The brief
The X3 had been losing coolant slowly, the expansion tank dropping faster than evaporation could explain, and the owner had let it ride for a couple of weeks until an overheating warning came up on a longer motorway run. He brought it in straight after.
The water pump is what circulates coolant through the engine and radiator to carry the heat away. It spins on a shaft sealed against the coolant, and when that seal wears it weeps. The slow drop in the tank is the leak; the overheating warning at speed is the pump no longer moving coolant the way it should under load. A cooling system that is both leaking and not flowing properly is not something to keep driving on.
The diagnosis
A pressure test on the cooling system traced the leak to the water pump housing, and there was a pool of coolant collecting on the undertray right below it, the dripping made obvious.
With the leak at the pump and the overheating warning showing it was no longer doing its job at speed, a partial fix was a gamble not worth taking. The right call was to replace the pump and its housing as a set, so the whole assembly was fresh and there was no old seal or gasket left next to a new one.
The work
The cooling system was drained, the drive belt section released, and the failed water pump and housing taken off. New BMW-spec parts went on with fresh seals and gaskets.
Then the system was refilled with the correct coolant and bled of trapped air, which matters on this engine, an air pocket left in there can cause the very overheating you just fixed. Pressure was held on the system to confirm it was sealed, and the pump checked working through a warm-up cycle with the bonnet open.
The outcome
No drips, the coolant level holding cleanly, and the temperature gauge steady through traffic and on a long road test. No warning coming back.
The X3 went home with the cooling system back to spec and the overheating risk closed off. For the owner that is real reassurance in stop-start driving, which is exactly when a tired cooling system gives up. And replacing the pump while it was still a planned job, before it failed outright at speed, kept this from becoming an overheated engine, the kind of damage that costs many times more than a pump.
