I have a 97 150 4.6 with 62K. I keep losing plugs to soot and misfires even though I replaced both coils, wires, and my plugs (twice for the plugs). I've tried Bosch, Autolite and mototcraft plugs. Cyl 3 it usually the culprit. I can tell when one of the plugs dies because it will shake violently at red lights, but I don't get a check engine light, and I lose alot of power, nd it hangs up at 3 on my tach. My program is YSL2. Any guesses? Thanks in advance, me and my dealer who charges me for a diagnostic and replaces one plug at a time would greatly appreciate it.

 

Try a compression test on that cylinder.
I had a 6 cyl Ford that did that to one plug and it was a bad ring on only that one piston.
A test with a scope would not show the problem. It fired good all the time. But, after a 100- 200 miles it would get sooted up with oil.