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Super Full Coolant Expansion Tank


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11 replies to this topic

#1 Chip Hedrick

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Posted 05 July 2004 - 10:10 AM

I drove my ’98 OBW (107,000 mi) from New Gloucester, Maine, to Albany, New York, and back over the weekend (300+ mi each way). Each way was hard, fast highway driving with the AC on.



Two or three hours after getting home, I changed the oil in the OBW. While performing the oil change, I noticed that the coolant expansion tank was full nearly to the top with coolant. I had never before seen it anywhere near that full.



This morning I went out to the garage and looked at the coolant expansion tank. The car had been sitting for nearly 20 hrs, but the expansion tank was still full.



The car has never exhibited signs of over heating, either on this recent trip or before. During this trip the temp. gauge read slightly below halfway, as it usually does once up to operating temp.



Is this a sign that one or both of the headgaskets are going?

#2 theotherskip

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Posted 05 July 2004 - 12:16 PM

it can be a good indication, but not positive proof. the phase i engines leak internally. pressure from inside the combustion chamber leaks into the coolant passages. normally, the coolant is a closed, pressurized system, operating around 12-15psi. as it heats, the coolant expands, and overflows into the overflow tank. as the car cools, the coolant contracts, and a vaccum is formed, which sucks coolant back into the block from the expansion tank. if a leak is in the system, where the cooling system does not pressurize, it will push coolant out to the expansion tank as it warms, but since it never pressurizes due to a leak, it never draws a vaccum to pull coolant back. if you remove your radiator cap (with the engine cool!), you will probably see that it is low.

a good check at this point is to fill the radiator (and burp it appropiately), then take the car for about a 20 minute drive on the highway. pull over, leave the engine running, and pop the hood. check the expansion tank. if you see a steady stream of bubbles, it is a good indication of a blown head gasket, and you can have further tests done (such as test for combustion gasses in the coolant) before comitting to the repair (or getting rid of the car...)

#3 joeo

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Posted 05 July 2004 - 01:49 PM

It could also be a bad radiator cap.
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#4 theotherskip

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Posted 05 July 2004 - 02:02 PM

It could also be a bad radiator cap.


also a possibility. this is usually accompanied by a collapsed radiator hose, caused by the vaccum the cooling has caused...

#5 Chip Hedrick

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Posted 05 July 2004 - 04:52 PM

Thanks for the info about the radiator cap and hoses.



Coincidentally, I have new hoses on hand, which I intend to install tonight (along with a new t-stat, gasket, and antifreeze/distilled water mix). I knew it was nearing time to replace the radiator hoses (they’re “squishy”), so I picked up new ones last weekend but didn’t have time to install them.



I didn’t buy a new radiator cap, however. I’ll have to get one tomorrow.

I hope the problem is not headgasket related and that simple cooling system maintenance will solve it.

#6 cookie

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Posted 05 July 2004 - 07:09 PM

and we are all chanting OHHHMMMM.

#7 elwood91

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Posted 06 July 2004 - 10:01 AM

I have been having same problem with 99 gt wagon 5mt 2.5l engine. I replaced the radiator cap and it got better and was going to have radiator flushed because it was time to replace coolant and at 90k miles thats when suby radiators start to have a lot of buildup inside causing bad flow.

Now it's in the body shop due to hit and run and I'm waiting on totaled or not settlement, so I doubt I'll ever know if it was headgasket problem starting or not.

#8 chadwick

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Posted 06 July 2004 - 07:06 PM

also a possibility. this is usually accompanied by a collapsed radiator hose, caused by the vaccum the cooling has caused...


there is a good chance that headgaskets are gone or perhaps your radiator needs to be replaced doa leak down compression test on your engine that will tell you that waopuld be the easiest way instead of trying to guess

#9 Chip Hedrick

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Posted 06 July 2004 - 08:14 PM

Last evening I changed the t-stat, radiator hoses, and coolant. I also burped the system.

Drove 8 mi to work this morning and 8 mi back this evening. When I got home, I popped the hood while the engine was idling and looked in the expansion tank for a stream of bubbles. I saw none. Not a definitive test, but so far so good.

I will keep my fingers crossed and keep a close eye on the expansion tank and temp. guage.

#10 99obw

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Posted 06 July 2004 - 08:22 PM

Skip touched on this, but I will add my $0.02.

Once the car is fully warmed up drive it really hard for about five miles, stop and immediately open the hood and look in the expansion tank, while looking rev the engine to about 2500 RPMs with the throttle body. That was the only technique that would show bubbles on our car when the gaskets went, and doing that the coolant in the expansion tank looked like it was boiling.

#11 theotherskip

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Posted 06 July 2004 - 09:57 PM

like what 99obw said, i had to give mine a good run to get it to produce bubbles. i could do stop and go traffic without any problems, but if i took it for sustained 65+ for 20 minutes or so, i would get bubbles.

hopefully you'll never see them...

#12 Chip Hedrick

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Posted 12 July 2004 - 08:38 PM

Well, so far, so good. It's been a week since I performed the cooling system maintenance and no problems so far. I've driven the car every day, including a couple of trips to Augusta and back.

I was at an independent Subaru repair shop today to get the ignition lock cylinder replaced. I mentioned the super full expansion tank incident to the proprietor (25 yrs experience with Subarus). He said it's common, and not necessarily a sign of a problem, to have the expansion tank get full to the top as the result of a long trip at highway speeds.

I hope I've dodged the headgasket bullet for the time being.




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