In a 1989 420 SEL, if I do not turn on the heater, the engine temp gauge shows 80, which is the proper temp. after a highway run. However, if the heater is turned on, the engine temp only reaches 65 or 70. Also, with the heater on max. heat and high fan, the inside temp. is not really hot.

The temperature outside is -20c. Is it a bad thermostat?

__________________
1989 420 SEL