in order to improve engine warm - up characteristics under cold climatic conditions and to safegard against possible unintentional overfilling of the of the cooling circuit of an evaporative type automotive cooling system wherein the coolant is permitted to boil and the vapor used as a vehicle for removing heat from the engine , the ambient temperature or a parameter which varies with the same is sensed and the control of the cooling system modified accordingly . one main feature comes in the control which is effected to reduce the surface area of the radiator of the system in which the coolant vapor is condensed is modified to avoid overfilling and possible damage due to overpressurization .