BMW introduced electric water pumps on the E90 for efficiency. The pump motor fails much earlier than a traditional belt-driven pump. When it fails, there is zero coolant flow ? the engine can overheat in minutes.
Symptoms
Overheating warning, reduced cooling, error codes for coolant temperature, possible limp mode
Fix / Remedy
Replace with updated electric water pump. Always replace the thermostat at the same time. Bleed cooling system thoroughly.