I've got a 1998 f-150 that has the 4.6 gas engine in it. The problem I'm having is with the engine. It knocks terrible if ya use regular gas. Switching to high test totally cures it. If I run mid-grade it knocks a little when you punch it to enter traffic or to pass. Would a sparkplug change cure this? Or is this another of those dang car computer problems? The original owner ran only high test in it.