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Old 07-19-2019, 09:50 PM   #1
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Question about pressure and level in coolant

Hi guys,

I notice recently when my car is hot the coolant level in the bottle would drop from Max to Min and sometime even below Min. The coolant would stay at the dropped level even the car cooled down. But when I open the cap and release the pressure the coolant level will go back up to the initial leval (Max). I pretty sure it not behaving like this previously. I only notice it recently, when outside temp reach 34C, as I get the low coolant flashing warning. I run three times, run car hot and cool it back, same thing happen. I searched the forum and I can’t find people with similar situation. Anyone know what is happening here?

2001 S Tiptronic with "04" cap and no moisture issue

Thx

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Old 07-19-2019, 10:26 PM   #2
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It’s normal for the tank to have pressure, but it’s possible that you have a faulty tank vent/bleeder valve. Remove the tray directly under the coolant and oil fill caps and you’ll see it.


Also, make sure that you have the most recent coolant cap.

.

Last edited by particlewave; 07-19-2019 at 10:28 PM.
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Old 07-20-2019, 03:42 AM   #3
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Thx for the reply.

I changed both the cap and bleed valve few months back when I have moisture issue which solved the moisture problem.

I can understand the pressure build up in the system but I can't understand why pressure pushed down the coolant level. I suppose coolant is not compressible so it has to go some where. I am thinking maybe I have a fail thermostat so the pressure pushed the valve open.
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Old 07-20-2019, 06:56 AM   #4
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Thx for the reply.

I changed both the cap and bleed valve few months back when I have moisture issue which solved the moisture problem.

I can understand the pressure build up in the system but I can't understand why pressure pushed down the coolant level. I suppose coolant is not compressible so it has to go some where. I am thinking maybe I have a fail thermostat so the pressure pushed the valve open.
Full is full and liquid is not compressible so I don't think thermostat open or closed or failed would be the issue.
Sounds like you have air trapped in the system or older coolant hoses that are expanding as pressure in the system rises.
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Old 07-20-2019, 10:08 AM   #5
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Thx for the input the old coolant hose expanded under pressure is something I not consider but it does make sense. Geez I suppose I have lot of hoses to check now and some of them are not easy to access.
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Old 07-20-2019, 04:04 PM   #6
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Thx for the input the old coolant hose expanded under pressure is something I not consider but it does make sense. Geez I suppose I have lot of hoses to check now and some of them are not easy to access.
Yes and if you don't replicate the conditions a bad hose may be hard to locate.
It May not show it's self as swollen if not fully warmed up and under pressure.
Sometimes you can feel a soft spot in the hose when you squeeze them but that is rare.
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Old 07-21-2019, 08:47 AM   #7
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Yes and if you don't replicate the conditions a bad hose may be hard to locate.
It May not show it's self as swollen if not fully warmed up and under pressure.
Sometimes you can feel a soft spot in the hose when you squeeze them but that is rare.
Actually, this is very easy to replicate, even with the system cold. You need a coolant system pressure tester, pump the system up to 18 PSIG and start looking at the components.
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Old 07-21-2019, 09:21 AM   #8
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Actually, this is very easy to replicate, even with the system cold. You need a coolant system pressure tester, pump the system up to 18 PSIG and start looking at the components.
I had thought about that but the OP being in Hong Kong I didn't know if he would have access to something like that as a DIY.

JFP glad you added to this post your feedback is always no nonsense to the point and on target. Thank you
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Old 07-22-2019, 01:34 AM   #9
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Like most of the people in HK I don't have a garage so I can only do limited DIY. Tool is not an issue as the next door to China mean lot of cheap tools. If you wonder the quality I would say "what you pay is what you get".

I can easily duplicate the situation by driving around for 10 min to heat up the coolant. I sent it to my usual indy today and explain to him one of the possibility is the expansion of old hose under pressure. However, he pointed out to me when the hose get old it get harder . So, old hose under pressure most likely crack instead of expand.

He called me few minute ago said found leak in the transmission oil cooler. I think a leaking transmission oil cooler would mean just lost coolant but not going have what I am experiencing. Unless the coolant would go into transmission oil and when the pressure release the coolant get push back up from the transmission cooler.

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