Radiator boiling?

[Zephrant]

My 2006 230k mile engine was leaking oil on the passenger exhaust header, and when it overflowed the radiator I figured the head gasket was blown.

 

Pulled, replaced all gaskets and hoses, but gaskets looked fine to me. Put it back together, burped it. Found the cracked radiator, replaced it. Drove it a few hundred miles.

 

Now, after a drive (temp gauge says in the dead middle), the temperature at the turbo tank is 100c. After 5 minutes it is 120c, and boiling in to the overflow tank. It eventually settles down, and pulls the fluid back in to the system.

 

I can idle with the turbo tank open and the funnel on it, without getting a stream of bubbles, so I don’t think my head gaskets are bad. I used the correct MLM gaskets, new bolts, and carefully torqued as per Chilton’s directions.

 

Fans are working, it doesn’t overheat when running, just boils after I get home.

 

Any ideas?

[All_talk]

These cars are designed to cycle coolant through the turbo after shutdown to avoid coking in the oil circuit (this is why modern cars don't need turbo timers). The turbo heats the coolant which rises to the turbo tank and cooler coolant from the block is drawn in the bottom, the coolant flows like this for quite a while after shutdown. If the turbo still has a good amount of heat in it some boling is a normal part of this process.

[All_talk]

Just re-read your post this morning, I don't think I've ever noticed mine actually pushing coolant past the cap into the recovery tank, but if you came in hot and the turbo was still cooking at shutdown I suppose it might. Normally there are just some gurgling/boiling sounds for a minute or two as the coolant circulates.

 

You might want to try a test, let the car idle for 5 minutes or so after driving then shut it down. With the turbo cooled down there should be much boiling.

[Zephrant]

Ok, will try that test. The overflow tank was foaming until I burped the system, now it just bubbles after driving, filling to the overflow hole. When it cools, it draws it back in ok.

 

I did add a temp measuring cap to the turbo tank, but haven’t changed the cap on the radiator yet. Maybe I should put the original back on the turbo tank.

[Keyan]

If the turbo still has a good amount of heat in it some boling is a normal part of this process.

 

incorrect, coolant when boiled is a vapor and heat can not transfer as well when its in vapor form as compared to liquid. boiling coolant is never a normal part of the process. that is why the rad caps apply pressure to the coolant system. higher pressure means a higher boiling point and if you can keep it a liquid then it actually transfers the heat properly.

[SubOperator]

Ok, will try that test. The overflow tank was foaming until I burped the system, now it just bubbles after driving, filling to the overflow hole. When it cools, it draws it back in ok.

 

I did add a temp measuring cap to the turbo tank, but haven’t changed the cap on the radiator yet. Maybe I should put the original back on the turbo tank.

 

 

 

 

Did you do the test to determine if there is any trace of exhaust gases in the coolant? Sorry but this looks like the original problem you were having before the HG were replaced.

[All_talk]

incorrect, coolant when boiled is a vapor and heat can not transfer as well when its in vapor form as compared to liquid. boiling coolant is never a normal part of the process. that is why the rad caps apply pressure to the coolant system. higher pressure means a higher boiling point and if you can keep it a liquid then it actually transfers the heat properly.

 

I don't disagree with anything you say. But its seems that under certain conditions (engine off, hot turbo) there is some small amount of localized boiling in the turbo. I have never seen it raise the system pressure high enough to vent past the cap. But I have clearly heard it on my old Turbo subies (1987 RX and 1991 Legacy SS), I've never noticed it on the Spec B but like I said, if you came in hot and shut it off I wouldn't be surprised or alarmed to hear a little gurgle from the turbo.

 

To the OP, I have had cars with slight head gasket leaks that would produce a slow stream of small bubbles in the recovery tank but never over heat in use. I would check for bubbles when cold, that's a dead giveaway for a HG leak. Or as suggested, test for exhaust gases.

 

Also, you mentioned a temp measuring cap, is it the same opening pressure as the OEM cap?

 

Gary

Edited July 8, 2021 by All_talk

[shralp]

Personal experience, I routinely see this after 20 sessions at the track once I pull into the paddock. A bit of gurgle localized from the turbo coolant tank. Can’t say I hear it much, if ever, in my stg.2 daily Outback on the street unless I’ve been wringing it out.

[Homedepotmade]

Something similar happened to me with my reservoir boiling over and my car would randomly overhead when I have my AC on when I am not moving. Eventually tt was my top reservoir cap (turbo side) was a little worn. I also noticed that Subaru changed their cap design from my original cap. So far 45min drive in traffic at 92 degrees with no issues. Best of luck.

[All_talk]

Might also be important to note that the cap pressure rating for the turbo tank and radiator are different, from memory, the turbo tank is about 15psi and the rad is around 20psi.

[Pleides]

Before you throw in the towel and call it quits on the old head gasket, go to NAPA and get a new radiator cap. Mine was original in my car when I bought it and leaked in some air and caused similar symptoms to you when I drove quickly enough or uphill long enough.

[Zephrant]

I may have to try the new radiator cap idea, or put the old one back on. The new one with the temp gauge in it is rated at 1.3 bar.

 

Twice I've let it cool down and have had to add 3-4 cups of fluid to the overflow. Not sure if that means I got out another bubble or two, or if it is overflowing on the ground as I drive. I keep it at "MAX" on the overflow when cold.

 

I'll try the test kit for exhaust gases if I can't find another solution.

[NORULZleggy]

Go get a OEM rad cap for the car and see what that does. Some times OEM stuff is the way to go. I personally do not run Subaru coolant or their additives. I run Amsoil coolant, Oil, Filters, and the rest of their stuff. I also have the big rad with stock fans. car stays really nice temp when I am driving.

[Rusty9]

the rad caps should always be replaced, they are cheap enough OEM.

[Zephrant]

I put the stock cap back on the turbo overflow, and it still does the same thing.

 

I really don’t think it is the head gaskets, but will order the test kit.

 

Any chance the turbo could be leaking in to the radiator?

[bdcvg]

Sounds like warped head/gasket didn't seal to Me. That can show as pressure in the system but not always pushing coolant into the oil.

[Pleides]

The text kit isn’t always a be-all, end-all tester. Most modern cars will leak coolant out head/block mating surface rather than always leak exhaust gasses into the coolant. You can pressure test the system to see if anything is leaking out somewhere.