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Subaru loses heat & AC when car warms up - Mystery!

Took a 9 hour road trip and after an about an hour of driving we noticed AC slowly lost both cool temp and air flow (makes not different if AC button depressed or not). We could hear the fan but NO air was coming out - as time progressed the high setting on the fan made no noise at all. The vent did not blow any outside air either. There was about a gallon of water dripped from under the car when we reached our destination. The next 2 days, EVERYTHING returned to NORMAL during short trips around town. AC was cool but not cold. On the return road trip, again we slowly lost the AC - as we drove north to MA it got cold outside and when we tried the heat, it was the same problem - we could hear something like a fan but no air at all, heat or vent. No warmth when hand placed on vent and high fan setting made no noise. The next day - a cold June morning - the heat works great - fan and all.
This 2005 Subaru Forrester (100 thou miles - car fax ok) - was purchased from a used dealer last Nov and has been a never ending nightmare (including - but not limited too - the dealer replacing the engine post purchase) - we noticed that the AC was not working at the time of purchase and they signed a paper saying they would fix it - just took it in to the dealer last week to have them repair the AC and then we took the trip. Think all they did was replace the coolant and it blew cold when I left the dealer. Before I go back to these guys AGAIN (Hour from my house) can someone give me an idea as to what is wrong? I think this has been a problem from day one but could not really tell with the AC during the winter and did not take long trips in winter when using heat…sometimes I did question if the heat was working though.

Maybe the fan is physically not connected to the motor. Clearly the motor is spinning because you could hear the fan so that’s working.

The gallon of water is a dead givaway. The system works by decompressing refridgerant into a heat exchanger, called an “evaporator”. It’s like a little radiator. As the refridgerant expands, it’s temperature plumets and it draws in heat from the air being blown through the evaporator coils, cooling the air being sent into you cabin (removing the heat FROM your cabin air).

As this air is chilled, it deposits its moisture on the evaporator coils as condensation. There should be a system for draining that moisture. If the drain system is plugged, and the water accumulates, it could be slowly immersing your evaporator coils and slowly making the system inoperable (the fan can’t push air through water), then when it drains the system works again.

I’ve attached a diagram that should help.

I think she said it drains from under the car and that the heat isn’t working properly sometimes.

Yup…a whole gallon after they reached their destination. That would have been the collected condensation.

The heat also works via the fan pulling air through the evaporator coils…except instead of them chilling the air, the coolant is going through its own small radiator called a “heater core” and heating the air. Thank of the two as “stacked” in the system, one after the other. If the evaporator containment is actually trapping water, that water will prevent air from being drawn through both the heater core and the evaporator coils.

The details that I focused on were
gradual reduction of AC function
being able to hear the fan but not feel air coming out
the gallon of water that drained after stopping
then everything went back to working right.

Gotcha. That makes sense.

Let’s hope I guessed correctly. Clearing a drain should be sooper inexpensive.

I think what’s happening is either your system is slightly undercharged or a sensor is malfunctioning, causing the compressor to not cut out when the evaporator gets too cold. Either situation can cause the evaporator to ice up from the moisture in the air freezing on it. The situation will be worse on humid days or if a car window is down slightly, and if the system is not on recirculate so that it is drawing in humid air from outside the car. When enough water freezes on the evaporator, the air flow starts to be diverted and not flow through it efficiently. Eventually there is no airflow through the evaporator leading to warm air output and reduced air volume coming out, though the A/C is still running, and you pretty much have a giant block of ice in the evaporator case. You may notice the fan sounds a little different too, kind of muffled when this happens.

When you park the car and shut it off, the block of ice melts and you get a large amount of water on the ground. What you can do to alleviate this until you can get it checked is to leave the blower running but turn off the A/C switch when you notice the air output getting less. When it returns to normal-ish, turn it back on. You will have to repeat this especially on highway trips.

Thanks all for the suggestions. I will mention the clogged drain when I take it to the (same) mechanic tomorrow. He seemed to think it was the “heat sensor” - I forgot to mention in my post that it does not seem to matter if the AC button is depressed or not - As long as the temp is set to the coldest vent you get the AC full blast during short trips. But maybe that is how Subaru’s work but I think I would notice some sound change in the ac motor or something. This has been difficult to figure out because 1) the outside temp has not really been all that hot and 2) It takes at least an hour+ of continuous driving before it starts to loose temp (ac or heat) and I don’t make that many long trips.
Thanks and I will post how (and if) he fixes the problem.

In your description of the problem you stated that you can’t hear the blower working, even on high after the motor has been on for a long time. The blower moves the air inside car for both the heat and the AC system. The blower seems to work ok in cooler weather from what you say. There is a relay for the blower motor which handles the fairly high current needed by the motor. Sometimes the relay contacts get dirty and go bad. I suspect that is what is happening to the blower. When the relay contacts heat up, resistance across the contacts goes higher and so power to the blower goes down. This is a classic problem with relays. I think replacing the relay will fix that problem unless the relay socket contacts have a problem, which I doubt, but could happen. A simple thing to fix if it is the relay. It is up under the dash on the driver’s side.

If the AC system isn’t cooling the air inside the car as well as it should be then the AC system may have a small leak in it. I wouldn’t worry about the water coming off the system. This is normal and in a way a good sign the AC refridgerant is working like it should be.

Sorry to hear the car has been such a lemon for you. Soobs are usually very good but when buying used it is hard to say what has gone on in the past and Carfax doesn’t have all the details.

Hi Again! My sorry subaru AC tale continues.
Sales/Repair (where we purchased the car) guy replaced the Blower Motor Resistor. But that did NOT fix the problem. My husband, in his frustration took the car to the local Subaru dealer (this may have been a big mistake) and they in turn told us that the resistor, blower motor and connector needed to be replaced for $475. I told the service rep that the resistor had just been replaced and she said that it was the blower motor was causing the resistor to short out. I TRIED to emphasize that the fan was working! That it only stopped “working” after an hour or so of driving (and actually SOUNDED like it was still running - just no air was coming out) and that the next day - after the car engine was cold - the fan and AC worked just fine. She keep repeating they were “trained” in Subarus, knew what they were doing etc etc" -
The story gets better - I picked up the car 2 minutes after the service desk left for the day and noticed they charged me $41 more than the estimate. When I spoke with the manager the following Monday, he first claimed the labor was for the cabin air filter which I had said “yes” to replace. When I pointed out there was no labor charged for the $50 filter, that the additional cost was built into the other labor, they then said - “Oh - that is because the filter is on order, we didn’t replace it yet” - No one - not even the cashier who went over the entire bill with me, knew that (I was never told).
Anymore suggestions??? THANKS!!!
I keep mentioning the clogged line but get the “you so don’t know what you are talking about lady” response.

The resistor didn’t need replacing to begin with. As you said, the motor is running. The resistor is really only there to make the motor run at the lower speeds. That’s not your problem. They are full of it.

I bet the motor is likely fine but the fan just is not attached to it somehow. Notice the attached drawing in this link:

Look at how the squirrelfan seems to be attached to the motor by means of a nut. I bet that nut is loose or there may need to be a star washer under that thing so it doesn’t walk off (item 15).
Also, notice the price. I’m sure that 475 must be two hours worth of labor, which this job does not take.

As far as the air filter is concerned, you’re right there when you service the blower motor. That labor should be included, if they are replacing the motor.

If your husband knows how to hold a nutdriver and a wrench, it is my bet he can fix this himself for probably free by just snugging up that squirrel cage’s nut or possibly looking at what mountainbike suggests. Regardless, you can get to that fan assembly by taking the glovebox out - a couple of screws et voila - you can see what’s going on in there.

I can only theorize as to the reason, but many dealers seem unwilling to do a proper diagnosis and repair job on older cars. I have a feeling that yours falls into that category.

IMHO you need to look for a reputable independant shop that does AC work. Often they’re shops that specialize also in radiators.

A 2005 isn’t that old of a car but it does appear you’re right - they’re not interested in supporting the cars they sell.

I think oblivion is exactly right.

The evaporator coils are slowly icing up, impeding airflow, and eventually no cold air comes out of the vents, even though the fan is turning. When stopped at the destination, the ice melts and drains, leaving a gallon of water on the ground.

As oblivion suggested, on the next long trip, when you notice the cold air flow starting to be reduced, turn off the AC for 30 minutes or so to let the ice melt, then turn it back on. If his theory is right, the AC will be working normally again.

Then, you need to find a mechanic who will listen to you when you tell him that your evaporator coils are icing up.

I absolutely 100% agree with those who said you were icing up, that is exactly what I thought when reading your 1st description… I HAVE NO IDEA what the dealership was thinking other then, “how can I get money out of their pockets and into my cash till”…There DIAG makes no since… Either your AC compressor is not cycling when it should, or your drain line is plugged up, or the system is slightly low on refrigerant .