When my car initially heats up, it goes beyond the halfway point then drops back below it. Usually there is still no heat at this point. Once the car is running at 35 mph or higher heat blows, but if I park the car and leave it running or sit at a stop light, it no longer blows warm air. Also, I do hear a noise that comes from the vents that sounds like a flap opening or closing when I first turn the heat on. Eventually, everything normalizes and the heat is maintained in all situations. But short trips are not much fun when it’s cold out.
Let me add that it continues to blow air but the air is no longer warm.
Sounds like a plugged heater core. After the car is warmed up, feel the heater hoses under the hood that go through the firewall. The should both be hot. If they’re not, you may have a plugged heater core or stuck water valve.
What car do you have ? It could be the blender door or vacuum controller, depending on the maker, model and year.
My guess is a vacuum leak. it may also be a air block in the radiator, or any of the other things suggested.
Telling us what make model and year car you have would help.
Joseph, The Original Post Indicates It’s An “Oldsmobile Silhouette”. However, No Model Year Is Given.
CSA
The noise that you have probably is a flap opening and closing - probably your “blend door” that controls the flow of air through various openings to provide heat or cooling, etc. The door is controlled either by engine vacuum or by a little electric motor.
My guess is that this is the problem. You either have a motor or hinge that is gunked up a little and won’t function well when it is cold. Then works normally when everything warms up. Someone will have to get into the dash to find and fix the door.
I’d See If The Symptoms Diasappear After Attempting To Purge Trapped Air From The Cooling System. Try A New Thermostat At The Same Time.
Should The Symptoms Reappear, I’d Pressure Test The System For External / Internal Leaks.
CSA
It is a 2001 Oldsmobile Silhouette.
Bingo! Purging the trapped air did the trick! Thank you all for your advice!
Thanks For The Update. I’m Glad We Could Help.
Do I get any stars? ***
CSA