Ok, im going to try and explain this without writing a book about it. I have a 2000 Ford F250 with the 5.4 engine. I have no heat at an idle. If Im driving at 60mph I have luke warm heat. If I raise the rpms while in neutral I can get luke warm heat. But if Im driving along with my luke warm heat and I stop at a light....it blows cold.

I brought it into the shop to have the coolant system flushed because it looked like rusty water. I had them throw a thermostat in it as well. I went to pick it up and they told me they thought the heater core was bad because it had no heat. I explained what it was doing and told them I was hoping the flush would fix that. So, I bought a heater core and replaced it. (Not sure if it matters but when I pulled the old one out it had barely any coolant in it). Even with new heater core......no change. Still no heat unless Im driving. Its driving me crazy cuz its gonna get cold soon and I am completely lost. I was thinking it could be the heater control valve but I was told by the shop that my truck doesnt have one. He said it was a door type? Also. I can reach into glove box and feel the heater core lines while its idling and they are hot but once I turn the heat (or defrost) on the air turns cold and it actually cools the heater core down. Im so lost!!!

Also, and im not sure if this is related in any way, no matter where I set my selecter (defrost, vent or floor) once I get to about 40mph it switches to defrost! and then once I slow down it goes back to where I set it. I pulled the glove box and watched the panel/floor door vacuum control motor actually moving on its own once I started accelerating. Im guessing I have vacuum problems but I have no Idea.

I can deal with it switching to defrost as long as I can figure out how to atleast get it to blow heat! Can anyone help??