A system for controlling coolant flow in an engine includes a housing where coolant from a radiator and a bypass passage are mixed, the housing enclosing: a water pump having an impeller, a heat responding element downstream from the impeller and having a piston that extends when a temperature is above a predetermined amount, a linkage connected to the piston, and a valve upstream from the impeller and connected to the linkage so extension of the piston causes the valve to open so radiator coolant flows. A bypass valve also upstream from the impeller is connected to the linkage so extension of the piston causes adjustment of opening of the bypass valve, coolant from the bypass passage flowing when the bypass valve is open. The two valves are ganged so total flow is kept constant. A thermistor accelerates the action of the piston by quickly heating the element.