Head Gasket or air bubble?

[lozanoa11]

So when i bought the car i knew the last owner thought he had the Head gasket going out. Well we ran a block test that tests for exhaust in the coolant and it passed. But every once and a while the temps will rise or wont warm completely. Also the coolant level in the res will go up and down. This sounds like an air bubble in the cooling system to me but i am still paranoid.
Am i just crazy or are these symptoms for a head gasket?
I think i may do a coolant flush on it on monday and see if that fixes it.

Is the best place to bleed the air out the screw on the pass side of the radiator?

Thanks

[hallowpoint]

burp your system. also check your thermostat. typically when you ave HG failure there will be black crap in your coolant.

[lozanoa11]

Oh well the res does have a black coating in it but i have not notice any black stuff in the coolant other places.
Edit: it does have 188k on it and the head gasket was replaced before i know.

[danci1973]

Oh well the res does have a black coating in it but i have not notice any black stuff in the coolant other places.
Edit: it does have 188k on it and the head gasket was replaced before i know.
My car had similar symptoms - it would overheat and throw out coolant whenever I drove over ~120km/h for more then a minute. I took it to several 'experts' (including Subaru dealer shops, specialized engine cooling shops, ...) but they all pretty much blamed the head gasket...

So I replaced head gaskets along with some other almost-due maintenance (clutch, oil seals, ...), but it didn't help. After that I simply replaced all the cooling components one-by-one (starting with the cheapest and working my way up) until it was fixed:


thermostat[/*:m:jelhdeuq]
radiator cap[/*:m:jelhdeuq]
water pump[/*:m:jelhdeuq]
radiator[/*:m:jelhdeuq]

However, the radiator wasn't blocked or junked up - it turned out that the 'mouth' where the radiator cap should seal was somehow 'eroded' , so the seal was no good. Because there was no pressure in the system the coolant would boil and expand so much, that it would overflow the expansion reservoir. After that it was a matter of seconds before the engine overheated...

You can read more about it here (https://sl-i.net/FORUM/viewtopic.php?f=11&t=13943) - there are some pictures of the cause at the end.

D.

PS: I'm not saying that's what's going on with your car, but it's a possibility that -apparently- nobody thinks of... ;)

[Benfolio]

I have a similar issue I've been chasing. I'm assuming my problems is headgaskets too, even though there's no oil in the coolant or vice versa.
Mine failed the gas test too, but ONLY UNDER A LOAD it will get air into the coolant and therefore too much coolant into the overflow. It's gotten worse now that it's colder out to the point that if I drive more than 15 minutes it pushes the coolant out and overheats. My radiator neck looks fine and I just flushed the CRAP out of my radiator and it didn't seemed blocked. Maybe I'll try my spare (AT 2.2L radiator, but it'll work) and see if it's an easy fix.

You can try burping first and see if it goes away when you do the flush.
I've heard it helps to have the coolant line off of the throttle body when you fill it, AND the vent screw on the passenger side of the radiator, but I've always just done the screw.

[ouch1011]

danci1971--I've repaired an early model Forester with that exact same problem before. It came in for an overheating concern and failed the block test and was pushing coolant out to the reservoir so another mechanic diagnosed it as a bad head gasket. Unfortunately it had been badly overheated and lost compression in one cyl, so he replaced the engine with a used engine. It was fine for a couple days, then came back with the same problem minus failing the block test. So, it came to me. I pressure tested the cooling system and found that I couldn't build any pressure because it was dumping it all out to the reservoir. The problem was the same, the sealing section inside the radiator neck was erroded to the point that it wouldn't seal anymore. New radiator and cap fixed the problem. You might just be able to polish it up with some sand paper and get it work ok if that is the problem, but I can't do that at work since we guarantee our repairs. I can't do something on a car unless I can guarantee it will fix the problem, otherwise I get to recheck it for free.

As far as issues with air bubbles in the cooling system, I've never had a problem with that on this car. Maybe I've just been lucky. I always just open the bleeder screw and fill until it comes out the bleeder screw, then closed it and top it off and run the car with the heater on. It will also help if the front end of the car is up higher than the rest of the car, like on ramps or jacked up in the front, because air will always try to get to the highest point in the system. I've also gotten in to the practice of using OE/quality thermostats because they almost always come with air bleed valves in them to keep air from getting trapped behind the thermostat. If you don't want to pay a couple extra $$ for a good therostat, you can drill a small hole in the metal frame of the t-stat and it will have the same effect without having any adverse effect on the t-stat operation.