This is for my 2017 jk.
A few weeks ago I had an issue where the thermostat seemed to be stuck open - coolant temp would stick at 184 - 186. The day before Thanksgiving the thermostat decided to stick closed for a bit (hitting 230) before intermittently opening up all the way. I replaced the thermostat along with a leaking oil cooler. Now, it seems like the new thermostat just doesn't open at all, with temps going up to 220 before I can get back to my driveway and shut down. I have opened the vent on the thermostat housing and filled coolant until it comes out. Placed the jeep on an incline on ramps, allowed it to cool, cracked open the vent and got coolant gurgling out, added a very minimal amount of coolant, squeezed the radiator hoses, and achieved the same hot result after not getting anymore coolant in.
What am I missing? Did I get a bum thermostat from the store? Do I have to go through this topping off thing for a few days? I believe the water pump works because the system does pressurize a bit plus I don't have any noise or leaks.
Please help.

 

Welcome to the Forum. What Thermostat was purchased. I've read multiple posts on several forums of issues with Aftermarket thermostats and they ultimately purchased MOPAR. Yes your description BTW; your burping attempt might not have been exactly correct.. Go to youtube multiple videos on burping the system..

 

Welcome to JKF. Lots of good advice and knowledge here and hope to hear about your exxperiences in the Jeep.

 

I typically have to top mine off for a few days after opening up the cooling system. i always go with actual mopar thermostat. can't rule anything out with an aftermarket. of course, fakes these days are common, so even buying a "mopar" off ebay or amazon is risky.

 

The 3.6 cooling system is a pain to bleed, if you aren't using a vacuum system to pull the air out. I'd be willing to bet you still have air in the system.

My 17 was constantly running 220-235, causing the fans to run all the time on high. I found the upper radiator hose was hot, and the bottom was cool to the touch. Radiator wasn't flowing properly. Replaced the radiator, upper and lower hoses, thermostat and had the coolant replaced and vacuum purged. Runs at 198-203 under all conditions.

Vacuum purge the system, make sure your radiator is flowing (clogged radiators are common) and run an OEM thermostat.

I'd also get a test kit and test for combustion gasses in the coolant as well, to rule out a head gasket issue.