Cliff's: with the AC on, the temp gauge would climb to the top after 10-12 minutes of driving. No matter the speed.
It will occasionally get to the upper mark on the gauge, then comes back down after a minute or two to where it normally sits. The ac doesn't blow hot when this happens. Tomorrow (today), I'm going to flush the cores out, and see if that makes a difference.
Mine has started doing the same. I initially thought it was my fault.
When I changed the power steering rack last winter, I had to pull the lower rad hose, so I changed the coolant while I was at it.
What drained was yellow/gold (not neon green). I didn't realize that Ford has it's own HOAT coolant, and thought the previous owner used cheap coolant, which is the same yellow/gold color at the local parts place, so I threw in the neon prestone "universal" stuff.
I also didn't realize that about half the coolant is stuck in the engine and can't be removed without pulling the engine, so ultimately, I mixed the Ford Gold and the Prestone Neon together.
I suspect that caused some rad clogging, so I've been running a flush though with distilled water. It's running about 5C cooler on average, but still doing some weird stuff. On some drives, it'll hover around 90C (normal operating temp). Gets to 97C and (I assume) the thermostat opens and it drops down to 90C again.
But on some drives, it'll get into the low 100s and stays there. With AC on it hits around 110C (Was 117C before flush) but it stays there even after I turn off the AC. Other times, I'll turn on the AC and the temp won't climb at all.
I'm hoping it's just a bad thermostat. I ordered one, and a new rad cap to be safe, so that's what I'm working on now.
The thermostat bolts are a pain in the ass to get out. The Ford manual doesn't say you need to pull the upper intake manifold, but I might have to. Another pain in the ass, because that means removing the fuel rails too, but I'll have access to the CHT sensor, so that might get changed too.