97 F-150 4.6 198K miles. I have been having a problem with the truck shaking during acceleration with no CEL. After scanning the mode 6, it seemed like there were sporadic misfires in the #5 and 8 cylinders. I just bought the truck a few months ago, so since I didn't know when the last tune up was, I figured I would change out coil packs, plugs, and wires.

When I got the #5 plug out, the threads were covered in oil. I haven't gotten to the #8 yet.

I'm going to press on with the rest of the plug/wire replacement for now, but after that, should I look at a possible valve cover gasket replacement?

 

sounds like a leaking valve seal on #5, not a valve cover gasket issue

 

I cannot tell from your description if the problem is from oil comming from inside or outside of the engine. Is your valve cover leaking oil? If yes, you could have a lot of oil and grease in your plug socket and when you take the plug out, oil on the plug threads. If you do not have a lot of burned oil on the electrode, most likely oil coming from the outside of your engine.

Start by cleaning the spark plug socket real good before you install new plugs. I recommend using a round wire plumbers brush, rags, and WD40. Sometimes you can snug down the valve cover bolts to stop or nearly eliminate a oil leak from the valve covers. Also look for any possible leaks from your PCV valve and fix that (its easy to fix). Put in the new plugs with everything cleaned up and see how well it runs. Keep an eye on it to see if any oil leaks continue and if they do, then you can get into going through the extra work and effort to fix them.

 

It is definitely a leaky valve cover gasket. Heads and all should be fine, had the same thing on my 97 4.6 with 385,000 kilometres. (Yes I'm Canadian)