Originally Posted by
pmkls1
The EGR system does reduce combustion temps and thus reduces NOx emissions, but at the same time it is effectively reducing the displacement of each cylinder by partially filling it with inert exhaust gasses that will not burn. This is why when the EGR system is active the fuel system is designed to deliver less fuel. That is where the old and partially true "myth" that removing the EGR valve from an otherwise stock engine would "burn it up" because it would run too lean. Of course, that myth was started back in the carbureted days where the EGR was completely vacuum operated and the carburetor was jetted leaner and the enrichment systems took care of adding the proper fuel under load. Although I have never personally seen a carbureted vehicle actually experience some sort of engine damage by removing the EGR valve, I could see where it might be theoretically possible if the carburetor were left untouched. Throw in fuel injection, though, and all of that changes somewhat. But, like I said before, different engine configurations are going to react differently and you could also go a few steps further by figuring in ambient temperature, the fuel, elevation, vehicle weight, converter stall speed, and gearing etc.... Which would explain why different people experience either mileage losses or gains by disabling the EGR system.
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