My 4.6 liter with 120,000 miles was missing badly. Had the mechanic replace the plugs and plug wires. Bought new Motorcraft at chain autostore for much less than the dealer. Ran much better but I then took the truck to another vendor to have "Motorvac" procedure done.
Now the truck idles smoothly and runs great and gets better gas mileage. They cleaned the idle control valve also and it starts up right everytime.