Okay, you asked...
I was getting random CELs for many months before I made the trip. Codes were for fuel trim and cat efficiency. Those never gave me any running problems or audible detonation, so I just kept driving it. Engine had 275,000 miles, and body was showing rust in lower rear fenders (east coast car) so I wasn’t gonna throw a lot of money at trying to fix it.
I started getting the major missing on a long uphill grade in the aAppalachians of western MD, and I was heavy and pulling a trailer at close to max power. My working hypothesis was that one cylinder was running lean, probably due to a faulty fuel injector, and the exhaust valve in the cylinder overheated. I didn’t hear any detonation, but remember these engines have knock sensors so spark timing was probably being retarded on that one cylinder by the ECU without me being aware of it. Also I said it had a burned valve, but it was not a piece missing, rather, the tulip was distorted from overheating so it was not sealing, resulting in no compression.
I just pulled the right head and replaced it with a rebuilt unit from NAPA. I didn’t have to remove the intake manifold, just loosen up both sides, take the bolts out of the right side, and then slip the head out and back in. That was about 10000 miles ago and it’s working great. I still have a CEL related to the cat, but like I said, I'm not gonna throw a lot of money at it, especially since I’ve decided to order a new F150 diesel.
I forgot to mention that a reman head from NAPA with exchange was about $350, plus the gaskets and new TTY bolts brought the parts total to about $500. Gotta love that. A few days of contortion and cussin’ and I was back on the road.