SteveF has it right…With all engine and cooling system designs, when you shut off the engine, you also shut off the water pump. Coolant trapped in the heads can boil as it absorbs the heat in the metal engine parts with no way of transporting this heat to the radiator. Running the fan for a few minutes can prevent this from turning into a boil-over and coolant loss…Car owners need to realize that 65-70% of the energy they buy in the form of gasoline is converted into waste heat by their cars engine…Car engines make a much better furnace than they do a producer of torque…