Exhaust Gas Recirculation (EGR)
The Exhaust Gas Recirculation (EGR) valve and its related components first appeared on automobiles in 1972. The valve may be controlled by vacuum or electronically depending on the year, make and manufacturer of the vehicle. The use of EGR is on the decline since manufacturers began to use fuel injection, which provides more accurate control over the fuel and air mixture.
EGR systems help reduce the level of harmful oxides of nitrogen emissions (NOx) in the exhaust. This type of emission increases with combustion temperature. Although at first impression it may appear counter-productive, the EGR system does this by introducing calibrated amounts of exhaust gas into the engine's intake system. Since the exhaust gas—for the most part—doesn't burn, it takes up some of the space in the combustion chamber of the air/fuel mixture. As a result, combustion chamber temperature drops and so do NOx emissions.
EGR systems usually do not require regular maintenance. Signs of a malfunctioning EGR valve or its related components include engine spark knock under acceleration, rough idle, stalling, and reduced power. To determine if your car has an EGR system, refer to the Vehicle Emission Control Information (VECI) label underneath the hood, which will call out this emissions subsystem if so equipped. If you suspect a problem with your vehicle's EGR system, have it checked out by a professional technician. A malfunctioning system, if neglected long enough, can cause engine damage.