The brief
The 216d had coolant leaking near the water pump housing, with visible puddles under it after parking, the coolant level dropping, and an overheat warning coming up on the dash. He brought it in before the engine actually cooked. That trail points straight at the water pump. The pump pushes coolant around the engine and through the radiator to carry the heat away, and it spins on a shaft that runs through a seal. When that seal wears, coolant weeps out past it, which is the puddle and the dropping level, and a pump that's failing also stops moving coolant properly, which is the rising temperature and the warning. An engine that loses its coolant flow runs hot fast, and a diesel run hot can crack a head, so it isn't something to keep driving on.
The diagnosis
A pressure check traced the leak to the water pump, weeping from its shaft seal, with the hoses, the radiator and the expansion tank all checking out clean. That's a pump replacement, not a re-seal. The seal is part of the pump assembly, and a pump that's started weeping only weeps more, so the unit gets changed, with the thermostat as the matching part in the same circuit so it isn't back in here soon.
The work
The cooling system was drained, the failed water pump and the thermostat removed, and new genuine BMW-spec parts fitted with fresh seals. The system was refilled with the correct coolant, the air bled out the proper way, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady and there was nothing dripping.
The outcome
Gauge steady through traffic and at speed, no coolant traces, the level holding, and no overheat warnings. The 216d went home with the cooling system back to spec. A weeping water pump only gets worse, and the failure at the end is an overheat that can cost a head gasket, so changing the pump and the thermostat together put the whole circuit right in one visit.