the coils are the weak point in ford modular engines. Looking at the top of the coil you usually see a crack in the potting which allows moisture to get in. You can find the bad one by removing the supply wire one at a time which will show a decrease in RPM for each good one and none for the bad one. A 6mm socket is all you need to replace it. I buy them on ebay for about 8 bucks each and keep a spare or two in the glovebax with the socket. Also I have never gotten a check engine light when one died. The loss of 1 cyl in a v10 can feel like the engine is jumping out of the truck
