Amateur Fix? for 2.5L HG fix. - Subaru Outback - Subaru Outback Forums
 
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post #1 of 4 (permalink) Old 06-07-2010, 05:39 PM Thread Starter
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Talking Amateur Fix? for 2.5L HG fix.

"Subaru located the thermostat at the lowest point of the Cooling System. This causes residual air pressure to become trapped in the upper-end of the cooling system. Consequently, when any accumulated air pressure is not regularly bled from the cooling system it will eventually far exceed the systems designed PSI tolerance and find the weakest point to vent. In the 2.5 liter version of the '96-'04 EJ25, that weakness has proven to be at the HG between the coolant journal and exhaust port."

So Two stupid questions for anyone to answer.
1. Would it be possible to relocate the thermostat above the pump? and would this fix the issue?
2. In warmer climates would removing the thermostat all together fix the issue as well?

Just brainstorming since I just bought a 2002 OB 2.5L

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post #2 of 4 (permalink) Old 06-07-2010, 06:22 PM
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[QUOTE=Valentine;243627. . . So Two stupid questions for anyone to answer.
1. Would it be possible to relocate the thermostat above the pump? and would this fix the issue?
2. In warmer climates would removing the thermostat all together fix the issue as well?

Just brainstorming since I just bought a 2002 OB 2.5L[/QUOTE]

No, not stupid questions. And No, to both.

The pressure in the cooling system should never get high enough to blow the HG between the cooling jacket and the combustion cylinder. This is because the pressure in the cooling system is regulated by the radiator cap, which for most Subarus is around 13 psi. Any build up of air in the cooling system will not cause the cooling system pressure to exceed the psi of the cap (provided the rad cap is working properly).

Moreover, the highest physical point in the engine cooling system is the cross-over pipe which connects the tops of the cooling jackets on the left and right sides of the engine together AND to the top of the radiator. So any air trapped at the top of the cooling system will eventually be pushed out to the radiatior (which is in fact yet higher) and will eventually be expelled out through the external reservoir.

This doesn't mean that the HG cannot fail between the combustion cylinder and the water jacket. In this case, high pressure gases from the cylinder will be forced into the water jacket. When this very hot gas travels up to the crossover pipe, it comes in contact with the temperature gauge sensor, which suddenly spikes up -- a characteristic symptom of a failed HG cited here unnumerable times.

Air is not a good heat conductor, so too much air trapped in the cooling system, until it is forced up to the radiator, can cause localized overheating, which in turn can damage metal components and compromise gasketed joints. But the cooling system will not normally accumulate air inside, and it should not be necessary to bleed air out of the system if it has been filled and bled ("burped") properly at the outset.

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post #3 of 4 (permalink) Old 09-09-2010, 12:20 PM
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Hi...How do I bleed or burp the air from the cooling system. When the radiator coolant is hot it spills over into the radiator overflow tank but when the coolant is cold it does not return to fill the radiator. Can anyone help me

Thank you, Gary
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post #4 of 4 (permalink) Old 09-09-2010, 12:29 PM
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Towards the bottom of the first post in this DIY are the steps Subaru recommends for purging air from the cooling system. If the coolant isn't returning to the radiator, it's probably because of a bad radiator cap that no longer seals.

https://www.subaruoutback.org/forums/...hermostat.html
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