[Davescort97]

You could be right about flushing the heater core. Hot coolant flows from the thermostat housing into your heater. It flows hot all the time and the temp. you want inside is determined by the blend door. The blend door directs air through the core for maximum heating. The air is first cooled though the AC evaporator to remove moisture and then is directed though the heater core for all settings except the upper vents only and lower heater.. The blend door position is operated by a vacuum motor. If the vacuum line going to the blend door is defective it will have a bearing on how much heat you are getting. The only 2 positons on the heater control that aren't cooled first by the AC are the upper vents and the lower heater mode. That out of the way, if not your problem then the heater core needs to be back flushed. Disconnect the two heater hoses and flush it both ways to get it cleaned out.