Car: 2005 STi, Cobb stage 2 93 octane, 81,000 miles. The only thing not stock about my cooling system is the hoses running up to the upper tank that Cobb supplies with the SF intake system. Coolant was flushed at 60K miles and again at 75K. 50/50 mix of clean, pure water and Subaru genuine full strength coolant.
I took the car on a ~500 mile drive yesterday on some awesome roads, I did a lot of high engine load driving (not high speed, just high RPM's/staying in the "power band" of 2nd, 3rd and into 4th. When we got the the end of the first leg of our route, my car smelled like coolant so I checked it out and saw that the overflow bottle was overflowing. I thought nothing of it because I figured it would get sucked back into the radiator, it gets full from time to time but the level always goes back down. We take an easy drive up to the next portion of our route (about a 30 minute break from running the car hard) and I start getting on the car again. Everything is normal but I notice the temp gauge starting to fluctuate. I turned the heat on and it didn't really make a difference so I pulled off on the first safe spot I could find to see the coolant overflow bottle was still at the top and overflowing slightly.
Sounds like I'm lifting the head(s) and combustion gasses are pushing the coolant out of the engine, right? I've seen the threads on here and on the other forums, it all seems to match up (no bubbles in my overflow bottle though?)
I took both caps off (the system was holding pressure, I had to use a towel and very carefully let the radiator vent before I could take the cap off). I added about 3/4 of a gallon of water, let the car run so the system would burp the air out, got back to the highway, checked the levels again and everything was fine. I then proceeded to drive 175 miles home with zero issues whatsoever. Temp gauge sat right above the first line like it always does, motor still pulls hard, no misfires, oil is clean and clear or any signs of coolant, coolant level is stable, the heat works, and the overflow isn't overfilling anymore.
If the HG was blown, it wouldn't just get better. I would think if I stretched the head bolts and lifted the heads it would still be showing signs of that as well? I'm at a loss here. I work at an auto parts store (not your typical store, the guys I work with and myself actually work on cars and know how to solve problems rather than stare at a computer) and one of the guys suggested that I may have boiled coolant, which would pressurize the system and force coolant out. It makes sense in theory but I don't know if this is possible or not. One of the other guys said I may have lifted the head and then it re-seated, but that doesn't sound right.
I just don't understand how I can be pushing coolant out of the system/overheating and an hour later drive 3 hours home on the highway at varying speeds and RPMs with absolutely no problem. I don't ever drive the car hard and the first time I do it does this. Maybe my constant "babying" of the car is the problem? I'm picking up a cooling system pressure tester and I'm going to send some oil out for analysis to see if there's any traces of coolant chemicals in it, but if anyone has any ideas or suggestions on other things to check out I'd really appreciate it.
I think I may have found the problem. If I did, it was an incredibly easy fix, but I'll also kick myself for making such a dumb mistake. Hopefully it truly is this simple though.
I was looking at my dad's Saab 92-x Aero, as he thinks he has a cooling system leak as well. I was looking at his radiator to make sure the cap was the same as mine so I could use the same adapter on it when I realized his caps were installed reverse of what mine are. I then proceed to Google search "STi engine bay" and lo and behold my caps are on bass-ackwards. The 1.1bar cap is on my radiator and the 1.3bar cap is on my upper tank. Best part of that? They've been like that since I flushed the coolant on July 6th, 2011 (checked my service records to see when I did it).
So follow me on this one: With the lower pressure cap on the radiator, it would vent coolant out to the overflow sooner. The boiling point of the coolant in the radiator would also effectively be lowered, as I understand it, because higher pressure raises the boiling point. Assuming that coolant was venting too quickly from the radiator and its boiling point was reduced, I could have boiled coolant in the system which would have pressurized it. This would explain why the system was still pressurized when I removed the caps after it started overheating. It would also explain why the car was perfectly fine after I refilled the radiator and drove it home like I normally would drive it, and shows no other sign of head/gasket failure or damage.
Again, let me reiterate that I do not drive my car hard on a daily basis, so the coolant probably never got hot enough to cause this issue before, even though the caps have been switched for over a year now. If you guys knew how I normally drove my car you'd take it away from me and tell me I don't deserve it because I'm forcing it to live a sad, miserable life. I drive my 93 Impreza L 100x harder than I drive my STi, mostly because you have to drive it hard to get it to go anywhere, but also because I'm not worried about anything happening to it.
Anyhow, I'm still going to look into some of the easy things to check that you all have suggested and take the car out to try to replicate the issue, but I think I may have already found it.
What do you all think?