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Heating/Cooling Mystery

2007 Subaru Outback, 110 K miles
I drive 200 miles each way, once a week to DC on Sunday and back home to the mountains of Western MD on Friday. All problems happen on that long drive, not on local trips. Only happens when I’m driving in the mountains.
Symptoms:
Sometimes overheats (needle gets close to red area, but not quite into it). I pull off and stop the engine for 5 minutes, start the engine, and needle goes to normal area. This happens only after driving about 2 hours. Continue the trip without problem. Never had to stop more than once.
A/C sometimes doesn’t cool. Harder to tell now that it’s cooler outside, but it put out only warm air one day two weeks ago. The next morning, it worked fine.
Heat does not work very well. Set at max (85), the air coming out is barely warm. Also, the passenger side is colder than the driver (dual controls, both set at 85).
May not be related, but I can hear a sound like water gurgling when the engine is running. It’s very soft and seems to be inside the dashboard behind the steering wheel. There are no apparent leaks anywhere. Seems to only do it when I am pressing on the accelerator.
My mechanic checked the A/C gas, flushed and filled the cooling system, looked at the temp sensor inside the car, and doesn’t know what to try next. Any suggestions? Could this be a computer problem, or is it mechanical?

The gurgling sound is an indication of air trapped in the system. The cooling system needs to be bled correctly. If the air returns, then you have another problem which could be as simple as a radiator cap or as expensive as head gaskets.

All this related to air in the cooling system like Keith is saying.

Good info. Thanks to both of you. One more detail: I recently hit a deer, and had to get the A/C condenser replaced. Could that have been when the air got into the system? How hard is it to bleed it? Is that something I could do?

Have you ever checked the radiator to see if it is full of coolant (when engine is cold)? Is the overflow tank up to the proper level?

Yes, and I have been checking every day. I have to add a small amount each week. Does that indicate a head gasket leak?

It does indicate that you’re losing coolant.

That, combined with the gurgling especially at wide open throttle,
combined with the occasional overheating,
combined with the make and year of the vehicle,
strongly suggests to me that you may have a headgasket leak. The next step would be to do a pressure leakdown test of the cylinders. You have enough symptoms to justify going directly to this level of testing.

I would do the test post-haste. The sooner you address this is that bettor off you’ll be.If you correct the problem before th eengine gets overheated, you might prevent a lot of additional damage end expense. You may have caought the problem before the head warped.

Do you have to add teh coolant to the radiator or the overflow tank, or both?

You need a cold pressure test. This may show other leaks that only occur while running like a bad water pump seal. If you are handy you can borrow the tools for this from a national chain. Cylinder leak down will not reliably show exhaust in the coolant from a head leak. The best test for exhaust in the coolant is a exhaust analyzer to see if the air in your coolant is in fact air or exhaust.