A one-way throttling valve for use in the vacuum line of a pneumatic membrane actuator which controls the combustion air temperature of a carburetor-euipped internal combustion engine by means of an air flow proportioning flapper admitting cold raw air and/or preheated raw air into a duct junction upstream of the intake filter, the throttling valve having a tiny throttling orifice which remains open, while the throttling valve itself is closed, thereby delaying the removal of negative pressure from the pneumatic actuator. The one-way throttling valve may be arranged in one of the two vacuum line connectors of the valve housing of a thermostat relief valve.