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Bubbles in coolant res

I have a Subaru outback and today I was doing a little check over the car. It’s been running great but I always like to just give it a little look.



Well today I noticed that the coolant level in the reservoir was higher than normal. So I popped open it’s cap while the engine was running and I noticed bubbles appearing in it. The car has been running great no overheating or anything like that, I am just curious as to what is going on. I also waited until the car cooled then I opened the radiator cap and noticed that the radiator fluid was just a little low, there was a small gap between the cap and the fluid level. I’m not sure if there is just a bubble or what is happening. I am asking this question as I know Subaru’s have headgasket issues and I hope that it not what is happening.

Well there have been a lot of Subarus that have had head gasket problems. Any good garage can test for exhaust gasses in the coolant.

I hope I am wrong.

This definitely could be a sign of a breached head gasket.

Look carefully at the coolant overflow container to see if there is a dark line indicating the highest level that the coolant reached in the container. If you see this, that dark line is motor oil.

I also would suggest that you check the dipstick for evidence of coolant in the oil. If you observe that the oil has even a slightly “milky” appearance to it, rather than the “normal” color of oil, that is a very bad sign.

Even if you don’t observe evidence of oil in the coolant, or evidence of coolant in the oil, this does bear watching, simply because once the coolant begins to invade the crankcase, serious engine damage will follow shortly thereafter.

You did not tell us the model year or the odometer mileage of your Outback, so we don’t know if it is of one of the model years ('96-'02, IIRC) that are prone to this problem after ~100k miles. But, you should be aware that Subaru of America has gone “halfsies” on repairs for many owners who experienced this problem–if the vehicle was maintained strictly in accordance with their maintenance schedule–and especially if it was maintained by a dealership.

Make mine another vote for a blown head basket. The bubbles are likely gasses from the combustion in the cylinders being blown past a headgasket breech and migrating up out the system as bubbles. That’s really about the only place bubbles can come from.

Check on the coverage that VDC referenced. But whatever you find out, as a minimum do a pressure leakdown test on the cylinders. That’s a test where air is pumped into the cylinders through the spark plug holes when the valves for that specific cylinder are closed and an air pressure gage monitors the cylinder’s ability to hold pressurized air. Leave the radiator cap off and you might even see the bubbles coming up out the radiator while you’re doing the leakdown test when you hit the cylinder with the breech.

Were all Outbacks prone to headgasket problems or just certain years? The OP does not specify a year model.

I did not see VDC’s post.

Well the problem gets more interesting now. I took the car in and had the system pressure tested and it passed without a problem at all. So again I am back to scratching my head a little as to what is going on.

The car is a 98 outback, bubbles in the coolant reservoir while running. No over heating, no oil in coolant reservoir and no coolant in the reservoir.

Potential problems already looked at

  1. Radiator cap: Replaced with new cap from dealership (2 weeks ago)
  2. Pressure test on system: Today and system passed

Suggested problems from mechanic

  1. Thermostat not opening or closing correctly changing boiling point of coolant (Thermostat was replaced 1 month ago, and after showing receipts mechanic advised no changes and actually said don’t worry about payment today)

So final word from mechanic today was that the head gasket is in working order no cracks. I am to watch the system and if I the engine over heats I am to bring it back in and he will look at it all again.

What does everyone think?

It’s still a head gasket leak … pressure test won’t show it up, you need an “exhaust gas in coolant” test.
How can the mechanic see that there are no cracks in the head gasket without removing the head?

I’m glad everything checked out. Bubbles could be normal or a sign of exhaust gases blowing by a bad head or head gasket. Since you passed the pressure tests your bubble were normal.

The purpose of the expansion tanks is to catch coolant and retain it due to expansion as the coolant warms up. If you checked over you car while it was idling and still warming up you noticed the coolant was expanding. Once the motor is hot then the expansion stops and you’ll see the coolant at the “hot” mark on the tank. When the car cools the coolant is sucked back into the radiator as the fluid contracts and when cold the fluid should be near the “cold” mark on the tank. A bit of air space just below the radiator cap is normal because the overflow tube is a bit below the cap. When the motor is hot you don’t want to pull the cap due to the pressure will shoot hot fluid on you.

So, you likely pulled the cap when the car was cold and noticed the air space. If you can’t see any coolant and inche or so down, that would be a problem. Everything is fine. You just happend to be looking at the right time to notice the bubbling due to the expanding coolant.

A pressure test of the cooling system will not show a headgasket leak. That is a different test entirely from the one I suggested of testing the cylinders for their ability to hold pressure. A pressure test of the cooling system looks for leaks in the cooling system. In order for the low pressures used in a cooling system to force coolant back through a headgasket leak the breech would have to be severe indeed, and then you wouldn’t see it because the coolant would go into the cylinder. Combustion forces are far higher than that produced in a cooling system pressure test.

I still suspect a headgasket leak. Unless I misunderstand and he actually did a cylinder pressure leakdown test.

If my understanding is correct and he only did a cooling system pressure test you may want to try another shop.