I changed my spark plugs in my1998 F250 5.4 two wheel drive. The truck lost its bottem end and the gas mileage went from 16/21 to 9/11. After installing BOCH super HR9DCY plugs I was told they were the wrong heat range. Now I installed motorcraft AWSF22E platinum plugs. The power is back now! Also I did not remove the fuel rail. Thank you all for the information that was a big help.