[rhandwor]

Their is a heater valve for these units in the bottom of the case this is a damper it has a vacuum operator.
Their should be a dial in the dash which turns from heat to def or ac. They sell them at the dealer. You can use a vacuum gage to determine if you have vacuum. Check the vacuum at the engine I have found this fitting plugged and had to remove and clean it. Normally it has a few branches for a few lines.
With the temperature you stated I don't understand how your engine is operating effeciently because it is like operating with the choke on.
I had a customer who was getting about 3mph because he was operating about 150 deg. A new 195 thermostat jumped his gas mileage back to I assume around 12-15 mph.