I have a '98 Outback Limited,.2.5L 145k miles.
I have read numerous posts on bubbles in the coolant, overheating, and "goo" in the overflow tank. The typical response is a bad head gasket. I have these same symptoms with the addition of the gurgle noises from the air circulating in the coolant while driving.
Air will accumulate in the radiator to the point that enough coolant is forced into the overflow tank that the engine overheats. It takes about a week to do this driving about 30 miles a day. I just top up the radiator about every 2-3 days, allow the accumulation of air to force coolant into the overflow, and once a week drain the overflow tank. Rinse and repeat. Something is pushing air into the coolant system. I replaced the water pump recently when I replaced the timing belt. I took it to the local Subaru dealer about a year ago who replaced the thermostat and radiator cap, checked the compression, and tested the coolant and air in the radiator for signs of exhaust leaks. The compression was normal on all cylinders and all cylinders were about the same. The coolant test were negative for traces of exhaust. I was assured by the shop manager that it was not a head gasket. They removed all the air from the coolant system and asked me to drive it for a while. It took about 6 weeks for enough coolant to be pushed into the overflow to overheat and has slowly gotten worse. I took it to my local mechanic about 4 months ago and explained the problem I did not tell I had taken it to the Subaru dealer. I asked him to do a compression check and drive the car for a while. After a month he said he couldn't figure out what was causing the air but he also assured me that the compression was good and it was not a head gasket.
This week, every evening, I have started opening the radiator cap just enough to purge the air in the system to the overflow tank. When the bubbles stop I tighten the radiator cap. I do this within a couple of minutes of parking the car. When the engine cools it draws coolant back into the radiator. It never gets all the air out (I can still here the gurgling) It saves me the trouble of adding water and draining the overflow once a week but it has to be done *every day*. I say this because it tells me the radiator cap is allowing coolant to flow to and from the overflow tank.
After over a year of dealing with this problem I would think a head gasket would have either completely failed or gotten bad enough the engine would overheat immediately. Certainly any mechanic should be able to detect a bad head gasket after driving the car over a year with this problem.
I'm stumped. Any help is greatly appreciated.
Well, that fixed it. Replaced both head gaskets and have been driving it for a while. Radiator is always full to the cap when I check it. The first 10-20 miles there was some gurgling while the rest of the air was being pushed through the system but that has stopped. After a good look at the old head gaskets I can see some places where combustion gas was probably passing. I’m not so sure of the bolt pattern. It looks like more head bolts would prevent this. I guess they designed it with a very small margin and it didn’t show up in testing. Setright, thanks for the feedback. Hope this helps other readers.