i have a 1998 f-150 4 x 4 and last yeari had the right side head replaced because it blew a plug, the dealer put a new head on it and used the old parts now since then i have had a loss of power like something is holding the truck back nad the fuel mileage sucks before i had averaged around 16 mpg now i'm down to 11 mpg i have a k + n filter, helix tower on the throttle body and a tornado fuel saver just after the filter box all three seemed to help. i also have a tapping noise and i have read before on the message board about piston slap but i'm not sure what it is i told the dealer and they said they cant hear it but when i even gome to a red light i can hear it tapping, that has happened before they replaced the head. any information would help