A retired mechanic stopped by yesterday evening and said to check for air pockets first before replacing the stat. One thing I did notice when removing the factory stat yesterday was that the jiggle valve was installed at 9 o'clock rather than 12 o'clock, so I installed the new stat in the same orientation. Was that factory installed position to blame? I don't think so because it had never overheated to this point.
How critical is the jiggle valve position?
The 5.0's stat housing angles downward as opposed to the 3.5, which looks to be perpendicular with the engine, from the pictures I've seen of the 3.5 in the Haynes manual.

 

TO get all of the coolant out AND to avoid any air pockets when refilling, I use a vacuum tool. Works perfectly. No mess, no issues. Great $90 spent.

 

Where did you suck the coolant out from?

 

The reservoir. Super easy.

 

With the reservoir (degas) tank being mounted up over top of the radiator, how are you going to suck out any coolant below the tank (ie, in the engine block)?

 

Vacuum. You’re essentially sucking everything out of the closed system. Then “blowing” it back in using that same vacuum. The result is zero air pockets.

 

Still not seeing it. To me, it's like sucking a drink through a straw. Once the level clears the bottom of the straw, you're sucking air.

 

 

 

The Ford WSM manual has both procedures for filling and air pockets; vacuum tool and purging like in the video below.