This system holds the choke open during warmup to prevent an overly rich mixture from emitting pollutants above acceptable limits. When the coolant temperature rises above 140°F, a thermostatic vacuum switching valve allows vacuum from the manifold to pull in on the choke opener diaphragm which is connected to the choke valve. Thus the choke valve is opened sooner than if just the automatic choke was operating it. When the coolant is below 95°F, the choke opener works with the automatic choke to close the choke valve and permit the choke to close until the engine is warm. Some models use two thermosensor switches and a speed sensor to trigger a miniature computer, which in turn triggers the vacuum switching valve.