An exhaust manifold heat valve forces hot exhaust gases to flow into the intake manifold to aid cold weather starting. A butterfly valve may be located in the outlet of the exhaust manifold. A heat sensitive spring or a vacuum diaphragm and temperature sensing vacuum switch may operate the valve.
When the engine is cold, the heat valve is closed. This increases EXHAUST BACK PRESSURE. Hot gases blow into an exhaust passage in the intake manifold. This warms the floor of the intake manifold to haste fuel vaporization. The heat valve opens as the engine warms up. The valve can be actuated by a bimetallic spring or by a vacuum motor (diaphragm unit). In vacuum motor operated heat control valve, a thermal vacuum switch is used to apply enough engine vacuum to match engine temperature. When the engine is cold, full vacuum is applied. The valve is pulled closed. As engine warms, the vacuum signal weakens. When the engine reaches operating temperature, no vacuum is applied. The valve is kept open by spring pressure.
The heat control valve must be checked periodically t make certain the valve is free.
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