means opening up the cooling system anyway, the thermostat went in at the same time, a fresh unit while the access was already paid for, rather than leaving an ageing thermostat to fail next.
The work
Released the system pressure, drained the coolant into a clean catch, and removed the auxiliary belt to clear access. Dropped the failed water pump off the engine.
Fitted a new BMW-spec pump with a fresh gasket, and a new thermostat with a fresh seal. Refilled with the correct coolant mix at the right ratio, then ran the bleed cycle to clear the air pockets a modern cooling system traps after a refill.
The outcome
No weep at the pump housing. No whining at the front. The coolant level holding over the test cycle.
The 216d went home with the cooling system reading sealed. For the owner, that means a car that holds its coolant and runs at the right temperature, with no more topping up every few days.
And catching a weeping pump at this stage keeps the engine clear of the overheating risk that comes if a coolant leak runs until the level drops too far.