Engine Thermostat
WARNING: This page does not describe the selected car, but rather 41 other vehicles, including the 2003 BMW M5, 2003 BMW M3, 2003 BMW 540i, 2003 BMW 530i, and 2003 BMW 525i. However, it is still accessible from the selected car via links, so may be relevant.
There are two types of thermostats in BMW cars, the mechanical type, and the type that is electrically heated. Both are designed to bring the temperature of the coolant up to operating temperature quickly.
When the engine is cold, the thermostat is closed; in the closed position, it sends coolant from the engine back to the engine, through a bypass; as a result, the coolant temperature rises quickly. As the coolant reaches the rated temperature, the thermostat gradually opens, to allow coolant to flow from the engine to the radiator.