For 1990-2009 cars

The Exhaust Gas Recirculation (EGR) System is used to reduce the amount of nitrogen oxide (NOx) emission levels caused by high combustion temperatures. At temperatures above 1 371°C (2,500°F), oxygen and nitrogen combine to form oxides of nitrogen (NOx). Introducing small amounts of exhaust gas back into the combustion chamber displaces the amount of oxygen entering the engine. With less oxygen in the air/fuel mixture, the combustion pressures are reduced, and as a result, combustion temperatures are decreased, restricting the formation of NOx.

### Exhaust Gas Recirculation (EGR) System Operation

The EGR valve is vacuum operated. Vacuum for the EGR system is created by a mechanical pump driven off the camshft called a vacuum pump. The EGR solenoid valve is located between the EGR vacuum pump and the EGR valve. The EGR solenoid valve is powered by ignition voltage through the main relay. The EGR solenoid valve is controlled by pulse width modulation signals from the ECM through the ground circuit allowing the EGR valve to open to the desired position.

The mass air flow (MAF) sensor signal is used by the engine control module (ECM) to detect the proper amount of EGR flow. The ECM will close the EGR valve, then open the EGR valve to 100 percent. The ECM will then calculate the MAF difference and determine if the proper EGR flow has been detected.