I have a 2017 F150 3.5l ego boast. Started have overheating issues when I got up to 60mph. Under that I was fine. Replaced the thermostat with a Ford updated one. Pulled a vacuum and filled system. Now I start overheating around 45mph. Fans are working fine, kick on high speed. Heard horror stories with this engine with overheating. Only had truck for 5,000 miles. For the first time it overheated sitting. Sounds to me like a flow problem, maybe radiator. Thanks

 

Sounds like whatever caused the original concern is still there. try flushing the radiator.. but whens the last time you did a waterpump?

 

When I drained the radiator not a whole lot comes out. Took the top hose off and pumped water in it and still not a lot coming out. I guess the water pump is original. It had 129,000 miles on it, I have had it for 5,000. When I replaced the thermostat it looks like someone put some leak stop or something in it. On the front of the thermostat and inside the lower hose going to it was full of a brown creamy stuff. I am thinking this is in the radiator and possibly restricting flow. Thanks for your help

 

Sounds like the Radiator is blocked from the stuff added.
Can't flow enough coolant to keep ahead of the engine requirements.
There may be some other issue to uncover besides a blocked cooling system.
Good luck.

 

Check the oil, if it looks like a chocolate milkshake, that's a sign of a failed headgasket, which can cause overheating. It can also cause combustion gasses to enter the cooling system, which can cause overhheating.