If your scanner can show live data, check the fuel trims. That will let you know if it's actually running rich, or what.
Maybe you've got an O2 sensor or two getting lazy, but not bad enough to throw a code?
Heck, maybe they're sneaking in more ethanol into your fuel! That won't help mileage any,lol!
1995 Blazer LT 4x4, 4 door. 4.3L, 4L60E, np233, 3.73 G80. 289,xxx+ miles
2006 Chevrolet Cobalt LS, 2.2L, 5 speed. 202,xxx+ miles.
1973 Chevrolet Laguna, 350ci, TH-350, 2.73.
1971 Volvo P-1800E. 2.0L B20, BW-35, 4.10