98 the plunger is biased by a spring 100 to keep holding magnet 94 in close proximity to switch 92 actuator 98 is connected to the vacuum advance or to an intake source of vacuum on the engine when engine speed is reduced the vacuum pull increases and the actuator 98 retracts plunger 96 and magnet 94 allowing switch 92 to open and de energize solenoid 82 this closes valve 78 and shuts off the water supply to the boiler pack a flasher 168 starts spasmodic operation of solenoid 82 if high vacuum persists longer than a predetermined time for the boiler feed to be shut off when the engine speed picks up vacuum is reduced and actuator 98 releases plunger 96 so that magnet 94 can close switch 92 control valve 78 is then again responsive to control by switch 84 pump 74 is driven by a motor 102 which is controlled by a temperature responsive speed control 104 the basic arrangement being well known a thermal sensor 106 is installed in the head water jacket next to outlet 158 to detect cylinder temperature and the speed control 104 adjusts the water flow rate accordingly to maintain proper heat balance.