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Old 02-19-2010, 06:35 AM   #1 (permalink)
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Default Coolant boiling over yet temp gague reads normal temp

(1998 2ltr engine)

As the thread title said having taken the car for a long trip during the week (mostly motorway driving) and stopped in a cloud of steam as coolant was bubbling out the overflo tube on the expansion tank and spraying on the hot engine. The odd thing is that the temp gague on the dash shows normal opperating temp! From cold it rises steadily as normal then stays in the normal opperating range.

I'm confused

The gague shows the coolant is at normal temp yet it's boiling over so what's the cause? thermostat? trapped air?

Thanks in advance!

:D

*edit* having read more posts this sound really horribly like head gasket faliure. However I can see no sign of gunge under the oil filler cap so hopefully not!

*edit 2* the title says 'boiling over' but actually it probably just overflowing.

Last edited by Kelleher; 02-20-2010 at 02:22 AM.
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Old 02-19-2010, 06:50 AM   #2 (permalink)
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Quote:
Originally Posted by Kelleher View Post
(1998 2ltr engine)

As the thread title said having taken the car for a long trip during the week (mostly motorway driving) and stopped in a cloud of steam as coolant was bubbling out the overflo tube on the expansion tank and spraying on the hot engine. The odd thing is that the temp gague on the dash shows normal opperating temp! From cold it rises steadily as normal then stays in the normal opperating range.

I'm confused

The gague shows the coolant is at normal temp yet it's boiling over so what's the cause? thermostat? trapped air?

Thanks in advance!

:D

*edit* having read more posts this sound really horribly like head gasket faliure. However I can see no sign of gunge under the oil filler cap so hopefully not!
If I were you would have a dealer or garage test the coolant tank for carbon monoxide. It sounds to be that it is a head gasket failure and that the bubbles are not the fluid boiling but exhaust fumes escaping from the combustion chamber into the coolant and working their way back to the overflow.

I had a HG failure and there was zero coolant in the oil. This was confirmed through an oil analysis by Blackstone.
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Old 02-19-2010, 06:52 AM   #3 (permalink)
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sounds like a headgasket leak where what is primarily happening is combustion gas is getting into the cooling system (rather than coolant into oil or coolant into exhaust).
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Old 02-19-2010, 07:01 AM   #4 (permalink)
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I agree sounds like a HG to me too. The only thing you can hope for though it maybe it's a bad radiator cap? I honestly doubt it but you may get lucky. If the coolant is boiling at a lower temp (~100C+) then maybe the cap isn't doing it's job. If the HG was not such a common issue with Subaru's that the cap would be my fist thing to look at. Here's to hoping...
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Old 02-19-2010, 07:05 AM   #5 (permalink)
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i did assume that the coolant system hadn't been recently serviced and that it actually had a decent mix of coolant and not just plain water, and was full.

if recently serviced air can be trapped and lead to overheating. it happened to me on the first subaru i changed the coolant on. i was going through one drive cycle with water in it and it overheated on me due to trapped air.

do you check the coolant bottle enough to KNOW the cooling system was full? slow hose leaks can lose enough coolant over time to overheat a car.

always good to hope for a less expensive problem.
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Old 02-19-2010, 07:12 AM   #6 (permalink)
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Good point about recently being serviced. Air pockets and how much a bugger it is to get rid of them is reason I let the dealer do is a coolant change.
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Old 02-19-2010, 08:11 AM   #7 (permalink)
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thanks for speedy replies couple of further points,

when I mentioned about the steam etc in the first post it was defo. liquid (not gas) coming out the top of the expansion tank as it was all over the engine.

The cooling system hasn't been serviced but I have topped it up recently.

Having read further posts I've given the rad cap a clean (there was grease and two tiny bits of 'dirt' (actually looked like metal filings).

I just took the car for a 5 min run to get it up to temp. Topped up the coolant (via the expansion tank, suprisingly only needed a little) and gave the top hose a few squeezes with the rad cap off. A few bubbles came through, not loads, but some never the less. Then replaced the rad cap and started the engine to leave it idling and watched the fluid level in the expansion tank. I gave the hose another couple of squeezes and a few more bubbles came through, I then left it and didn't see any more bubbles coming through but tbh. I only watched for less than a minuet. I'll try and get a buit more time to observe it tommorow, (would I expect to see lots of bubbles of just the odd one or two? I guess it depends on revs, higher revs = more bubbles)

I guess if it is worst case scenario and HG faliure then gasses could be escaping into the system, forcing the coolant up and overfilling but I'm still bemused by the temp gague showing normal. Or is that actually proving that the coolant's not over heating but is most likely being forced out by gases?

Are there any other obvious signs of the dreaded HD faliure that I might find ?

Last edited by Kelleher; 02-19-2010 at 09:29 AM.
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Old 02-19-2010, 08:18 AM   #8 (permalink)
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I had a similar problem with my 2005 XS, except mine never blew steam. I would find the coolant surge tank too full and also found very small amounts of coolant splattered about the tank. I could even smell coolant when stepping out of the car from time to time.

At the Subaru dealer, the coolant was checked for combustion products, but none were found. A visual inspection of the exterior of the head gasket joint showed no signs of blown HG.

The coolant system was drained and refilled, as part of the dealer's 90,000 mile service and I've had NO problems since. The coolant in both the radiator and the surge tank never vary when checked with the engine stone cold.

The only thing I can think of is there was an air bubble in the system and was purged with the drain and refill.

Even with the suspected air bubble in the system, the temp gauge never varied from the "factory set" position it attains once the engine was warm. Heat was always available from the heater, too.

However, I still want to see what happens once we get summer temps back up near 100 degrees F. Hopefully nothing will happen...

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Old 02-19-2010, 10:57 AM   #9 (permalink)
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when refilling my coolant now, i leave the cap off the "tee" in the heater hose until i see coolant about to come out. then i put the cap on and continue filling. never had another incident of overheating due to air pocket.

mine has manual controls, so i know hot is really hot. not sure how the control valve on the electronic climate control works, but it should be set to hot to purge the air from the system after any service.
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Old 02-19-2010, 11:20 AM   #10 (permalink)
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There is always coolant flowing through the heater core. There is no valve in line. The heat is controlled by diverting air with flaps through (or bypassing) the core itself.
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Old 02-19-2010, 01:04 PM   #11 (permalink)
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Quote:
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There is always coolant flowing through the heater core. There is no valve in line. The heat is controlled by diverting air with flaps through (or bypassing) the core itself.
i remember reading that somewhere now. too much working on 80s toyotas with a valve.
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Old 02-19-2010, 07:47 PM   #12 (permalink)
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Replace the head gaskets.
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Old 02-19-2010, 10:53 PM   #13 (permalink)
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I was going to say radiator cap until you mentioned greasy stuff on it; definitely suspect HGs now.
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Old 02-20-2010, 02:21 AM   #14 (permalink)
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Right...

this morning I had a mate come over and hold engine revs at 3000 whilst I peered in the expansion tank. No bubbles. I then got him to rev it back and forth up to about 3500 revs and again no bubbles.

I'm really hoping this means not HG faliure :). It doesn't overheat (on the gague), the coolant doesn't seem to smell of exhaust and there's no residue on the oil filler cap. Are there any other signs of HG faliure that I could pin point that will confirm a yes or no (other than taking it to a garage to check coolant and pressure etc?)

Since topping up with coolant yesterday it's lost a little bit overnight so could it be a leak in the system somewhere letting in a bit of air and causing the rise in fluid?
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Old 02-20-2010, 04:33 AM   #15 (permalink)
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Firstly, the expansion bottle is there to allow the coolant to flow there as the engine/system heats up. Secondly, the coolant flows back to the radiator as the system cools.

My bottle is nearly empty when cool, and about half full when warm.

If the bottle is full when cool then coolant can/will flow out of the top onto the engine when the engine heats up.

I only mention this because I know of a person who thought there was an over heating issue and would fill the bottle when cold and then it would blow out the top all over the engine reaffirming thoughts of overheating.
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