Replaced thermostat now it overheats

[MSkrastins]

I just replaced the thermostat in my 5.4 because it was running too cold and throwing a code. Now it overheats and shuts down.
I tried burping the system to get air out, and the truck wouldn’t overheat with the overflow cap off, but as soon as I put the cap back on it overheated.

I’m lost, thinking maybe I got a bad thermostat.

 

[ManUpOrShutUp]

One of the local shops I use told me they get so many bad tstats out of the box that they now bench test every single one before installing. That said, these systems are a PITA to get the air out. I always park my truck uphill in my yard w/ the heat on full blast to get most of the air out and I still end up adding more coolant for a couple days after. 50/50 mix?

 

[MSkrastins]

Figured out the problem, half of an old thermostat had flipped over and fallen into the coolant passage under the thermostat and was blocking flow. Every time the thermostat opened the truck overheated, now it’s working fine.

 

[FX4FOUR]

Isn’t strange how turds like that happen to us all ?