Exhaust Gas Recirculation: Description and Operation

On California vehicles, exhaust gas recirculation is regulated by the EGR Valve which is computer controlled through a solenoid. California vehicles also incorporate an Exhaust Gas Temperature Sensor within the EGR valve. The sensor is used for detecting exhaust gas recirculation flow as an input to the MPI Control Unit. If the EGR Valve should become stuck in the closed position, the sensor would not be able to reach its specified temperature, causing the MPI Control Unit to set a trouble code and illuminate the Malfunction Indicator Light on the instrument panel. The same procedures would be followed if the temperature sensor were to malfunction.




The EGR Solenoid is spring loaded open to vent port vacuum back to the air intake chamber, until the MPI Control Unit grounds the circuit. After the engine has reached operating temperatures, the MPI Control Unit will internally ground the solenoid circuit and vacuum will be routed to the EGR Valve. Before the valve can open, additional vacuum is needed. The additional vacuum is supplied once the throttle is past the idle position; then exhaust gas is allowed to recirculate. When the throttle is fully opened (WOT.), the lack of vacuum to the EGR Valve causes the EGR Valve spring to close the valve. The MPI Control Unit will cycle the EGR Solenoid ON time based on engine RPM and Engine load.