I had the rough idle/engine miss in my 97 4.6L that many seem to experience. It became so bad that my Service Engine light came on. I made an appointment with the dealer to have the truck looked at, of course it couldn't be seen for 2 weeks.... In the mean time I decided to buy new Autolite platinum plugs just for grins. I found that my 48,xxx mile plugs were shot. I was checking the gap as I was pulling them and found they ranged from .06x-.08x, stock is .054. The electrodes were rounded, worn and looked like he11. I'm surprised it still ran at all. Bottom line is check the plugs, they may be worn out even though the manual says they should be fine for your mileage. Now the truck runs great and my around town mileage is up from 11MPG to 15.5MPG.
Tom

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1997 F150 4.6L 5speed SC 4x4