I have a 96 gmc sonoma2.2. 5 speed my old thermostat is stuck open bought two new ones the new thermostat won't open and engine runs hot
The reason why your new thermostat does not open may be because you installed it not facing the right way. The best thing to do is to contact a mechanic to help you fix the problem.
Ricky is right. put one of the new thermostats into a small pan of water. put the pan on your stove and boil the water to see that the thermostat opens. put it in the car the other way.