TonyM,
Try running some top end engine cleaner through the intake. Carbon built up on the intake valves actually acts like a "sponge" sucking up, and retaining fuel, causing a lean condition in the combustion chamber. This lean condition will cause higher combustion chamber temps increasing your NOX output. I have used this method to reduce NOX on several different occasions. With the high mileage you stated, I can garuantee, you have a carbon problem.
