Mechanic Glossary

Exhaust Gas Recirculation

An emissions control method that routes a small portion of inert exhaust gas back into the intake manifold to lower combustion temperatures and nitrogen oxide levels.

Exhaust Gas Recirculation (EGR) is an emissions control technique. Nitrogen oxides (NOx) form in large quantities when combustion temperatures rise above 2500F. EGR lowers these temperatures by introducing a small amount of cooled, inert exhaust gas into the fresh air intake. Because the exhaust gas has already burned, it does not burn again. It acts as a cooling filler, lowering the peak combustion temperatures in the cylinders and reducing NOx emissions.

EGR systems use a valve to regulate this flow. Because exhaust contains soot and carbon, EGR valves clog over time. Carbon build-up jams the valve open or closed, leading to rough idling, engine stalling, or severe engine pinging under acceleration.

Frequently Asked Questions

Exhaust gas will leak into the engine at idle, when the engine needs pure oxygen to run. This causes a rough idle, engine shaking, stalling, and a check engine light.
No. When driving at highway speeds, EGR actually improves fuel economy by reducing throttle pumping losses and allowing the computer to advance the ignition timing safely without pinging.
Yes. You can remove the valve and spray the internal ports with carburetor cleaner, scrubbing the carbon crust away with a wire brush. Replace the gasket before reinstallation.
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