I need a little help determining the problem that is happening here, I just replaced my thermostat last night after the previous one got stuck open and after the repair was completed and the coolant was replaced, I took the vehicle for a short 30min drive around town and I pushed it hard to see if the thermostat is functioning as it should (I tested it in boiling water and it worked). During the drive, the thermometer on my dash never rose above 120*F even though the new thermostat is rated at 180*F. I’ve bled the system of any air and topped off the coolant and my heater was so hot it could literally cook a steak but yet the temp remained the same. I’ve even felt the top radiator hose and it was hot to the touch. Could this be a false sensor reading or something more serious?

 

It's most likely the temp sensor. If you are getting heat and everything is opening fine.
You never indicated what year or engine you are working on here?

 

Sorry my bad should have mentioned that, 97 with 3.1L V6
Thank you for the reply though, i figured that was the case

 

Most vehicles require a 192-195 thermostat. The computer system usually goes into closed loop at 180 so odds are it will also affect fuel mileage because it makes the vehicle run rich and dump excessive fuel into the engine. This might not apply to some car's but does involve most. Unless you computer has been programmed it's not advised.

 

I'm pretty sure the 3100 only needs to be 120-130 and running to 10+ seconds to go into closed loop.

I would lean towards a bad sensor or your gauge in general. Was your gauge reading correctly before the thermostat swap?