On some gasoline engines, a valve is placed in the exhaust manifold to deflect exhaust gases toward a hot spot in the intake manifold until the engine reaches operating temperature (fig. 4-48). This valve is a flat metal plate that is the same shape as the opening that controls it. It pivots on a shaft and is operated by a thermostatic coil spring. The spring pulls the valve closed against a counterweight before warm-up. The spring expands as the engine warms up, and the counterweight pulls the valve open.