In order to assure that the cooling system of a so called "vapor cooled" type engine remains free from contaminating air, a control system is provided which enables air to be purge both during initial engine warm-up and during normal operation if the temperature of the coolant rises excessively. The control system further provides for the temperature of the coolant to be varied with factors such as engine speed and engine load and further provides that, before the system is switch to an "open state" when the engine is stopped, the coolant temperature is caused to rapidly fall via continued operation of a cooling fan or like device for a brief time, so as to eliminate any super atmospheric pressures which would otherwise tend to displace large quantities of the coolant out of the system.