The brief
The X3 had been overheating, the temperature gauge climbing high, with a loss of engine power as it got hot and a coolant smell under the bonnet. He stopped driving it and brought it in, which is the right call, an engine that overheats shouldn't be run on. Overheating has a few possible causes, the coolant level, the thermostat, the radiator, the fan, or the water pump, so it needs diagnosing rather than guessing. On these BMWs the water pump is electric, and when it fails it stops moving coolant properly, so the engine can't shed its heat, which is the rising temperature, and a hot engine pulls power back to protect itself. A failed water pump only stays failed, and a hot BMW engine is one stop away from a warped head.
The diagnosis
A pressure check and a look at the cooling system traced the overheating to the electric water pump, which was failing and not circulating coolant properly. The radiator, the hoses and the expansion tank checked out, the level was fine, and the fan worked, so the pump was the fault. That's a pump replacement, with the thermostat in the same circuit done at the same time, fresh seals on both, rather than being back in here when the thermostat lets go.
The work
The cooling system was drained, the failed electric 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 following the procedure for the electric pump, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady, the power was back, and there was no coolant smell.
The outcome
Gauge steady through traffic and at speed, full power, no coolant smell, and the level holding. The X3 went home with the cooling system back to spec. An electric water pump gives little warning before it quits, 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.