IMHO if you're already going in to do plugs why not just spend a lil bit more money and do the coil packs at the same time, even if they are still "good."
I went the cheap route also when I first bought my expedition and just did the plugs, and after all my hard work a cpl months down the road the coil packs started going, one at a time.
The truck had over 300,000 kms on it with original plugs and coils so the coils were due for a change, I had the same mentality, the coils are still good, why change them?
After 2 coils went Individually I said f it and night the other 6 and replaced them. The thing that surprised me the most is how much better the truck ran after. It was good after doing the plugs but after the coil packs it team even better, so even if your coil packs are "good" it makes sense to change the coil packs, since you're already in there may as well change them out, I wish I did the first time instead of having to do the job again a few months after doing the plugs.
I found a seller on eBay that sells coil packs that are advertised as "performance" coil packs with red plastic on the coils for much less than OEM of other aftermarket brands, and his feedback trading was almost 100% positive with thousands sold, if I had to do it again I should probably go this route.
I realize that everybody on here is pro motorcraft, and most people that are passionate about their vehicles usually prefer OEM but I believe good parts can be found if you look around enough.
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