actuated type and operable in response to a vacuum regulated by a vacuum regulator 21 the flow control valve 20 includes a valve element 23 connected to a diaphragm 25 via a rod 24 and a spring 28 biasing the valve element 23 in a valve opening direction the valve element 23 cooperates with a seat portion 22 formed in the outlet passage 15 to open or close the outlet passage 15 the diaphragm 25 has a righthand side thereof as viewed in fig5 exposed to the coolant discharged from the water pump 17 and a lefthand side thereof as viewed in fig5 defining a vacuum chamber 26 which communicates with the vacuum regulator 21 the spring 28 is disposed in the vacuum chamber 26 and has one end bearing on the diaphragm 25 so as to bias the valve element 23 in the valve opening direction the vacuum regulator 21 comprises a pressure regulating unit 30 for keeping engine intake manifold vacuum supplied thereto at a predetermined value such as 120 mmhg and a solenoid unit 32 for modifying the vacuum on which the flow control valve 20 is operable in response to an electric signal supplied by a later described controller 31 intake manifold vacuum of the engine.