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Old 06-26-2014, 08:28 AM   #1
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Coolant Tank- not stupid, just ignorant about some things

I and my indie mechanic are chasing a small coolant leak under the right rear.

I would like an explanation of the Boxster cooling system that an overeducated farm boy can grasp.

The tank in the trunk is pressurized- pretty obvious

Since it is under pressure, if that tank has a leak, the leak should occur all the time if the engine is warm and you will have a wet trunk, yes?

However, if you have a bad or incorrectly seated cap, it will dump a load of coolant AFTER you shut off the engine (personal experience). what is that all about? Why not as you are driving a warm car? Is that a feature Porsche added so it will only dump fluid after you park it (I am being a little sarcastic here)

There is also a small black object with a relief valve on it. What is the point of that. If open, is it depressurizing the whole system or just the coolant tank (assuming the coolant tank is somehow isolated from the rest of the cooling system while the car is warm and running)?

I have heard on some blogs, that some people keep this valve open for a few days to "burp" their system. If that is also part of the pressurized system, wouldn't it bleed pressure and leak fluid all the time (after the car get warm?

Anyone who can enumerate (an eighth grade word- thanks Mr. Stuckey) and explain the various parts of the cooling system in the trunk and what a failure in each of them looks like would be a hero to many, or at least to me.


Last edited by RBrummer; 06-26-2014 at 08:56 AM.
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Old 06-26-2014, 12:29 PM   #2
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I have had the overflow on parking issue.
Cause was that I overfilled the coolant level and so out came the extra as I was parking. Problem has never recurred.

The pressure valve next to the fill cap allows the expanding when hot coolant to expand into the coolant reservoir thereby expelling any air that might be in the system.

These expansion pressure valves have been known to leak as the seals get older and the leakage goes down the overflow tube by the right rear tire.
You will see moisture only on the trunk lid above the Cap area if either the Cap or the Pressure valve are leaking, also a crusty residue will form around the pressure valve.

The Coolant Caps have also been redesigned so you want the latest version.

If you don't see any overheating issues and the puddle is by the rear right tire, I would suspect the Pressure valve or the coolant cap.

The coolant cap is very easy to screw down incorrectly and leave a gap so very carefully re-secure it making sure it looks perfectly parallel to the opening.

A coolant tank leak would leak when the tank gets pressurized and forces the crack open and the coolant will end up in your trunk in that scenario.

Just because the engine is warm does not always mean the tank is pressurized as the expansion valve seems to be the gateway to the coolant tank and is not always open.

Hope that helps.
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Last edited by jb92563; 06-26-2014 at 12:33 PM.
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Old 06-26-2014, 04:58 PM   #3
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Thanks for the info
Cap is new and just replaced the valve
Will keep looking
I had the same massive coolant dump under the right rear due to a loose cap.
Still do not understand why it waited to leak with a bad cap until I parked the car
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Old 06-26-2014, 06:54 PM   #4
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Doesn't water expand when it cools? How long after you stop is the "dump"?
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Old 06-28-2014, 07:30 AM   #5
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"Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink. The liquid contracts as it is cooled; because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other. Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed.

Water is one of the few exceptions to this behavior. When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%."

Looks like the water/coolant level should go down as it cools unless it is near freezing outside but then antifreeze probably affects this behavior.

I was curious when you asked if water expands as it cools so I did a little research. I knew that it contracted when it cooled and expanded when freezing because of the crystalline structure of the molecules but didn't know what at what point the cooling water started to expand.
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Old 06-28-2014, 07:42 AM   #6
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If water didn't expand when it freezes, ice would not float, think what that would mean to ocean and lake water levels in the winter.
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Old 06-28-2014, 10:32 AM   #7
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when the engine is "running", the radiator is cooling the coolant. when engine is turned off, the heat built up by the engine will continue to heat the coolant but with the water pump off, there is no cooling interaction with the radiator.
if the system is running at pressure just under the relief point, with little bit of heating, the pressure may be exceeded. my $0.02 worth.
also water is not compressible, it changes stated at the liquid/solid temperature. with coolant added, no idea
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Old 06-28-2014, 02:18 PM   #8
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Sb01box is right. The water heats up after shutting down the engine and then exceeds the ability of some part to hold the pressure,

I did a little troubleshooting this morning and a few minutes after I shut her down she dumps some coolant so now I know the timing

It is coming from the drain built into the black top of the tank so it has to be the cap, the relief valve or some hairline crack in the top itself and not the body of the tank or it would be leaking into the trunk

The cap and relief valve are new so now that I know how long to wait for the leak i am doing another test run tomsee if I can pinpoint exactly where the fluid Is leaking

Would be simpler if I had the eq to pressurize it and look for the leak that way

Experienced owners are chuckling but maybe other novices can learn from my struggles

Last edited by RBrummer; 06-28-2014 at 02:21 PM.
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Old 08-30-2015, 01:08 PM   #9
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I'm dealing with this same issue, and this thread appears to be one of the more complete ones on the subject, so I thought I'd post here asking for an update. Any solution identified?

I too have upgraded to the newer expansion tank cap, and have tried to "burp" the system a few times. I always hear some air gurgling when I open the valve in the trunk. Mine started dumping coolant after I had the IMS done, wherein the mechanic topped off the coolant because he said it was a bit low. Probably the engine was totally cold at that point and he overfilled it.

Today the car dumped a lot of coolant at the right wheel after a 30 minute drive on a slightly cooler day. The expansion tank shows an above "full" level now. Looking at the anatomy of the expansion tank, it appears that when you exceed the "full" level the coolant just runs down an overflow hole for this purpose, which is why it always dumps in the same spot and why the trunk is never wet. I noticed that the temp gauge was pointing straight up after running the car at idle in the garage for a few minutes before I turned it off (it's usually closer to the 180 degree mark), and the fan had just come on right when I turned the car off. A couple minutes later it dumped a lot of coolant.

How can you differentiate between an old leaking valve vs. other problems?
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Last edited by tommygunn; 08-30-2015 at 01:35 PM. Reason: correcting detail
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Old 08-30-2015, 11:07 PM   #10
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This sounds like a classic air in the cooling system problem.

When you "burped" the system, did you do the following:
1) Lift the D shaped expansion valve lift ring to its vertical position (open).
2) Make sure the coolant level is on or just below the max when engine is cold.
3) Leave the expansion valve D ring open for at least 2-3 complete heat cycles - stone cold to hot with the t'stat fully open (at least a 10 mile drive).

If you did the above, there is another problem with either the valve or the expansion tank cap.
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Old 08-31-2015, 08:26 AM   #11
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Search for tool called "AirLift" .
You can bleed the system w/o it but it could take many attempts.
The Airlift/Uview threads will include lots of useful details - like elevating the rear end of the car as high as possible.
There are youtube vids on this subject.
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Old 08-31-2015, 02:54 PM   #12
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Please lift the carpet and board in the trunk and see if there is coolant there. The carpet is not always wet, as it is a good inch above the metal base of the trunk. If there's a puddle sitting there, you have a tank leak.

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