Realized I posted this in the wrong sub-forum originally.
I have an '05 Impreza rs wagon, with 360k miles.
A few weeks ago, I changed my timing belt and water pump with an AISIN kit. I attempted to drive the car to work a few days later, and within ten minutes of driving, the temp gauge creeped up to 3/4, and blew coolant out the overflow tank. The gauge only stayed at 3/4 for maybe a minute or so, and slowly lowered back down to the normal position.
At this point I believed that I didn't "burp" the coolant system well enough, and purchased a spill free funnel and tried that method. I let it run with funnel attached for about 30 minutes or so and watched air escape through the funnel. I was able to drive the car for approximately 800 miles before I encountered the same problem mentioned earlier.
I then replaced themostat and radiator cap with Subaru parts, "burped" the system the same way with a spill free funnel, let it run that way for about an hour that time. This time I was able to drive the car about 1200 miles before encountering the problem again. During this 1200 miles, I would check the coolant level often in the radiator and overflow tank, and would sometimes see bubbles in the overflow tank, still at the proper level, and other times, the overflow tank would be filled to nearly the top. I would check the levels usually after driving approx. 80 miles to work.
Throughout the 1200 miles, I did not have any issues with temp gauge, and it always read at the correct level. Each time I encountered my initial problem, it was always within the first 5-10 minutes of driving.
Today I decided to try the combustion gas tester, which remained blue and did not change the fluid to yellow, which leads me to believe that the head gaskets are good. I noticed that a clamp on I think the small hose coming of water pump (return hose?) was a little wet, so I snugged the clamp up some more. I wouldn't think that would be my issue though.
Aside from that little return hose, there are no visible leaks.
I pressure tested the cooling system with a DIY technique, I pressurized the system to roughly 15 psi, and found the upper radiator hose bubbling again the radiator. I tighten the clamp and repressurized the system with no problem. I again pressurized to 15 psi and the system held that pressure for about 30 minutes, maybe dropping .5 or 1 psi over that time. After that 30 or so minutes I released the pressure.
Without anymore evident leaks, I decided to top off the coolant and burp the system, let it run for atleast an hour, going through several heat cycles. In that time, minimal air came up through the funnel, if any.
My only concern is that once I removed the funnel and let the car cool down, I topped of the radiator again, taking minimal coolant, however I gave the upper radiator hose a couple quick squeezes, and had a few air bubbles come up. Is this just a coincidence?