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Discussion Starter · #1 ·
Hi all,

2010 2.5l Subaru Outback with around 90k miles on it (had in since around 50k). At the end of a 7 hour road-trip a few months ago the car overheated, blowing coolant everywhere (seemingly out of the top of the overflow tank). No signs of a leak anywhere.

Turns out the fuse was blown on one of my radiator fans. Replaced that and on advice of the mechanic replaced the entire fan as they said the bearings were going out.

Fast forward a few weeks, overheats again, again blowing coolant everywhere after a 2 hour drive on a fast highway in hot weather (AC on for most of the trip). Take it back in, they detect a leak in the water pump. Seeing as I'm approaching 100k, I have them replace the water pump, timing belt, thermostat, radiator cap. Ran multiple block tests to look for a head gasket issue (all negative), checked temp of coolant after running it hot for awhile and they said it all checked out. Very confident that the issue is resolved.

Fast forward to today, overheats again on a 2 hour trip (at the very end), blows coolant everywhere, overflow tank is full to the brim and bubbling out/over. Call the mechanic and now they're saying it might be the coolant temp sensor is bad and prevented the fans from working properly. The fans are running as soon as I turn the car & AC on so I can't imagine the fans are not running when it's on the highway.

As far as I know this mechanic is extremely reputable. There have also been 3 separate mechanics by now that have ran tests to determine if it is a head gasket issue and have given the all-clear. I just spent $1200 on the water pump, timing belt and fan replacement, sill overheated today.. really have no idea what is going on and afraid that I'm doing lasting damage to the engine from all of this overheating.

Any advice before I go insane?
 

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2006 OutBean, 2005 LGTW
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Check your radiator for an external blockage before you do anything else. Pull the two brackets at the top of the radiator and push it forward to look between the radiator and the back of the ac condenser. Is there a bunch of **** in there?
 
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2006 Saab 9-2x 2.5i N/A M/T
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What he said. ? So what does the dash gauge read during all of this? Normal till it overheats? Steady climb? Or is it always high?
 

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'10 3.6R Outback Limited, 2zr swapped Toyota Yaris track toy, '12 Mazda3 skyactiv
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Test your rad cap! Seriously or just replace it with a new oem. You will blow coolant out of your overflow tank if your rad cap doesnt hold enough pressure. I learned this the hard way on the track due to a faulty aftermarket rad cap.

And as other stated check for proper airflow by looking for obstructions
 

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What he said. ? So what does the dash gauge read during all of this? Normal till it overheats? Steady climb? Or is it always high?
No gauge in that model. Only a light. Blue when cold, red when hot and blinking red when it's cooked.
 

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Discussion Starter · #6 ·
Test your rad cap! Seriously or just replace it with a new oem. You will blow coolant out of your overflow tank if your rad cap doesnt hold enough pressure. I learned this the hard way on the track due to a faulty aftermarket rad cap.

And as other stated check for proper airflow by looking for obstructions
I did replace the radiator cap and thermostat after the first overheating incident. It was not with OEM parts though so the mechanic has put new OEM parts in today. My only concern here is that the issue happened originally with the original rad cap / thermostat and it would surprise me if the exact same issue happened after replacement, regardless of manufacturer.

Check your radiator for an external blockage before you do anything else. Pull the two brackets at the top of the radiator and push it forward to look between the radiator and the back of the ac condenser. Is there a bunch of **** in there?
Thanks for this tip, the car is at the shop right now so I will call and have them check it out - they seem as lost as I am though so they may have already looked into that.
 

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Discussion Starter · #7 ·
No gauge in that model. Only a light. Blue when cold, red when hot and blinking red when it's cooked.
Probably not worth harping on this but despite not knowing much about cars I really don't understand why they would replace a temperature gauge with a MPG gauge in this model...

It's like having a fuel light that just flashes when you're near empty instead of a proper fuel level gauge. Ridiculous
 

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Probably not worth harping on this but despite not knowing much about cars I really don't understand why they would replace a temperature gauge with a MPG gauge in this model...

It's like having a fuel light that just flashes when you're near empty instead of a proper fuel level gauge. Ridiculous
Agreed. I'm a big Subaru fan but I think the decision to exclude the temp gauge was foolish.
 

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19 2.5i OB LTD w/SSD Strt Twr Brc + OEM 19mm RSB
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Wow, you've replaced everything but the... radiator. Maybe the radiator is blocked? Have someone rev the engine, while you squeeze the hose. If the radiator is clogged, the hose is going to get stiff because of the restricted flow and thus pressure buildup.
 

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Discussion Starter · #10 ·
Had the mechanics check the radiator, they said their best bet for determining the issue might be to just run it hard and take it immediately to the shop for tests once it boils over because they've been chasing a ghost.

Ran it today, got it to boil over, did tests and they're saying that it is likely a head gasket issue (they said that the compression test liquid SLIGHTLY turned less blue / from "royal blue" to just blue, if that makes sense).

My only question now is, if it is a head gasket issue, should they have caught it by now? Three mechanics have ran compression tests (when engine is cool) and said that it is definitely NOT a head gasket issue. Is it possible that they only would have been able to see the head gasket issue if the car was very very hot, and even then only barely see it?

Does the car only overheating after driving it hot for 2+ hours sound like a head gasket issue? I always assumed a head gasket problem would be more noticeable
 

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Yes a head gasket issue can be intermittent in the beginning. What happens on these is usually that the combustion gasses get pushed into the coolant a little at a time. A pocket of non liquid forms in the cooling system, pushing a lot of coolant into the overflow bottle. This process continues of pushing coolant out, replacing it with combustion gasses, and raising temps the longer you drive until it reaches a boil over and it could take some time to get to that point. Its a shame there is no gauge but I guess that is what it is. These can also be hard to confirm or rule out with just a compression test unless you happen to have a large leak, so I think that telling someone that had these symptoms that "its definitely not head gaskets" might have been a little premature. Compression tests and leak downs occur at relatively low pressure. The gaskets may seal fine at a couple hundred psi. After ignition on the power stroke however, cylinder pressure can rise many hundreds or even to 1000 psi. This is where the combustion gasses "fart" past the gasket at high pressure into the cooling system.
 

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Wow - that is very interesting and very educational.

Two questions: (1), as gasses enter the system, might they pool under the thermostat and prevent the thermostat from sensing the temp of the coolant to open and close properly?

(2), doesn't someone make a pressure monitoring kit that one could add to the system? I imagine the sensor to be inexpensive. The trick would be, what do you hook the sensor to? There must be multimeters like this one with a USB interface for hooking to a computer, if you needed to capture data:
 
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If combustion gasses were getting into the cooling system there would likely be numerous pockets of "air". They could be anywhere including around the thermostat as they are circulated through the system by the water pump. Much of it would try to get to high points in the system like the radiator cap where it would then be pushed into the overflow. Both bubbles and liquid will be pushed out, whichever happens to be at the rad cap at that moment. If it gets to this point the system would be hot enough that the thermostat would probably remain mostly open anyway.

The problem with monitoring pressure is that a functional radiator cap regulates the system pressure. If you have a 16 psi cap for example, anything over that pressure will be vented into the overflow tank, so even if combustion gasses are being pumped into the cooling system it all goes right back out the rad cap and the pressure stays the same. About the only thing monitoring the pressure would tell you is if your rad cap is working properly.

In this case, being able to monitor the temperature would be much more useful than the pressure.
 
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Oh, I see thank you again for taking the time to explain what is likely happening.
 

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Discussion Starter · #15 ·
If combustion gasses were getting into the cooling system there would likely be numerous pockets of "air". They could be anywhere including around the thermostat as they are circulated through the system by the water pump. Much of it would try to get to high points in the system like the radiator cap where it would then be pushed into the overflow. Both bubbles and liquid will be pushed out, whichever happens to be at the rad cap at that moment. If it gets to this point the system would be hot enough that the thermostat would probably remain mostly open anyway.

The problem with monitoring pressure is that a functional radiator cap regulates the system pressure. If you have a 16 psi cap for example, anything over that pressure will be vented into the overflow tank, so even if combustion gasses are being pumped into the cooling system it all goes right back out the rad cap and the pressure stays the same. About the only thing monitoring the pressure would tell you is if your rad cap is working properly.

In this case, being able to monitor the temperature would be much more useful than the pressure.
Thanks everyone for the great information in this thread. I just have one more question, I browsed through the main head gasket issue channel without seeing much opinion either way.. Given that the car has overheated / boiled-over 4 times in the past few months, Is it likely that there was additional lasting damage done to the engine in some way? I am basically trying to determine if getting the head gaskets replaced (as well as the water pump, timing belt, radiator fan etc that I had done already) will put me in the clear for another 100k miles, or if I should expect further complications down the road as a result of this whole experience. If that's the case I may just sell the car as I don't owe anything on it at this point.

Best regards
 

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Discussion Starter · #16 ·
I should say that the mostly likely sequence of events is this:

7-8 hr drive with a blown fuse on one of my radiator fans likely caused the first overheating

This overheating was pretty extreme and probably damaged the head gaskets

All subsequent overheating was caused by the head gasket issue injecting exhaust into the system but only noticeable after 2ish hours of driving
 

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These little motors are pretty robust in general, but head gaskets are one of their weak points. Large numbers of them have overheated due to the gaskets failing and went on to drive many problem free miles and years once repaired. You were getting close to the timing belt water pump interval anyway. If its the gaskets, which seems likely, it will probably ok. There are no guarantees though. I'm wondering if the issue with the fans was the original culprit here causing it to run hot and develop a gasket leak. No way of knowing. If it were mine, and I liked it, and it was paid for, I would have it repaired. Think of it as investing what you would have spent on your first few car payments and continue to put miles on it
 

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You beat me to it lol. That seems like a probable sequence of events.
 

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And just think, a simple coolant temperature gauge might have prevented most of this. Thanks Subaru!?
 
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