I've been fighting overheating while towing issues on my 2012 for a couple years. I monitor the water, trans and charge air (IAT2) temps digitally over the OBD2 port. The factory gauge doesn't move from 130* to 230*. It hits yellow at 240* and red around 245*~250*. I went for the most expensive thing first, new bigger blingy full race aluminum radiator.
$850. Didn't change a thing. 😥 Might have made it worse. 😭 Since it only gets hot when I'm in the boost and cools off very quickly as soon as I crest a hill, I decided to do some testing. If the outside temp is under 70* I can drive however I want and it won't get above 230*. Obviously when it's close to 100* outside it heats up much faster.
So, I got a 4 channel wireless thermometer and strapped the probes on each side of the radiator, the thermostat housing and the underhood air temp. They were strapped on the outside so obviously not accurate water temps, but I was able to make decent comparisons. What I noticed was at times I saw a differential temperature across the radiator, but at other times both sides of the radiator were exactly the same temp. I believe this is indicative of the position of the thermostat (open = temperature drop, closed = same temp). What I found strange was that at times the stat would be closed (presumably) when the engine was still reading 220*+ with the factory 188* thermostat. So I started looking in the service manual at the coolant flow diagram and noticed that Ford decided to put the thermostat on the cold side of the radiator.
The wax pellet side of the thermostat is on the engine side and it also has a restrictor plate that partially blocks off the (rather large) bypass pipe that allows hot water to short circuit the radiator when the thermostat opens.
My theory is that when the thermostat starts to open, the cold water from the radiator rushes in and pushes all the hot water from the engine away from the wax pellet on the thermostat, causing it to close, even though the engine is still hot. So, I installed a Reische Performance 170* thermostat. This helped, but didn't solve the issue. The next thing I tried was an inline thermostat housing from JEGS. I installed it with a 180* thermostat in the hot radiator hose, removing the stat in the OEM location. I screwed up and installed it in the middle of the hose, which meant it never got hot enough to open. So, I dropped another $50 on a new radiator hose and promptly cut it in half, this time as close to the engine as I could. This functioned, but still had some issues. A lot of water was bypassing the radiator and sometimes (cool outside, quick heating of the engine from startup), the stat wouldn't open at all. So I drilled a small hole in the stat so it would always see engine temp water and I gutted the factory stat and put it back in so that the bypass restrictor plate was still functional. That brings us to today.
I've been driving the truck like this for a couple months but I have not had a chance to pull a heavy load yet. That opportunity will come this Friday, when I will be pulling a large enclosed car hauler (should be 7000-9000 pounds once I get it loaded up) from Lehi down to Arizona (about an hour north of Phoenix). I will update after that.