Summary Normally, the air entering the air cleaner is cold. Cooler air is more dense, that is it contains more oxygen, and that should help the fuel burn more efficiently. But fuel doesn’t vaporize very well in cold air. Since an enormous amount of heat passes through the exhaust manifold, many carbureted engines capture some of it with a shroud around the exhaust manifold. This heats the incoming air to improve fuel vaporization. This heated air then enters the air cleaner. Once an engine is hot, it may no longer need the incoming air to be hot. So the amount of hot air entering the engine needs to be controlled. On vehicles with emission controls, air cleaners use a thermostatic valve to control how much hot air enters the air cleaner. When the engine is started, only heated air is used. As engine temperature rises, the valve opens, and provides more and more air at normal temperature. This ensures that the temperature of the air supplied to the engine stays fairly constant. This valve can be a simple thermostatic type too. Another way to control the amount of hot air is to use a vacuum control unit and a control valve mounted inside the air-cleaner unit. The vacuum control unit has a diaphragm attached to the control valve. When a cold engine starts, vacuum from the intake manifold moves the diaphragm. It opens the valve and hot air flows into the air cleaner. A heat-sensitive valve in the air cleaner responds to changes in air temperature. Below a certain level, it opens, letting hot air flow into the air cleaner. As the temperature rises, it slowly closes, reducing the flow of hot air and blending in cooler air.