1996 Mercury Sable. Engine Size : 3.0
Question: My cars heater is not working. The temperature will fluctuate constantly from 3/5ths of the way up to 1/4 the way down back and forth constantly.
I have changed the thermostat in this car. There is no leaking water anywhere either. I have burped the entire system as well but with no luck. Please any advice I got $400 to fix it.
Answer: Fluctuating engine coolant temperature is a normal condition of a functioning thermostat. If it is very cold where you are this sounds about normal. You should at least be getting heat when the temp is at its highest. If you do get heat when it is at the highest, but then none when it drops, then this would be a coolant temp issue. If no heat at all, then this is not a problem of the coolant not heating up, but another problem. Your Mercury could have a problem in the temperature door in the dash not directing heat over the heater core, or even a plugged heater core.
Put the HVAC fan on high, ear up to a vent and switch from cold to hot- you should hear a change in the airflow in one of the vents. If not, it is an issue with the temperature control of the dash. If you do, then suggest getting the heater core back-flushed.