Replacing the thermostat was definitely the correct repair direction for your initial slow to heat up condition. However, based on the fact that the engine never overheated until the new thermostat was installed, I'm lead to believe that it may be defective. I would suggest having the same shop who installed it, remove it verify that it's opening properly. That's really the only thing that makes sense at this point. I've attached the thermostat replacement procedure just in case you need it.
Let us know how you make out!
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Saturday, November 9th, 2019 AT 6:58 PM