Re: 5.4 sparkplug replacement
Some guys have said they have had success running a couple tanks with Marvels Mystery Oil through the truck prior to doing a plug change. Here is someone's formula, run Marvels Mysery Oil in every tank for a month prior to changing the plugs, and use 6oz per every 10 gallons of gas. Some people have used a fuel system cleaner as well. You want to try and get the carbon build up to break down. It's the carbon build up that causes the plugs to seize up. Whatever you do, make sure you use Motorcraft plugs. Fords motors don't like other plugs very well. Once you get your new plugs in, document the milage, and change them more often. Good luck.
2001 Ford F250 Super Duty Lariat, Crew Cab, 5.4 liter V-8 gas motor, 2WD, 410 LS Rear Axle
2004 Ford Taurus SES, 3.0 liter 24 valve DOHC v-6 Duratec engine