Got to disagree with you dirtytorquer..
What is an Exhaust Gas Recirculation (EGR) Valve?
From Christine & Scott Gable, former About.com Guide
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Definition: An EGR (Exhaust Gas Recirculation) valve is an emission control device that sits between the exhaust and intake manifolds on a vehicle engine and regulates the amount of spent exhaust gas that is mixed into the intake stream. Its purpose is to cool combustion chamber temperatures to the threshold that reduces the formation of nitrogen oxides (NOx). The higher the combustion temps, the higher the formation of NOx. In conjunction with the OxyCat and DPF, the EGR valve significantly reduces unburned hydrocarbons, soot and NOx in diesel exhaust emissions.
Pronunciation: ig-zawst gas ri-sur-kyuh-ley-shun
EGR valves cool engine exhaust gas temperatures to help lower oxides of nitrogen emissions.