bubbles in coolant - Subaru Outback - Subaru Outback Forums
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post #1 of 7 (permalink) Old 05-16-2019, 09:31 AM Thread Starter
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bubbles in coolant

I have a 2006 outback with the 3.0. I just bought the car. It had a cracked radiator so I replaced it. I bled the air with a radiator funnel for an hour. When I was doing the radiator I put in a new thermostat and a new cap. I did not use OEM because I was in a hurry and it is a long way to the dealer. Now I have bubble in the expansion tank and it will over flow after driving 15 miles. I used a block tester on the tank as it was bubbling and it came back with no change to the fluid. I don't see how there could be this much air in the system. Does anyone have any ideas?

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post #2 of 7 (permalink) Old 05-16-2019, 09:49 AM
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OEM thermostat is a requirement. Or, as an experiment, run without a t'stat. Amazon and on-line dealers' parts depts. sell caps and t'stats for the car. or, check here; https://www.subaruoutback.org/forums...des-vip-s.html

The car may have blown a headgasket that 'cracked' the radiaotor. Or, the cracked radiator led to an overheat event that ruptured the HG....might never know.

clogged heater core could be an issue.

The chemical block test seems to be unreliable for a lot of folks. You could try sending a coolant sample to Blackstone labs. they can look for combustion chamber products.

It can be tricky to fill Subarus. You might try filling the block through upper radiator hoses and cycling the engine warm-cool-warm-cool a coupla times with its nose up on ramps while assuring the overflow tank doesn't go empty. You can search for other posts about tricks for filling and modifying the cooling/overflow system.


PLENTY of posts about cooling issues.


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post #3 of 7 (permalink) Old 05-16-2019, 10:28 AM
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Replace tstat and cap with Subaru parts. Cheap 1st step. Bleed system of air again.

That said, an cracked radiator if it lead to an overheat could have popped the HG. If this is the case, it is cheaper and easier to drop in a used JDM engine than do a HG repair on the H6.
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post #4 of 7 (permalink) Old 05-16-2019, 10:45 AM Thread Starter
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Could the stat or the cap be what is causing the bubbles, and overflow issues?
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post #5 of 7 (permalink) Old 05-16-2019, 11:10 AM
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Yes, an aftermarket cap could not be fully sealing. This would allow air to enter the system.

Aftermarket tstats are typically a significantly different design than the OEM (see multiple pictures here on the forums) and wouldn't cause air entry, but may not allow it to purge well.

Have you considered the Google search bar at the top of the forum?

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post #6 of 7 (permalink) Old 05-16-2019, 11:27 AM
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Originally Posted by marc3800 View Post
Could the stat or the cap be what is causing the bubbles, and overflow issues?
It could, but it's probably not.

Seems pretty obvious the previous owner dumped it knowing they had a big debacle on their hands and you bought it with existing headgasket issues.

But let's assume I'm wrong:

First - let's clarify "causing bubbles" might be:

The stat and cap are not causing bubbles to be *introduced into cooling system full of coolant*
The stat and cap could potentially cause overheating which causes "bubbles" by boiling.

We need to determine if the bubbles are being introduced or an artifact of overheating.

1. top off the coolant in the radiator and get all the air out
2. monitor coolant loss - check the radiator coolant level every time before you drive and see if you're loosing coolant.

The goal is to see if the radiator level is dropping without overheating.
Once it overheats then there's no telling how it lost coolant since it may have pushed out, boiled out.

If it's constantly streaming bubbles from a cold start and you can't burp it - then you have a leak or headgasket failure.

If you finally get it topped off and full with no more bubbles and the coolant level drops each day in the radiator - you're loosing coolant somehere. Find the leak, or it's consumign it through the headgaskets.

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post #7 of 7 (permalink) Old 05-16-2019, 11:29 AM
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your car is basically an unknown - by using outside variables like incorrect thermostats, it is almost impossible to diagnose it. Plus, we aren't there so, we often tell folks to start with a 'base condition' as close to nominal as possible. Repair broken stuff, get proper parts and fluids installed, and go from there. It does occasionally mean a little 'good money thrown after bad', but, what else can we do?

the oem t'stat has a larger wax capsule, likely meaning it is slower to open, but slower to close. Aftermarket caps, as long as they e the correct pressure rating and well-made, may be less of an issue. But, I have personally had a Motorad cap fall to pieces (I was idling in a lot waiting for my wife when our 03 started to over heat. Broken cap had been allowing coolant to evaporate - unknown to me). And motorad or rebranded motorad seems to be all that is stocked by the parts stores - even Stant seems to be special order.

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