Exhaust Gas Recirculation (EGR)

This system is used to reduce the level of oxides of nitrogen emitted. By venting a regulated amount of exhaust gas to the intake manifold, the formation of NOx is inhibited during the combustion cycle. The EGR system uses a vacuum operated valve to control exhaust gas flow.
The diesel engine EGR control system allows the EGR valve to operate only when engine is at normal operating temperature, and provides maximum EGR at idle, with the amount of recirculation progressively decreased as engine speed increases. EGR valve vacuum is controlled by an electronically operated Vacuum Switching Valve (VSV) and a Vacuum Regulator Valve (VRV) which is operated by the injection pump lever. The VRV regulates vacuum pump output, providing maximum output when the pump lever is in the idle position and progressively reducing the amount of vacuum applied to the EGR valve as the lever is moved toward the full throttle position. The VSV opens and closes the vacuum supply line to the EGR valve in response to coolant temperature sensor, idle switch and control unit signals.
Voltage is applied to the VSV through the control unit. The VSV ground circuit is completed through the coolant temperature sensor and idle switch and full vacuum pump output is applied to the EGR valve when coolant temperature is above 78°C (172°F) and the injection pump lever is in the idle position. When the pump lever is moved out of the idle position, the idle switch opens, the VSV is de-energized and vacuum to the EGR valve is blocked. However, when engine speed is 1000 - 2800 RPM, the VSV ground circuit is completed through the control module, the VSV is energized and vacuum regulated by the VRV is applied to the EGR valve.