Abstract: A thermally responsive valve for detecting the temperature of an engine of motor vehicles includes: an upper housing having an inlet port, and outlet port, and a cylindrical chamber; a lower housing including thermal expansion member in a thermal transmitting cover; and a movable shaft piston movably disposed within an elongated aperture in the lower housing. The movable shaft piston is operatively connected at one end to the cylindrical chamber for operatively closing or opening the inlet port, and at the other end to the thermal expansion member through a diaphragm for moving the movable shaft piston upward or downward responsive to engine temperature.