Own a 88 Sentra 4wd 1.6L overall good shape. This little car is running super rich. My last check on the mpg was 12 and i might as well drive my truck around at that milage. I do not think it is a valve, becasue it doesnt run that bad respectively. Timing is good, plugs new (however they are black as coal). I think it might be a sensor or a vacuum leak. Looking for clues to fix it. Are there fault codes which might help me? and how do i look them up.