First, check your codes. I had a similar rich-running problem and did not check trouble codes until I had done many other things. Codes showed a bad air flow meter and barometric pressure sensor (both are one unit). Replaced that unit and mileage went from 9 mpg (stinko) to 14-15 mpg, so it can make a lot of difference. The car operated normally except for the rich mixture and there was absolutely no indication that the MAS meter was not functioning properly.