The H2O flows through the whole engine, radiator and heater unit. When the engine is cold the water only flushes to the heater unit (when you need warm air), and not through the radiator. If the temperature rises to about 80-85 degrees Celsius, the thermostat opens smooth. At this point the engine will keep the operation temperature.
The only function of the water pump is to transport the hot (engine) water to the radiator and the heater unit in the car. When it is bad (broken fan inside) no water will be transported anymore and the temperature gauge in the cluster goes into the red zone.
BMW '96 M3 (SOLD) June 2005 Car Of The Month!
Originally Posted by Torque
I was just dropped off here by aliens one day. They got bored with me.