Coolant overflow tank full, overheating?

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Greetings -

Our 2003 Subaru Outback is overheating, however when I opened the
hood... the overflow tank for the coolant was full to the top and
spilling over some.

Any idea what could be going wrong here?

Appreciate the help or advice.

Re: Coolant overflow tank full, overheating?

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Welcome to the 2.5L "Blown Head Gasket" club!

Yep, you read correctly. These engines blow headgaskets
like timebombs.

Re: Coolant overflow tank full, overheating?

Can you be more specific in what your suggesting this as the issue.
I'm not sure i get that feeling as much as I feel the water pump or

At further examination, the engine runs fine... When I observe coolant
from the radiator top, I don't see it flow anywhere.  It just sits
there in contrast to the past.  I tested the radiator cap and seems
functional.  However, once the engine overheats, it pushes into the
overflow.  It would seem water is not flowing anywhere and only backs
up into the overflow tank.  Water pump? Thermostat?

Interesting as well I notice the bottom hose on the radiator is
leaking a little and could pressure be the issue with no flow?

Sorry to challenge you or ask so much...
Mechanics in the past have scammed me before... The more I know the
more I feel confident interpreting a mechanic suggests so I can save
myself time and money and actually get the right problem solved..

Thanks for the response!

Re: Coolant overflow tank full, overheating?

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It's *possible* its a stuck T-stat, but do a headgasket leak test (the one
where it looks for combustion gases in the coolant).
Water pumps usually do not fail to push water, but leak and finally grenade.
Check the upper and lower hoses to see if both are hot.
The 2.5 engines are VERY prone to "Hot spotting" if there is any air in the
system,or coolant missing in the system. This will generate steam in the
system and destroy the gaskets.
The waterpump should be replaced at the same time as the timing belt.They
are in the same location and it's actually cheaper and better to R/R them
with the belt.
Are you hearing *Gurgling* in the heater coil?(Blown gasket) Are you seeing
wild "Excursions" of the temp gauge?(Stuck T-Stat)
Squeeze the top hose when cold, it should be soft. Start the engine and see
if it gets stiff in a few minutes.If it does, its pressure from compression
and not water pressure.(Head Gasket)
Look for bubbles in the coolant, does it smell like exhaust?(Head Gasket)

My money is on the Headgasket.....

Re: Coolant overflow tank full, overheating? wrote:
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Porgy could be right, however, his diagnosis is more appropriate to a
late 90s engine. OH - do you have a 4cyl or an H6?
Somtimesa radiator cap will fail to allow coolant to flow back into the
radiator during engine cooldown. That 'should' happen even if the t'stat
is not opening. The lack of flow is probably your best observation and
likely the t'stat is 'stuck' closed. Please use OEM when replacing as
some folks claim some aftermarket units are inferior. If you DO
experience overheating that CAN of course lead to a headgasket leak.
There are tests for that as mentioned.
Was any work dobe recently that required opening the coolant system? If
air is not very thoroughly 'burped' from the system it can fail to
circulate properly leading to hot spots and overheating. Make darn sure
anyone that refills the system knows Subarus require xtra attention.


to reply, change  ( .not)  to  ( .net)

Re: Coolant overflow tank full, overheating? wrote:

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My hypothesis would be that as the engine cools, it sucks air in through the
leak hole rather than sucking the coolant back from the overflow tank.

When next you start the engine, the air trapped in the engine heats up quicker
than the coolant, and expands quickly, forcing the coolant out the easiest way
- through the radiator cap.

Next time it cools, it sucks in more air, and blows out more coolant.

After a few cycles, you end up with a lot of air in the cooling system. Air is
not as good at conducting heat as coolant, so you get hot spots inside the
engine. This often leads to blown head gaskets. (And a blown head gasket leads
to exhaust gas in the cooling system, making the problem worse.)

Having gone through this recently on a 1991 L series, I have been talking to
lots of repairers. Several of them volunteered that if a Subaru engine has
been overheated through loss of coolant, it is almost certain that the head
gaskets (and even the heads) are damaged.

Bad luck if this is the case. Make sure next time that leaky hoses are fixed


Re: Coolant overflow tank full, overheating?

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There is a simple way to test the thermostat:
* Remove it and place it in a pan of water on your kitchen stove.
* Heat the water and observe the thermostat.
* If the water boils and the thermostat fails to open, or doesn't open
fully, then it's time for a new one.
If you have a thermometer you can place in the water with the stat, you will
get a good idea of when the stat opens and closes.

Re: Coolant overflow tank full, overheating?

Yea, you could also just watch your temp gage for awhile after you start the
car for the first time that day, while driving.
The gage should rise slowly and eventually drop back down to normal, before
it hits the hot mark.
If it does, it's ok. Much easier this way, yes? :O) Of course I assume all
Subs have temp gages. All mine have, so far.

Bob Noble
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