91 Legacy: uneven idle-cold or warm, loss of mpg & power, occas fumes in car.

What is your question?

After complete tune-up and timing belt change, noticed loss of mpg. CEL indicated knock sensor. Had that replaced. Now 500 miles later, mpg is down to 19-20 (from original 28-30), idle is noticeably uneven, cold or warmed-up, and pretty fumey, sometimes seems to belch into car, occas had strong raw gas smell outside car near driver's door. No CEL indicators on, had it checked by 2 mechanics who couldn't find anything.