Replaced water pump, thermostat, and radiator cap last week. I can see the heater/coolant valve is rotating, coolant hoses get warm on both sides of valve. Seems like heater core output hose is not as warm as the others, but that makes sense because heat is transfered to air in the car.

I hear the cold/hot air door flipping when I slide the heat control back and forth... but this brings up a related issue. If I leave the control on cold overnight, the door will be stuck next time I try to turn heat on. I can only slide the lever about halfway before feeling a spring-like resistance. I either have to wait half hour with heat control in middle for door to unstick... or reach under dash and manually rotate plastic thingy while sliding lever to heat... and also avoid getting my finger pinched in the process. After all this, heat blows lukewarm. Will blow hot if I let car idle for 20 minutes, but as soon as I crank up fan it cools off. Far left vent always blows warm. Middle vents blow warm on left side, cold on right site (yes, cold)... and far right vent blows lukewarm.

I replaced blower motor a couple years ago and noticed that almost all of the foam gaskets had rotted off the joints and doors. I replaced what I could but never got around to finding the hot/cold door.

At this point I can only suspect the heater core.... or possibly heater door issue. Is there a way to figure this out without ripping apart the heating system under dash?

Its about 15 degrees in minneapolis so I'm hesitant to reverse flush heater core and risk water freezing in there.

 

Forgot to mention....

I flushed the coolant system about 6 weeks ago (had previously added orange GM coolant by accident and needed to get that out of system)... I didn't add any flushing chemicals before, just cut the coolant hose before heater valve and inserted T-valve for garden hose hookup. Will a flush with garden hose clean out heater core? or do you need to reverse flush core only to really get it clean?

Also, car operates at normal temp range once engine is hot. Needle is slightly below half.

 

I would do a back fluch of the heater valve after being sure the control valve is opening all the way. I've seen them fail inside....which means the cable will move the arm but the inside will not move or only move a small amount.