My heater has not been right for a couple years. Its a manual AC/heater, and for the last couple years, it would only blow cool air unless it was reving or driving. I though low coolant to be the problem, but that has been fine. Then suddenly, this year it works. Actually, it is blowing warm air sometimes when I have the slider to cold. Any ideas? It at least made sense before, that coolant wasn't moving unless the car was at a high RPM. But what would make it change all of a sudden and blow warm air while set to cold?

As a note, it also only blows on 4. I just haven't gotten around to changing the resistor yet.

 

Have you tried burping the system?
Take the radiator cap off, run engine on idle till fully warmed up, continue to run on idle with HEATER ON FULL HOT for 5 minutes, topping off with distilled water as needed.

You may have air pockets in the line, that will fix it.

 

There is a part called the Air Mix Door Motor that moves a "door" that either allows or prevents air from blowing through the heater core. The heater controls in the dash tell the door how much to open. So the heater controls in the dash or the motor could be bad. Pray that it is NOT the motor.

 

I had the same issue.

After replacing the thermostat, outside temp sensor, I found out it was due to a bad radiator cap.

It would let air in the system, so I could never bleed it properly.

Replace your radiator cap and bleed the system as trooplewis proposed.

 

Exactly. I waited for about 10 minutes and the heater was still only warm, after changing my radiator. I had to give it several runs up the tach before the system had no air pockets.