Sarge is right. It's just like watering down beer....the recirculated exhuast gasses are inert (at least in terms of combustion) so they dilute the mixture: less O2 buring = less heat, just like less beer = less buzz John - that makes perfect sense and the engine does just that....it closes the EGR at above-idle RPMs to get a richer burn. It is only the low RPM combustion that results in a much higher temperature and more greenhouse pollutants...that's why it used to be the worst thing in the world for an engine to idle for long periods - it was pre-EGR and the combustion temperature would be much higher, resulting in all sorts of carbon deposits and other crap. Hope this helps.