The vacuum line for the heater control valve doesn’t seem to do anything the heater control valve seems to always be open I did just noticed that one heater hose to the heater core is sometimes cold in the other one is hot so I’m thinking I need to unclog the heater core
Years ago I had problems with low heat on a Chevrolet I owned, that had a vacuum operated heater control valve.
The valve seemed to work - the lever would open and close. My father suggested I remove the valve and inspect the inside. I did and found that the “flap” that would block the coolant flow inside the valve was loose and not moving to the full-open position, although the vacuum operated lever would move back and forth. I replaced the valve and that cured my low heat issue.
If your valve is stuck open, it may have a partial blockage inside the valve - which is overcome when you increase the engine’s speed.
You may want to remove and inspect the valve. Good luck.
