Yep, T-stat. Mine is doing it with a new water pump and verified no-air vacuum coolant refill. Kind of torqued me off because they had it for 3 weeks due to backlog, and I still got an aftermarket thermostat installed with a Motorcraft part number on the invoice. They're fixing it, but still...

 

The 5L engine has a different reverse cooling system design configuration.
The tip-off is the WYE connection in the front.
It uses a Ford special design for fast cold start heating then on the road temperature control.
Don't use any other Thermostat ,and the Ford design has already had a design revision.
With these new truck generations the owners has to stop comparing to the old system operation.
If other vendors stats are not designed to meet Ford spec, you will have a problem.

 

Thank you everyone for your input. New Motorcraft thermostat has been ordered, installed, and it is working great. No overheat at all. Temps not wildly fluctuating anymore. Definitely some design differences between the stant, Motorad, and Motorcraft thermostats.

 

From what I understand the 5.0 uses a reverse flow cooling system and the thermostat is a crucial part of system and is a uniquely designed thermostat. Normally, the thermostat controls coolant flow into the radiator from the engine. In the Coyote, the thermostat controls coolant flow from the radiator to the engine. It also utilizes a bypass to let coolant flowing through the engine to recirculate back into the engine at the Y pipe when cold and mix with the cooled coolant entering the engine from the radiator when the thermostat opens.

 

Thanks for the update and resolution! Glad it's fixed.

 

When the coolant temp drops, before you reach the thermostat closing temp, the computer is going to command the shutters to close. The computer aims to keep the coolant at least 20 degrees warmer than the thermostat point.

This image I put together last year shows the physical differences between the Mishi thermostat (the only non-Motorcraft thermostat that should be used), a standard aftermarket, and the Motorcraft thermostat.



The main takeaways are the Motorcraft and Mishi use a much larger wax pill, use a progressive force spring (aftermarket uses cheap linear springs and try to emulate progressive with alternating force), and have both a longer travel for the close off plate and more open intake, which provides less of a restriction through the thermostat when it opens.

 

The Motorcraft stat returns hot water up the middle of a tube through recirc block off, so the stat can sense the engine temp, instead of the radiator temp.
The aftermarket stats have small holes in the recirc block off, that are supposed to do the same, but the water gets pulled back into the pump before the stat. So it doesn't sense the engine temp. The radiator temp keeps it closed.

 

This has turned into a really great info thread for guys not familiar with the new cooling system on these trucks. Great info and replies, guys!

 

I had this same problem with my 2018 3.5 EcoBoost.
Putting in the Motorcraft Stat and vacuum purging the cooling system prior to sucking in the coolant did the trick.
Such fun!