just because the check engine light is not on does not mean there is not codes.You may not have a code to read. But some readers also read some engine perimeters such as engine temp. oil pressure and timing.
Guess I'm never sure of anything, had my ego get the best of me many times! Drove that stang for two years like that! Worst case, you run lean and you'll know cause the motor will knock under load. You can back off the timing a tad to correct that and still get the added MPG. I did experiment with a vehicle that had a front and rear EGR, extending one and not the other, and found both could be extended in the summer, but only the rear in the winter.Running too lean is worse than running rich. Jeffcityguy, are you sure that's safe?