It has one of a few things. Either the blend air door isn't working properly, the engine isn't fully warming up, the heater core is partially plugged, or (worst case) there is a head gasket problem. However, I would think you would notice a coolant loss, or air would be getting into the system.
If you start the engine, turn the heater on, and let the engine warm up, do both heater core hoses get hot at an idle or does one side get much hotter than the other?
Thursday, October 7th, 2010 AT 9:31 PM