Q: Running the Heat OR the air conditioning will cause this car to overheat. on 1999 Subaru Legacy Outback

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Other wise there is no problem. The car can be driven ontill you use the heat or air conditioning.

I have looked for bubbles in the coolant when warmed up, indicating a head gasket leak.

This is a 160,000 mile "new" car with less problems that the "old" car.

No radiator leaks, hose leaks, cap holds pressure.

Antifreeze appears to boil out of reservoir but I am not certain that this is where it goes. Enough antifreeze is lost to cause the heat to stop working.

(1) Answer
(6) Comments
ok, bubbles are not an indication of a head gasket. a shop inserts a dye or does a leak down test to verify.

have you checked or changed the thermostat. the fact that it boiuls indicates overheating. radiator may b clogged, head gasket issue, water pump. just some possibilities that nees to be checked and verified before you do major damage to the motor.

Ok, I will drain the radiator and change the thermostat. The Thermostat is in the lower hose.
correct. lower hose and 2 bolts. easy repair. make sure you properly bleed the system

Ok I changed the Thermostat.

Not so sure that I have "Properly bled the system"

We took off the upper hose, and added as much antifreeze as we could. put it back
We idled it for a long time until the lower hose got warm, with the radiator cap off. Then we topped off the antifreeze when it went down as the thermostat open. We also opened the air breather on the left hand side of the radiator,

1st time I may not have had the heater on. Air seemed to come from somewhere after 20 miles, the overflow tank overflowed again. The car will

2nd time, we did the same with the heat on to make sure that we didn't have any air in the heater.

Worked fine for a long time yesterday. Idling a long time and a short trip, no problem.
Today, my son went 20 miles again, and the temp gauge rose up and down a few times, and the coolant had been pushed out of the overflow tank. (The level was way down at the mark when the trip started.)

So either we don't know some trick with this car, or we have another problem

you better get a leak down test for a bad head gasket. the only other suggestion is to replace the rad cap but it sounds like it is working.

We took this car to a mechanic who believes that there is a head gasket leak. He is trying a sealing additive that he had had good luck with. One day in a row...its good