EGR Valve (Exhaust Gas Recirculation Valve)

What it does

The EGR valve is an emissions control device that recirculates a carefully measured amount of exhaust gas back into the engine's intake. Why would you want exhaust going back in? Because mixing exhaust with fresh air lowers the peak combustion temperature, which reduces the formation of smog-causing NOx gases. The valve opens and closes precisely based on engine load and speed. The problem: it's exposed to hot, sooty exhaust gas every minute the engine runs. Carbon deposits build up and can either clog it shut (causing a rough idle and check engine light) or freeze it open (causing poor power and black smoke). Cleaning often fixes it; sometimes replacement is needed.