What is the purpose of the EGR valve on a 2002 Ford Expedition?

Answer 1

EGR stands for Exhaust Gas Recirculation. If your EGR valve is not functioning properly it can result in poor performance, stalling, rough idle, and/or diminished fuel economy.

EGR's sole purpose is lowering emmissions- most importantly NOx. In standard combustion N2 does not react, O2 reacts with the fuel to form CO2 and H2O. However, at higher temperatures, around 2500 F, N2 begins to react with O2 to form NO and NO2- NOx, which then recombines in the atmosphere to form smog. By running some exhaust gas into the cylinder again you raise the specific heat of the gas mix- H2O has the highest specific heat known- thereby cooling the combustion reaction and significantly lowering NOx emmissions. EGR lowers power at full throttle, and can stall your car at idle, or cause a very very rough, shaky idle. Thus it should turn off at these points, which is the purpose of the valve. If the valve has issues, and leaves EGR on, you will stall/idle roughly, and lose top end power. If EGR is off you will just have more emissions. In fact, many people before the system was improved would simply disable EGR, but these days it doesn't make a difference. So if it's stuck on, or you wanna feel good about the environment, you need it fised, otherwise, it shouldn't harm your engine.