Exhaust Gas Recirculation: Description and Operation

The Exhaust Gas Recirculation System is designed to reduce Oxides of Nitrogen (NOx) by lowering the peak combustion temperatures with a diluted air/fuel mixture. There are two methods used for controlling EGR operation. While Automatic transmission equipped vehicles use a coolant temperature sensitive Thermo Valve to control the EGR Valve, Manual transmission equipped vehicles make use of a computer controlled EGR Solenoid Valve in order to control the EGR Valve. California models also incorporate an Exhaust Gas Temperature sensor within the EGR Valve. The sensor is used for detecting the flow rate of exhaust gases as an input to the MPI Control Unit. If the temperature sensor should malfunction, the MPI Control Unit will set a trouble code but the EGR system may not be affected. The Exhaust Gas Recirculation Valve controls the flow of exhaust gases back into the induction system.

Vacuum Hose Routings:

MANUAL TRANSMISSIONS
The EGR Solenoid is spring loaded open, venting ported vacuum back to the air intake chamber, until the MPI Control Unit grounds the circuit. After the engine has reached operating temperature, the MPI Control Unit will internally ground the solenoid circuit, holding vacuum within the hose and the EGR Valve. This vacuum is not strong enough to open the valve against the EGR's opposing spring, until the throttle is opened past the idle position. Once the engine's rpm is above idle, additional vacuum is supplied and the EGR Valve is opened. When the throttle is fully opened (WOT.), the lack of port vacuum is overcome by the EGR's spring and closes the EGR Valve. The MPI Control Unit will cycle the EGR Solenoid ON time based on engine RPM and engine load.


AUTOMATIC TRANSMISSIONS
The Thermo Valve on Automatic Transmission equipped vehicles, works 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 to the atmosphere. Once the engine reaches operating temperature, the valve closes and vacuum is held within the hose and the EGR Valve. Once again the throttle needs to be past the idle position for the EGR Valve to open. When the throttle is fully opened (WOT.), the lack of port vacuum is overcome by the EGR's spring and closes the EGR Valve.