im sorry that you believe that. i know when my fox mustangs egr didnt work my mpg dropped. i know that all the cars that were brought in for repair for decreased mpg, that had non functioning egr systems, got their mpg back when the system was repaired. im a car and light truck mechanic, not a heavy truck mechanic. i can not speak for your heavy trucks egr system. as for the original posters problem, most likely the egr wont get him more mpg.
That was likely due to the computer trying to compensate for the non functioning EGR. Remove it altogether and reflash the computer and the power and mileage should both go up. A non functioning EGR could well have been stuck in the "open" position, introducing exhaust gas at all times. It's only supposed to introduce exhaust gasses at low rpms when the engine is cold and running rich. Never supposed to do this at all times.
here is a quote that explains egrs purpose and why it increases mph from this page http://www.allpar.com/mopar/emissions.html The purpose of EGR (Exhaust Gas Recirculation) is to reduce the NOx emissions. Air is mainly made of oxygen and nitrogen (O2 and N2). At temperatures above 1300°C (2372°F), these molecules split apart and rejoin with each other to make nitrogen oxides (like NO, NO2, etc...). The nitrogen oxides contribute to smog formation. EGR puts a portion of the exhaust gas back into the intake manifold, so it mixes with the fuel and air. (Note that the exhaust adds to the fuel and air; it doesn’t replace any of it). The added mass in the cylinder is harder to heat up, so the combustion events have lower peak temperatures. The lower temperatures prevent the O2 and N2 from splitting and combining. Even though the exhaust is hot, about 600°C (or 1112°F), it's much cooler than the 1300°C required to make NOx. In summary, the exhaust adds mass, increasing the heat capacitance of the mixture (i.e. making it harder to heat up the mixture in the cylinder). Peak temperatures are lower, reducing NOx formation, which ultimately reduces smog in the environment. The reason EGR improves fuel economy is because it reduces the engine's pumping losses. For the cylinder to move down on the intake stroke, the piston is working against the intake manifold vacuum. Another way to say it is that the vacuum above the piston tries to prevent the piston from going down. EGR increases the mass in the intake; more mass means higher pressure, or less vacuum. Now the piston has less resistance during each intake stroke, which results in better gas mileage. it doesnt get any clear than that.
It's a pretty much case of "wishful thinking" if you ask me. To say it has some kind of effect on reducing pumping losses, is pure BS. When you add exhaust gasses to the intake tract, you ARE displacing a certain portion of the intake charge with exhaust gas(the piston traveling downward will only draw in "so much" in volume or "mass") that does not burn, and takes up space in the intake charge (because it has "mass") , thus it does nothing to contribute to the thrust on the piston. This, based on the info you posted above, confirms this. Thus, it reduces the power production and fuel economy, as your foot presses the accellerator harder to compensate for the lost power. How much power is lost, depends on how much exhaust gas is mixed into the intake charge. It can be a little, or a lot, but no matter how much is mixed, it's a simple fact that it decreases power production and thus, MPG.
