1991 Nissan Maxima thermostat

I changed the thermostat because the heater would not heat. After installing it, the temperature goes up and will not come back down. I assumed the new thermostat was stuck closed. I removed the thermostat from the housing and started the motor to check to see if the water pump circulates through the radiator and it does, the temperature stays low. I purshased another new thermostat, Installed it and had the same results as the first one. I have been very carefull in the instalation, installed it facing the same way the original one was installed. What could I be overlooking. Normally changing a thermostat would be a simple thing to do. By the way its a 170 degree thermostat.
Thanks