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Discussion Starter #1
I have seen this overlooked by even seasoned technicians. I see tons of posts about overheating problems and even some misdiagnosed head gaskets. Unobstructed heater core circuit flow is essential to the proper operation of the cooling system. I have created a drawing to illustrate why it is essential.

For the thermostat to open, it needs to have coolant with a temperature above the opening threshold at the sensing bulb shown on the lower left of the drawing. With the exception of an insignificant amount of coolant that flows through the throttle body heater circuit, all of the coolant flowing to the thermostat sensing bulb must come through the heater core.

A restricted heater core can block high temperature coolant from reaching the thermostat bulb. Without that high temperature coolant reaching the bulb, the thermostat will remain closed regardless of the coolant temperature in the engine. Even a partial restriction that allows some flow but is cooled significantly when passing through heater core will affect the thermostat operation due to it seeing a lower temperature at the thermostat than the than the actual engine temperature.



One problem with a blocked heater core is that it presents itself with the same symptom that you may see with a head gasket that is leaking combustion to the cooling system. You will have overheating with poor cabin heater performance.

My advice when you run into a "tough-to-pin-down" overheat problem, is to always bypass the heater core to see if it eliminates the overheat condition.
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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This is very good information. I have been party to another member working through the very same above issue. The absence of air bubbles is key to this diagnosis tho.
@BioDoc did NOT have air intrusion in the system. Many of the HG / Overheating threads are wrought with air intrusion.

I think the suggestion to bypass the core as an elimination procedure is a great idea. Providing folks just don't blindingly drink a new kool-aide.
 

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Discussion Starter #3
This is definitely something to consider after you have eliminated the other more common problems.

I have seen bubbles when the engine is warm due to boiling off in the engine or if its recently been filled. Which, cars that have chronic overheats, it's never safe to assume that all of the air is purged from the system. Its often hard to tell if it just insignificant air in the system or an actual problem. I have been doing this for a long time and I haven't found the silver bullet for making an absolute determination when all you are seeing is an overheat condition and and a small amount of bubbling in the cooling system.
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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This is definitely something to consider after you have eliminated the other more common problems.
I think it is a great 1st elimination.
 

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Discussion Starter #5
Its pretty easy to do... but I might consider easier stuff like the fan operation and air flow across the radiator before I went to the trouble of opening the cooling system.
 

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2001 Forester, 1997 Legacy wagon, 2.2
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Good info thanks!

What follows is not exzackly on topic, but useful for anyone seeking overheating tips:

(correct me if this advice is not advisable)

Some have recommended drilling a small hole or two in the thermostat to ensure some water flow in case the thermostat ever goes on vacation or retires permanently.

I did it when I was still in the denial stage. Desperately ( I drilled 3 little holes) avoiding the painful truth about the head gaskets in my 97 OBW last year.
 
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