conditions as the system is completely filled with coolant very little heat can be removed from the engine and the coolant quickly warms before reaching a predetermined temperature for example 35 u00b0 c any air in the system such as that dissolved in the coolant per se tends to be forced out of solution by the heating and rise to collect in the riser portion 170 at this time as the level of coolant falls below that of the level sensor 168 the control circuit energizes the electromagnetic valves 160 162 amp 176 and the pump 134 this energization may be continued for a predetermined short period of time e g three or four seconds after the level sensor 168 indicates the level having risen thereto this procedure closes valve 160 moves valve 162 to the position wherein communication is established between the pump 134 and the reservoir 154 and opens the overflow conduit 174 via opening of the third valve 176 accordingly the pump 134 draws coolant from the reservoir 154 via conduit 158 and forces same into the system overfilling same the excess coolant displaces the air or other non condensible matter out through the overflow conduit 174 upon the previously mentioned predetermined temperature being exceeded this 34 purge 34 mode is terminated and the valves 160 162 amp 176 and pump 134 are de energized subsequently the coolant temperature continues to rise and begins generating vapor.