I have an interesting overheating problem on my '04 FX4 5.3 3V.

Saturday ran from Denver to Kansas City, truck ran fine.
Monday was a return trip. 6 hours in towards Denver, temp gauge slammed to hot and a code was thrown about cylinder overheating.

Found a local shop guy and we didn't find any leaking or anything abnormal going on. I had my phone hooked up the OBDII and could read temps real-time. In the guys shop it was 198 at Idle. I hit the interstate again and it would climb up to 235. I would get off the throttle and it would drop. The whole way back to Denver it was up and down between 208 and 227, but slowly. No big drops. Slow to creep up and slow to creep down. Having heat or air on didn't seem to change anything for better or worse.

Today drove to New Mexico with a trailer hauling a motorcycle from Denver. Going over the mountain passes about 40mph the temp never went above 208. But driving flat it would go up to 230 again. Down hill would drop it.

I was for sure I was going to overheat going over the major mountain passes (wolf-creek) but the darn thing ran like a dream over that.

I'm thinking the radiator is clogged up. While I'm in NM I was thinking of replacing radiator and thermostat because I can't think of anything else.

 

System flush wouldn't hurt. Maybe check the fan clutch while you're at it?

 

^ I would try this

 

New radiator / water pump / thermostat / fan clutch installed. Nice 196-198 temps.

Now chasing a dreaded IWE issue

 

I'm thinking your water pump is gone...Miles?

 

Sorry, but this is a pet peeve of mine . . . people always say the fan clutch, but if it is overheating at highway speed, it's not the fan clutch. When he had it sitting the shop being tested, that's when the a bad fan clutch would make it hot by slipping and not turning the fan fast enough.

If the clutch were to lock up completely, it would make more fan noise and use more gas by being more of a drain on the engine, but would still be pulling air through the radiator.