2016 XLT 5 liter V8 - changed the factory spark plugs yesterday at 116,000 km (slightly over 60,000 miles) and job went well. I found rust in the threads of the two back plugs removed on the passenger side. No evidence of water infiltration or dirt on the coil packs or in the spark plug cavities. All other plugs good. Any insights as to how rust gets on these plugs?

BTW thanks to the info gotten on this forum, the video from FordTech M. and another video which shows how to use a 90 degree pick tool to easily remove the coil wiring connectors!

 

My Hemi Durango does that due to some of them being right under the cowl and where the cowl seal against the windshield is so that's where water is most prone to get in and drip. On it, I noticed the plug boots have a little slit at the top and I positioned them with the slit pointing down last plug change. Don't know if it'll help or not. I think the last cowl panel I bought for it was like the eighth redesign. I noticed the left side cowl panel on my XLT was not popped in well either so water could run down between the seal and windshield on it.

 

Link to my plugs at 60,000 -you can compare to yours. hard to know if what mine look like are what you are describing of yours.

https://www.f150forum.com/f12/spark-.../#&gid=1&pid=1

 

He's got a 5.0L which I don't think has that nice engine cover like the 3.5L too keep water off it....if it's an intrusion issue.