Exhaust Gas Recirculation: Description and Operation

There are two methods for controlling the exhaust gas recirculation. On Federal and Canadian vehicles, the EGR Valves vacuum supply is controlled by a coolant temperature sensitive Thermo Valve. On California vehicles, exhaust gas recirculation is regulated by the EGR Valve which is computer controlled through a solenoid valve. California vehicles also require 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. The same procedures would be followed if the temperature sensor were to malfunction.


CALIFORNIA
The EGR Solenoid is spring loaded open venting port vacuum back to the air intake chamber. Once the MPI Control Unit detects that the engine coolant has reached operating temperature, the MPI Control Unit will ground the solenoid circuit. Vacuum will then 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 allowing exhaust gas to recirculate. When the throttle is fully opened (WOT.), the lack of port 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 temperature and engine load.


NON-CALIFORNIA
The Thermo Valve on Federal and Canadian vehicles, operates on the same principles as the solenoid Valve, but is not computer controlled. Until operating temperatures are reached, the Thermo Valve is open, venting port vacuum back to the air intake chamber. When the engine reaches operating temperature, the valve closes and vacuum is applied to the EGR Valve. As with the solenoid EGR System, additional vacuum is needed and supplied once the throttle is past the idle position and will close at WOT.