Pontiac Repair: heater blowing cold air, upper radiator hose, pontiac grand prix

Question
I need to access and check to see if my blend doors are working properly. I have a 2006 Pontiac Grand Prix GT S/C 3800 with dual climate controls. This last winter my heater started to not blow hot air when it got cold. It seems to work better when the engine is revving at 1500 RPMS or higher either sitting still or driving, this is when I notice warmer air coming out of the vents. But if it just idles lets say under 1500 RPMS, it is cool to cold air. I have replaced the water pump, had the heater core and engine block flushed and back flushed several times, flow and heat to heater core is good and hoses to core are hot. Changed the thermostat, no leaking of anti freeze any where out of car or into oil.

Could this be the blend doors not working properly or closing all the way? Are they operated on a vacuum or electronic system? The air conditioning works just fine, it is just the heat.

Where are the blend doors and the blend door actuator located on the car as I'd like to see if they are working properly myself. Any help would be appreciated, thanks.

Answer
Hi Dan:

did you replace the thermostst with the proper temp one? usually they are 195 degrees and the factor one has a small orfice in it. I perfer to use only original equipment replacemnet parts. Next I would actaully measure the temp of the heater hoses. The inlet should be slightly hotter then the outlet. Measure the temp of the upper radiator hose when the engine is hot it should read above the temp of the thermostat. next the temp door(S) being a doal climate sustem it has 2 tem actautors they are electric and usually they just fail altogether and get stuck at full hot or full cold they do not stick. when you replaced the heater core did you replace that with an OEM part? were ther any seals on the old core that you didn't transfer over to the new one. Ther are foam pieces on the core to stop the air from leaking arond the core and not flowing thu the core. that is where I would start looking it's all over the board at to where to look. Good luck :)