Exhaust Gas Recirculation (EGR)

The EGR valve, typically mounted on top of the engine, reduces NOX (a tailpipe emission). When open, it reroutes a small amount of inert (chemically inactive) exhaust gas to the intake manifold. This inert gas has a cooling effect on the air-fuel mixture’s peak combustion temperature. Nitrogen and oxygen bond under high heat, so the formation of NOX is reduced. The EGR valve on older systems is operated by a vacuum from the engine. Newer electronic EGR systems use the PCM with engine load and temperature input to adjust the EGR valve. An EGR valve stuck open can result in a rough idle or stalling. An EGR valve stuck closed may produce high NOX and knocking.