Mercury Grand Marquis Forum : “Why does my heater fan only blow cold air?”
Why does my heater fan only blow cold air?
When I turn the heat on in my car it only blows cold air. The AC works just fine and the fan does blow at full power but no matter how long the car has been running for the air still remains extremely cold. We have taken the whole dash area apart and everything works fine so i assume the problem is coming from under the hood.
If both heater hoses under the hood are hot, the blend door in the hvac plenum is not moving to direct airflow through the heater core. If one is hot and one is not, you have a plugged heater core. If both are cold, you have an air lock in the heater core. Check the coolant level and run veh at 2000 rpm in park with heater on full till you get heat, then top up coolant again. If this continues to happen you are getting air in the cooling system. Only comes from one place -- the head gasket(s) have failed and you are getting combustion pressure pushed into cooling system.
You have a heater conrol valve that directs hot water to the heater core. If that valve is faulty it would cause your problem. I would first inspect all the hoses under the hood to ensure that a previous owner has not disconnected the heater core. Also inspect coolant level as low coolant level will effect proper heater function.
I have had this same problem. My car will blow hot air for awhile, then suddenly it will be stone cold. I have had it checked by 2 different garages (one was the dealership). The first garage identified a problem with the rheostat (sp?) which was truly broken, so they put in a new one. Due to the on and off again nature of the problem, this seemed to fix it. When it started blowing cold again, they referred me to the dealership, where they replaced the thermostat (which the first garage had said was okay, but the dealership showed me it wasn't working). The first garage is the guy I trust, and he told me if it wasn't the part he fixed, it was probably the diverter up under the dash and he referred me to the dealership to have that fixed. I think the dealership messed with the thermostat (which worked fine at the first garage) and just replaced that, since it was quick and easy to do and would also be the obvious place for the problem to be.
What I get sick of, is being treated like a "girl" who has to take them at their word. I asked to see the non-functional thermostat, but they already had it off the car, so how do I know that was even my thermostat? I asked to see the manager of the service department, and he assured me his staff is honest. Yeah, whatever. The car was blowing hot air at that point so I paid the bill and left. That was early spring, I haven't used the heat again until now and it is STILL DOING THE SAME THING it was doing before I had ANYTHING fixed.
I am frustrated, and don't know who to take it to.
My father was a career auto mechanic, so I grew up around this stuff and find it infuriating to be treated like a stupid woman who doesn't understand the difference between an diverter, a heater core, and a thermostat.
Grrrrr.