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.

Emission Controls Vacuum Hose Diagram:

Exhaust Gas Recirculation is controlled by a coolant temperature sensitive Thermo Valve and port vacuum. 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 ECU. If the temperature sensor should malfunction, the ECU will set a trouble code but the EGR system may not be affected.

The EGR Thermo Valve is normally open, and closes as engine coolant temperatures increase. Until operating temperatures are reached, the Thermo Valve is open, venting port vacuum back to the air intake. When the engine reaches operating temperature, the Thermo Valve closes and vacuum is held within the hose and the EGR Valve. This port vacuum is not strong enough to open the EGR Valve against the internal spring. Once the throttle is opened past the idle position, additional vacuum to the EGR Valve is supplied causing it to open. When the throttle is fully opened (WOT.), the lack of vacuum is overcome by the EGR's spring and the EGR Valve closes.