Abstract: In an internal combustion engine air cleaner, a thermal sensor delivers a vacuum signal to a vacuum motor which actuates a valve unit controlling admission of ambient temperature and heated air. The sensor receives induction passage vacuum and has a bimetal strip which positions an acetal resin valve ball in an air bleed to create a vacuum signal which varies inversely with changes in the induction air temperature, the bimetal strip and valve ball being enclosed within a protective metal housing which permits prompt response to changes in induction air temperature. This construction permits the induction air flow to be maintained at a predetermined temperature.