Abstract: In an internal combustion engine air intake temperature control system a pivotal valve is movable between two extreme positions of angular movement to open one and close the other of a cold air inlet and a warm air inlet to an engine air intake passage, said valve being biased in a direction to close said cold air inlet, and a thermo-responsive device in said intake passage includes an actuator constructed so as, in response to increasing temperature in said intake passage, to engage said valve and move it against said bias to open said cold air inlet and close said warm air inlet, said valve also being movable to open said cold air inlet, against said bias, in response to a predetermined pressure differential across said valve, and said thermo-responsive device including a stop member constructed so that, when the temperature sensed by said thermo-responsive device is below a predetermined value, the stop member is positioned so as to prevent complete closure of said warm air inlet by said valve.