My probe is overheating but pressure tests fine and didn't have a thermostat. I burped it for ten minutes.

Question

I used a snap-on pressure tester. I havn't run it since putting the thermostat in it.

Answer 1

It Was Overheating With No Thermostat in Cooling System!??

1; With Engine Cool Remove the Radiator Cap, on the Underside of the Cap do you see a Pinkish Foam? (Transmission Cooler Lines in the Radiator are Cracked, Replace Radiator!) or Rust? (Replace Cap!). With the Cap still off Start the Engine, After a short time do you Smell Exhaust from The Radiator? (Replace Head Gasket!). Cooling/Engine System may need to be Flushed/Boiled Out, "Remove New Thermostat Before Flushing to Prevent Damage to the Thermostat!" Make sure new Coolant Mixture added is 50-50 Clean Water & Anti-Freeze!

NOTE; Also Check Condition of Belt and Hoses!

P.S; "If System was Overheating Without a Thermostat adding a Thermostat will Increase the Problem, There is some other Problem!"

Answer 2

Unless there was air trapped in the sysytem,
and you removed all of the trapped air,
adding a thermostat will cause the overheating
to get worse.
Look for:
something restricting coolant flow,
something obstructing air-flow over the radiator,
something wrong with the water pump.