EGR — Exhaust Gas Recirculation

The EGR system takes a small amount of exhaust gas and feeds it back into the intake manifold. This sounds counterproductive — why would you put exhaust back into the engine? Because the exhaust gas is inert — it has already burned and will not burn again. Adding inert gas to the intake charge dilutes the air-fuel mixture and lowers the peak combustion temperature. Lower peak temperature means dramatically less nitrogen oxide formation.

The EGR valve is a PCM-controlled valve that opens to allow exhaust gas into the intake and closes to stop the flow. At idle the EGR valve should be fully closed — introducing exhaust at idle causes a rough idle and possible stall because the engine does not need EGR dilution at low load. At cruise and moderate load the valve opens partially to reduce NOx. At wide open throttle the valve closes because maximum power requires maximum fresh air.

EGR valve stuck open — rough idle, stalling, poor low-speed performance because exhaust gas is diluting the mixture when it should not be. EGR valve stuck closed — NOx codes set because exhaust temperature is too high without EGR dilution. Carbon buildup in the EGR passages — the most common problem. Exhaust gas carries carbon that deposits in the EGR valve and passages over time, eventually restricting or blocking flow. Cleaning the EGR valve and passages often resolves EGR flow codes.