Lesson 1/Learning Event 2
A valve is placed in the exhaust manifold that directs the flow of exhaust gases to the hot spot. A
thermostatic spring controls the operation of this valve. When the engine is cold, the valve directs gases into
the hot spot. As the engine warms up, the thermostatic spring loses its tension, and the gas pressure (aided
by a weight) turns the valve so the hot gases do not enter the manifold hot spot.
On some vehicles, the heat control valve does not have a thermostatic spring. In this case, the valve is
manually set to the OFF, INTERMEDIATE, or ON position depending on the weather and operating
Another method is to circulate the engine coolant around the manifold hot spot. This hot liquid, instead of
the exhaust gases, heats the intake manifold.