The temp gauge fluctuates.Sometimes it want come up to normal temp even after its been run awhile. Other times it will go to normal temp.But when you turn on the heat it will drop out. I have changed thermostat and coolant level is good.
No Heat on 1996 Mercury Sable
Thermostat is still sounding like an issue. Did you use the appropriate temperature replacement? Could be too cool a unit. Also, is your cooling fan maybe running non-stop? It should not...if it sticks on, it could be a fan relay issue. Soemthing is not allowing your engine (apparently) to get and stay hot enough...that's what I would focus on. Good luck.