Originally Posted by DJR96

The ECU can calculate gas flow through the EGR. During a period of steady cruising speed it will close the EGR valve, takes note of the air flow going into the engine via the MAF, the opens the EGR valve again, it is looking for a decrease in flow through the MAF because some gas should be coming through the EGR. If it doesn't sense an appropriate drop in air flow it assumes the EGR it clogged (or otherwise tampered with) and will run the engine with fuel injected a little retarded (effectively richer) in an attempt to reduce NOx exhaust emissions. That's the EPA for ya!
The result of this is that fuel economy may suffer a little. But you would need to keep good fuel consumption/mileage records to confirm if that's the case for your truck. Some people have noticed a difference, some haven't. It might depend on which strategy is loaded into your ECU's memory.

Hope I got that all right. I'm sure someone will correct me if I'm wrong..........