Okay , so I've done this mod TWICE today so far .Used the Duralast stat #4128 the first time and got it back together and refilled with coolant . Took it out for a test drive to get it heated up . Had the EVIC on so I could watch the temp went to 210 and still didn't open . Figured I got a bad stat , went to Napa and had them cross the duralast number and got a NAPA part THM-165 says its a 180degree on the box . Replaced and refilled again . Took it out for a ride and same thing 213 and no drop . What the hell , any ideas ? Didn't remove the mechanical fan or reset the electric fan . constantly ran about 203 with factory stat .
More than likely the tstat's are fine and you are just not purging the air out of the cooling system.
First of all, the thermostat should have the temperature stamped/engraved on it somewhere.
After you reinstall and fill the coolant with a 50/50 mix of water/antifreeze(too much antifreeze WILL cause overheating, since it doesn't cool as well as water), start the truck leaving the radiator cap off, and put the heater on(floor vent, fan motor on, turned to hot). Run the truck all the way to operating temperature, it may take a while idling. Also, squeeze the radiator hoses repeatedly to "burp" the air out. Add coolant as needed to maintain a full radiator.
You then can put the radiator cap back on and drive it a little. You will need to check and top off the coolant level for the next few heat cycles...