Cooling issue, but this time it's to cold...

Guys , I tested my old thermostat, my new thermostat, I did this on the stove, with a thermometer, they both functioned perfectly, they both open when specified and they both close all the way. I installed the new one. I tested my new Sunpro mechanical temp dash gauge against a thermometer in the radiator, they both read the same. All that checks out.