More details: Last year, the gauge suddenly started to read off-scale high temp shortly after starting- like less than five minutes, with no work being performed. I used an infrared temp gun to read all surfaces- thermostat housing, hoses, block, all registered ~180F. So I figured the sender had failed, all else seemed to be working.
To get to the sender, one must remove a quite a bit of other stuff, which lead to the thermostat replacement, new water pump belt, all new hoses. I looked inside the radiator after using the coolant system flush from JD, and saw remaining white deposits on the internal tops of the tubes. No true radiator shops near me anymore, none will boil it out like the old days, at least, which is fine. So i ordered a replacement radiator. The tank on top is plastic, and actually communicates pressure with the system vial a hole in the bottom and a tube protruding from the top of the radiator, not just an overflow hose. The cap for this tank os a pressure cap, which I saw no reason to replace. There was a small crack in a corner of the tank, but not all the way through, and it did not leak when first filled and pressurized.
I replaced the thermostat the way the original came out. With 20-30 hours on it, I think any air would have been swept out now, the tank is far above the thermostat housing and the hose does not create a loop seal of any kind.
I don't think it's cylinder pressure, it does not overflow right away, but will get top-of-scale hot after working for a while, then fluid comes out of the overflow.
I have a water pump repair kit already, but after reading the instructions and not having a special tool required, I left the pump as it was. Yes, probably should have taken it off, but it was at that point when I got distracted by the turbo/valve guide seals, and worried about making more unnecessary work, as I already had.
Since the turbo had to come off to get to the thermostat, I looked into it and saw oil. I took it to a shop, but they diagnosed the seals as fine, almost no wear on the bearings, but that the oil was likely from a leaking valve guide seal getting oil in the exhaust side, none on the charge side.
I bought valve guide seals and removed the valve cover, but my older engine does not come with the seals, it was a retrofit or stock on later serial numbers. I had a mechanic look at it and he felt the valve guides were within tolerance to leave well enough alone, rather than pulling the head off.
I had to do a lot of cleaning on the fan when it was off- there was this gritty, black layer of stuff on the fan and shroud, almost like a spray undercoating. I don't know if the variable fan unit uses a fluid that leaked out, but when it was overheating, I didn't think the fan was moving much air, even with the front side shields off.
My first thought was a water pump failure. Then I considered a hydraulic heat overload, like if there was a high-flow open loop causing the hydraulic fluid to overheat, sending the heat the coolant-cooled oil cooler, overloading the radiator. The PTO is hydraulic on this tractor, but I don't think it is a constant flow thing. AND, the temps on the oil cooler were low, lower than the engine temps.
I'm headed to the parts store to eat the cap, and maybe bring the tester home to test the radiator (if it fits the threads)