tipmar

2006 F-150 5.4L Triton, 160k miles. Replaced spark plugs and COP about a month ago (first time since new and I didn't break a single one!), used Champion 7989 plugs and Accel 14033 coil packs.

Ran great until maybe a week ago.

Now noticing intermittent misfire at highway speeds which is new, along with the same misfire at 45-50mph with torque converter locked that plagued me before changing the plugs. No CEL.

Could it be a bad coil? How would I figure out which one if so? Other possible causes?

techrep

iTrader: (1)

I would say bad plugs... install Motorcraft SP-515’s .. does the CEL flash when you feel the mis-fire condition ?

tipmar

Turns out the problem was, indeed, the Champion 7989 plugs. The misfire eventually got bad enough to throw a code and the CEL so I pulled the coil and plug on the cylinder it was complaining about. The electrode on the spark plug was completely gone. Bought a new set of SP-515's and replaced the Champions, 6 of 8 that I pulled showed damage to the electrode. And this was after less than a month of driving. No idea what happened or why, but hoping the Motorcraft plugs will solve the issue for good.

techrep

iTrader: (1)

^^^ that's a month of being installed ^^^