Abstract: Ambient inlet air and preheated inlet air are supplied to an internal combustion engine in proportions controlled by a proportioning valve to determine the temperature of the resultant inlet air mixture. Means responsive to the inlet air temperature downstream of the proportioning valve cooperate with an engine vacuum source to apply a temperature modulated motor actuating pressure to a vacuum motor, which in turn operates the proportioning valve to increase the proportion of ambient air as the inlet air mixture approaches a predetermined engine operating temperature. A restricted temperature responsive one-way check valve means retards the rate of flow of motor actuating pressure to the motor when the inlet air temperature downstream of the proportioning valve is above a predetermined minimum, prevents said flow entirely when the inlet air temperature is below said predetermined minimum, and enables the reverse flow of pressure freely from the motor regardless of temperature.