196 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Exhaust Gas Recirculation (EGR) System As discussed earlier, high temperatures in the com- bustion chamber cause oxygen to combine with nitrogen to form oxides of nitrogen (NOX). If combustion chamber temperatures are lowered by reducing the overall engine temperature, gasoline will condense on the cooler cylinder walls and head surfaces. This results in increased HC lev- els. The exhaust gas recirculation (EGR) system is used to reduce NOX formation without raising levels of hydrocar- bon emissions. The system reduces combustion tempera- tures by recycling exhaust gases into the intake manifold. The exhaust gases entering the combustion chamber con- tain almost no free oxygen (O 2 ), since the O 2 has already reacted in the combustion chamber to form carbon monox- ide (CO). If free oxygen is not present, the nitrogen cannot react with it to form NO X . At the same time, the heat in the exhaust gases keeps the combustion-chamber temperature high enough to prevent HC formation. The EGR valve is not needed at idle speeds and is closed to prevent rough idle. The EGR valve is a vacuum-, electrically, or computer- operated valve made of cast iron and steel to withstand the heat and corrosive effects of exhaust gases. Figure 10-24 is a diagram of the EGR valve and passages. The intake and exhaust manifolds contain passages for the exhaust gases. The diaphragm in a vacuum-operated EGR is moved by ported vacuum. The vacuum signal to many newer EGR valves is computer controlled and valve position is monitored by a position sensor installed on the valve. Figure 10-25 shows an EGR valve with a position sensor. An electrically controlled EGR valve uses one or more small solenoids to open and close the EGR pas- sages, Figure 10-26. No vacuum diaphragm is used. Some electrically controlled EGR valves have only one solenoid, which controls a single valve in the EGR assembly. Others have two or three solenoids, each of which controls a sepa- rate valve. Using multiple solenoids and valves allows a more precise control of flow through the EGR valve. Computer-controlled EGR valves are directly operated by output drivers in the ECM. The solenoids have integral position sensors that allow the ECM to instantly deter- mine valve opening and move the solenoids as necessary. The solenoids are pulsed open and closed as needed. The EGR opening can be read on a scan tool as a duty cycle or percentage. On vehicles equipped with OBD II, the flow rate through the EGR valve is monitored. A sensor monitors system pressure and flow. The sensor detects the amount of exhaust-gas flow by measuring the pressure differential Figure 10-24. The EGR system recirculates some of the exhaust gas in the engine to reduce NO X . This EGR valve is vacuum operated, but most EGR valves are controlled and monitored by the ECM. Vacuum connection Vacuum diaphragm Intake manifold Exhaust manifold Exhaust gases are drawn into the intake manifold EGR valve controls exhaust gas flow Figure 10-25. This EGR valve is equipped with a pintle position sensor. The sensor signals the ECM, which operates a vacuum solenoid to control the valve opening. This allows precise con- trol of the EGR valve in response to engine needs. EGR valve with a position sensor Figure 10-26. This electrically controlled EGR valve is installed on a V-6 engine. Notice that it has an electrical connector, but no vacuum line. Electronically controlled EGR valve