Deceleration Valve: Description and Operation

Fig. 86 Throttle Return Control (TRC) system schematic:

This system, Fig. 86, is used on some vehicles. During coasting, the control valve opens to allow vacuum to operate the throttle lever actuator. The throttle actuator lever holds the throttle valve slightly open, thereby allowing more air to enter the combustion chambers to lean out the over-rich mixture and reduce hydrocarbon emissions. When manifold vacuum drops below a predetermined value, the control valve closes, allowing the throttle actuator to return the throttle valve to normal idle.