This past summer I noticed a couple of times when the temperature setting was on cool vent that I would get a 10 second burst of hot air. I ignored it because it didn't occur often enough to really aggravate me.

Now that cool weather has arrived I need to fix it. I took a long trip last week and realized that I had either cool air or if I turned the temp control 1/3 of the way off cool air I got full heat with no regulation anywhere in between. I found a leak on a vacuum line that I thought might be contributing to the problem, but I have the same problem after repairing that.

What kind of control device is used to actuate the valve on the heater core?

 

Its a cable that controls it. Vacuum controls the vent positions.

 

No valve on the heater circuit. A cable controls a flap called a blend door. It blends either vent air or air that comes across the heater core.