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I started a thread about a week ago regarding an overheating problem I've been having. The consensus seems to be that it's the head gasket, but the symptoms have changed a lot since my original post and I want to make sure that the problem couldn't be caused by something else before I go ahead and replace the HG. Here are the symptoms:

 

'98 Outback, 2.5L DOHC. Problem first encountered after day of heavy driving in hilly environment: coolant seems to have overflowed from coolant tank & car overheated after coolant level dropped below critical level. After filling the radiator with water, we were able to drive ~30 miles home without incident. Took the car to our mechanic, the radiator passed the pressure test & there was no sign of oil in coolant. Since then, the problem has gotten worse: now, soon after the engine has warmed up, the radiator starts gushing coolant. Not only is it overflowing from the resevoir, but I have also detected a leak on the left side of the radiator which seems to start an inch or two below the bleeder plug. I have observed yellow particles in the coolant (approx. 1-3 mm in diameter) as well as a few larger black flakes (approx. 3-5 mm in diameter). Finally, I have seen large bubbles (1-2 cm) rising out of the coolant resevoir every 5-10 seconds.

 

I know it's probably a head gasket problem, but here are my questions anyway:

 

1. Why does it seem to be leaking underneath the bleeder plug? Could this leak somehow be causing the problem? Alternatively, could this leak somehow be caused by a blown HG?

 

2. Could the problem be caused by a bad water pump, as in the following post?

http://www.ultimatesubaru.org/forum/showthread.php?t=34301&highlight=particles+coolant

 

3. What could the yellow & black particles be caused by?

 

4. Could the problem be caused by either a bad thermostat (my mechanic put in a Stant instead of an OEM part), a bad radiator, or air in the radiator?

 

5. Finally, if it is the HG, could someone explain to me how such a leak could cause the radiator to force fluid out of the resevoir tank? I'm still a bit unclear on the mechanics of it all.

 

Thanks in advance for your help,

Chris

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1. Why does it seem to be leaking underneath the bleeder plug? Could this leak somehow be causing the problem? Alternatively, could this leak somehow be caused by a blown HG?

 

The pressure generated by exhaust gas getting into the cooling system could be responsible for that leak. Maybe there was already a small crack that got bigger and/or wider

 

 

5. Finally, if it is the HG, could someone explain to me how such a leak could cause the radiator to force fluid out of the resevoir tank? I'm still a bit unclear on the mechanics of it all.

 

Exhaust gas gets into the cooling system and push the coolant out into the overflow reservoir.

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4. Could the problem be caused by either a bad thermostat (my mechanic put in a Stant instead of an OEM part), a bad radiator, or air in the radiator?

A few years ago, there was quite a lengthy post on an aftermarket thermostat. The guy tested the aftermarket unit and a new OEM one in a pot of hot water and found them to act somewhat differently as they opened up. His troubles were not resolved with the aftermarket unit. When he put in the OEM unit, his problems went away. (Sorry, I don't recall just what his problems were, or what brand the aftermarket unit was.)

 

More concerning though is that I've heard of aftermarket thermostats which will strike against the housing as they open, therefore, not opening fully. The OEM unit has the valve offset. It is not centered. The thermostat goes in in one position only. (There is a small notch for alignment.)

 

I'm doubtful that the thermostat is your only problem however, given all your symptoms. But, I would highly recommend that you put an OEM unit in just to rule out potential issues with the Stant unit.

 

Commuter

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