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. During specified engine operating conditions, the ECU will recirculate some of the exhaust gases back into the intake manifold. Since exhaust gases are inert (inactive) and do not contain the needed oxygen for the burning process, the air/fuel ratio is less by volume which lowers combustion temperatures and reduces NOx formation. If too much exhaust gas is recirculated or recirculation occurs at the wrong time, such as during engine warm-up, at idle or at Wide Open Throttle, power and engine performance will be affected.

There are two methods used for controlling EGR operation. Non-California vehicles use a coolant temperature sensitive Thermo Valve. California vehicles make use of a computer controlled EGR Solenoid Valve. California models 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 ECU. If the temperature sensor should malfunction, the ECU will set a trouble code but the EGR system may not be affected.

California EGR System Vacuum Hose Routing:

CALIFORNIA
The EGR Solenoid is spring loaded open and vents ported vacuum back to the air intake chamber, until the ECU grounds the circuit. After the engine has reached operating temperatures, the ECU will internally ground the solenoid circuit holding vacuum within the hose and the EGR Valve. The valve will remain closed until the throttle is opened past the idle position. At this point additional vacuum is supplied and the EGR Valve is opened. When the throttle is fully opened (WOT.), the lack of vacuum is overcome by the EGR's spring and closes the EGR Valve. The ECU will cycle the EGR Solenoid ON time based on engine RPM and Engine load.

Non-California EGR System Vacuum Hose Routing:

NON-CALIFORNIA
The Thermo Valve on Non-California 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 and vents port vacuum back to the air intake. As the engine reaches operating temperature, the valve closes and vacuum is then held within the hose and the EGR Valve. This port vacuum is not strong enough to open the valve against the internal spring until the throttle is past the idle position. At this point additional vacuum is supplied and the EGR Valve is opened. When the throttle is fully opened (WOT.), the lack of vacuum is overcome by the EGR's spring and closes the EGR Valve.