I own a 2006 Subaru Outback wagon 3.0 liter R, 130k miles. Car is randomly overheating (not red-lining, but almost). Sometimes you can drive all day, no problem. Other times, temp needle creeps up to near the red. Several months ago (before the problem started), the radiator hose burst (car was turned off before overheating). So, shop put in new hoses, belts and coolant. Recently, this problem started, so I drained the coolant (was very clean), flushed the system with plain water, including the hater core. Installed new Subaru thermostat. Filled 50/50 with Subaru Long Life coolant, and Subaru Coolant Conditioner. Burped the system per instructions. Also replaced the radiator cap with a new Subaru cap.There are no obvious leaks that I can see.Typically, needle will be steady in the middle, but will suddenly start climbing after a lot of driving, then go back to middle. Both fans are running, even with the A/C off (expected if engine is hot). Advice I've been getting is that this is a HG leak, especially because often the overflow tank will boil -even if the car car hasn't overheated.Here's an update, after a few weeks. Now I'm more puzzled than ever! First, a little more info. The car never gets above-normal hot at highway speeds, even if I've been driving for a few hours. It can start when I get onto local traffic AFTER having been driven fast. (Note that I have never allowed the temp gauge to go all the way to red. The one time the needle touched the red line, I pulled over and stopped the car). The usual behavior is that the the needle will slowly go up and then back down to the middle position. Later, if I'm back on the highway, it goes back to its normal position a touch below middle, and stays there. If I pull over and look under the hood, the overflow tank is full and boiling.After these incidents, I always top off the coolant. The boiling causes coolant to spill out of the overflow.I know you are thinking that this is definitely a head gasket
failure, like I've already been told. Well, stay with me because this gets interesting.Yesterday, I took the car out and was in local traffic for approximately a half an hour. I was on a highway for about 5 to 6 minutes during this drive and reached a speed of 50-60 miles an hour. When I arrived home, I opened the hood and saw that the overflow tank was bubbling. (Note that the car was not overheating at all during this drive). But I was glad to see the bubbling, because I could now do a test. I am in possession of a block tester kit that I had been loaned from AutoZone. This is one of those gadgets that allows you to suck air into a clear cylinder into an indicator liquid. If the liquid turns yellow, you know that there are combustion gases present. I stuck the device into the overflow tank (not the radiator itself, of course, because the engine at this point was hot). The coolant in the overflow tank at this point was 4 to 5 inches below the level of the device, so there was no chance of sucking any coolant into the device. Bubbles were still coming out of the the overflow tank tube. Following the directions, I squeezed the tube for two minutes, probably more like three. And … (wait for it) …Nothing. Nada. The liquid did not change color at all. It stayed the same shade of light blue the whole time. This should indicate that NO combustion gases were present coming in the overflow tank.Now, the cynical among us may say "well, how do you know that that indicator liquid even works? Maybe it's just water with blue food coloring." Well, I thought of that. I did something else as a control. I walked to the back of the car, while it was still idling, and put the block test device near the tailpipe and squeezed the bulb in and out a number of times. Within seconds, the liquid changed from blue to yellow. This indicates to me that the block tester is working.Oh, and one more thing: the bubbling continues for about a minute or two AFTER the ignition is turned off. How could combustion gasses be bubbling out of the overflow tank if the car's not running?So, (considering all this info, sorry) do you think I'm right that the issue here is not a blown head gasket? Is air getting into the cooling system, or is a leak causing the coolant to boil (because the system never completely pressurizes)? What's the next step? Pressure test the radiator?As a reminder: new hoses, new Subaru thermostat, new Subaru radiator cap, recent radiator, Subaru coolant (50-50 mixture with distilled water). Subaru OB, 2006, 3.0 liter, 6 cyl.