Heater blowing cold. HELP!!!

I can figure this thing out! It blows luke-warm at idle, and will get somewhat warmed at cruise speed, but then after driving for a bit then stopping, it is blowing very cool. I have burped the system several times, so I don't think I have an air bubble. Full of coolant, heater core is not plugged, both hoses are very hot right up to the firewall, but little to no heat.

My only thought it a door/flapper is hanging partialy open (or closed) and blending cold air from the outside. I have chased the vacuum lines the best I can, but still am scratching my head. :banghead:

HELP!!

Check the door operation if they work probally check the thermostat if OK try this un hook the heater hoses and try to blow the heater core out with compressed air. Ive had this happen to me on 2 others. Same symptoms as yours.

I replaced the thermostat in the spring, so I ruled that out. I also did blow out the heater core, even filled it with hot water and blew it out, 1 gallon worth. As far as I can tell the doors are working normally, but I could be wrong there. Thanks!

make sure you have vacuum on the heater switches

if hot water/coolant is moving correctly, your problems is in the door itself, the vacuum or the switch.