My 1998 F-250 5.4 auto. had 40000 miles on it so I tuned it up. Big mistake my mileage went from 16/20 to 12/16 and I lost botten end. It runs smooth. I put bosh plugs in and reset the computer. any ideas what went wrong?

 

What did you do that was considered a tune-up?
Are you sure you were sold the correct heat range plugs?