One often neglected task on many cars is the maintenance of the cooling system.  In general, BMW recommends that you flush and clean out your cooling system once every 36 months, or approximately every three years. A properly maintained cooling system must have a few things in order: adequate supply of coolant, a radiator that acts as a heat exchanger with the outside air, a fan or air flow source, a water pump to keep the coolant circulating, and a thermostat to regulate the engine at its optimum operating temperature.  The coolant must also have the correct mixture and chemical compounds to promote heat transfer, protect against freezing, and also inhibit corrosion.