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Discussion Starter #1 (Edited)
Two weeks ago I noticed the heat didn't get hot at idle but would get hot once I started to drive- this ended up going away temporarily. Last week the car's temperature gauge would rise to the 2/3 mark after idling for a few minutes and then slowly return to the desired reading. I thought this was strange and could be related to the heating issue. I have a Mishimoto radiator and stock expansion tank. The car currently sits at 116kmi; the radiator, water pump, thermostat have all been replaced 14kmi ago (mid yr 2012).

So from what I have read on here that could be related to the thermostat. I thought this was weird because the thermostat was replaced 14kmi ago. It is a 20$ part so it could be possible I guess. With these things in mind I decided to get the car into the shop where the mechanic swapped out the 2 year(14kmi) old thermostat on 11/21/2014 (yesterday), as well as new Subaru coolant and OEM Subaru radiator caps.

When the mechanic brought the car back from the test drive, the coolant reservoir tank (right side of the radiator as you face the car) was overflowing with coolant. The mechanic and customer service rep explained to me that it may be because of a failed head gasket. I have never had a coolant leak, and the reserve tank always had coolant inside, so let's not go there just yet. So, I asked about their process for burping the system and the answer was that they back fill the system so that most of the air will escape that way. I also asked how the air would escape if there is air in the system because obviously this isn't good. I was then told that the air would eventually work its way out through the expansion tank (turbo side). I was a bit confused because the system is pressurized and designed to hold pressure; so how would air escape? Anyways, I am not a mechanic so I took it for what it was worth and thought nothing of it. However, at the same time I was a little skeptical about this process because after reading about similar issues here on the forum I came under the impression that it is more complicated than that. So I continued my research on this topic.

Today I decided to buy the Lisle spill-free funnel and burp the system . I burped the system using this process that I found on YouTube:
Coolant Burping Subaru STi - YouTube

After I have finished burping the baby I took it for a spin with the AC on full blast- no fluctuations in temperature. I get back and pop the hood to check the reservoir tank for bubbles- I was getting roughly one bubble per second. I turned the heat on full blast, gave it a few revs, then switched the AC on full blast and got the fans moving- no more bubbling.

For the time being I will be keeping a close eye on this and will be reporting my findingsso that we can, hopefully, tackle this problem together. What is going on here? Tips, recommendations, anything would be greatly appreciated!


Edit: Compression Test Results are as follows;
Cyl #1: 143, #2: 138, #3: 140, #4: 140
 

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A friend of mine is a Subaru master tech and though I'm not saying it's wrong that video is not how he told me to fill the cooling system. He said he sees a lot of STI's come in with air in the system. I used his method and it worked great for me.

The way he told me to fill the system was take off both caps. Fill from the upper reservoir and watch the radiator. The coolant will eventually make it's way to the top. Here's the trick. Many times there is an air bubble behind the first bit of coolant that comes out of the radiator so you have to let it overflow a bit. That funnel comes in handy for this step capturing the overflow, but you still have to be quick with the lower cap to minimize the mess. Anyway I didn't have that funnel so I made a bit of a mess. He told me that was OK. I actually didn't have an air bubble because I filled it very slowly. After I let it overflow for a bit I installed the lower cap and finished filling slowly until the upper reservoir was full. I then filled the overflow to the full mark. The car did suck some coolant in from the overflow, but after a couple run cycles it stabilized and has been fine since. My needle is rock solid just above the first notch at operating temp and moves very little regardless of ambient temp.
 

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Discussion Starter #3
That sounds like a solid way to do it. I'll get out there first thing tomorrow and try this method. Thanks
 

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You should keep one thing in mind. That method may not work to purge air on an already filled system. Air has a way of moving around once the car is running and becoming trapped in other places namely the heater core.

I was at my friends shop the other day and he was trying to purge air from the heater core of a turbo Forester. I asked why didn't he just use his Matco Airlift. He said it sprays coolant everywhere when used on an already filled system. The way he purged the heater core was to pull off the upper hose and let it run a bit like the method I described above with radiator. It's easier to try the fill method I described for sure, but if that doesn't work you may need to use this method. Good luck.
 

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There's not much to explain really. The heater core hoses are located just left of the clutch master on the firewall under the intercooler. There is an upper and lower. If air gets trapped in the heater core itself the upper hose can to be disconnected and then you would bleed the air out from that point (coolant will spew out too). The downside is the hose can be very stubborn to remove and you'd have to remove the intercooler too.

You can tell if air is trapped in there because at idle the heater will blow cold or luke warm, but when the engine is revved the air will get warmer just as you stated in your first post. It's also exactly what the Forester was doing and why it was in the shop. Though it also needed an oil change which was way over do on a new engine because the owner didn't want to stop when the radiator started to leak and ended up cooking the original engine, but that's a whole other story.

Back on track... The problem is the hoses going into the core are slightly higher then where they go into the block. Since air rises to the highest point it can get trapped in the core. I guess when filling the system initially the air can be pushed out because the hoses are not yet completely full of coolant and the upper reservoir is higher then the heater core. After the system is full though the air can't escape through the hoses and becomes trapped. So the only way to get it out is either drain the system down and start over or pull the upper heater core hose and bleed the air out from there. Both ways are a bit messy, but the latter method wastes less coolant. Unfortunately though there's more mechanical work involved since the intercooler has to be removed which isn't all that easy.
 

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Discussion Starter #7
Thank you for the elaboration. I have a fmic set up so it should be much easier. I'll take a look and let you know how it went. Thanks SubySal!:tup:
 

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Discussion Starter #9
I went out this morning and tried the first method that you told me about. I chose this one instead of the heater core way because after looking at the routing of the hoses for the heater core, the hose would always have an air pocket towards the end of the hose. So anyways, I used the first one that you had explained.

After completing the fill I noticed a bubble per second after starting the car. I shut the car off and popped the turbo tank reservoir and a small woosh of air came out. I put the cap back on and started the car once again. No more bubbles.

I let it idle, gave it a few hot/cold cycles for the fans and fluid to work its course. I then took it for a ~25 mile drive. I didn't romp on it or anything but I did try to keep the rpm's up so that I could work the engine a bit so that maybe if it were a gasket issue it would be exploited that way.

I returned from the drive, got a car wash to get rid of the overflow of coolant from the underbody and just to get the grime off the car, so far so good. Needle stayed right where it should. In the car wash bay I put the AC on high and the needle actually dropped so that the black part of the needle was resting on the 1/3 mark on the temp gauge. From what I have read this is the exact opposite of what it would do if there were bigger issues than air in the system. So I finish my ride home and needle stays at normal operating temp. Once I parked I checked for bubbling- Nothing. Let it idle for a bit still no bubbling. So thank you for your assistance Sal:tup:!

Now my question for you is what are some things I should be watching for in the future relating to HG failure? Also, are there any metal nipples that have welds that may have weakened with age? - Just a reminder 116kmi, and has been in Connecticut its whole life-
 

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I'm glad your car is doing better. I haven't experienced HG failure (and I hope I never do) so I don't have any first hand info on that. There are some basic signs like oil and coolant mixing turning the coolant a nasty brown color. Coolant can also smell like gas if the gasket fails between the combustion chamber and water jacket. I'm sure there's a great many threads here that go into detail about every kind of HG failure.

As far as the second question. I don't know. I live in CO so rust is not an issue. My car only has 40,000 on it, but I just went through the entire car for my STU build last year and didn't see anything like metal parts weakened by age. The car had a rougher life then I had originally thought too.
 

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Discussion Starter #12
After driving my car hard I noticed that the bubbling has returned and coolant is overflowing my overflow tank. This overflow tank is a powerade bottle with a hose going from the cap to about 1" off of the bottom. If there is air in my system, will the overflow aid in releasing the trapped air?
 

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I'm not sure about a powerade bottle, but trapped air generally doesn't overflow the overflow. It usually bubbles out air then sucks in coolant until all the air is bled out. Overheating or a bad cap that can cause a spillover though. In the STI's case having the caps reversed can cause issues too.
 

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Discussion Starter #15 (Edited)
After some thought, I came up with a good theory I think. Since I have a larger radiator, shouldn't I need larger overflow tank that can handle the excess coolant that the larger radiator holds?

Also, is the overflow bottle supposed to be air tight as well?

Edit: found this link (Coolant overflowing - NASIOC)
The last post by Element Tuning covers these two questions.
 

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Hello everyone
I'm new to the forum and this question may have been answered but my 05 Sti started to overheat and started dumping coolant out of the passenger side front of the motor any thoughts? It looks like a little faucet pooring out of the plastic cover on the bottom left facing the engine
 

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Coolant bypass pipe rusted and leaking would be my first thought.
 
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