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Dual STi radiator cap?
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Old 06-18-07, 04:58 PM   #21
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Default Re: Dual STi radiator cap?

Quote:
Originally Posted by snorky
your understanding of the cooling system is incorrect. the thermostat is what keeps fluid from circulating into the sytem. that is on the other side of the system. once the coolant reaches the thermostat's opening temperature("ideal" operating temperature), the thermostat begins to open.

the high pressure cap allows more pressure to be built up inside the system before it is dumped into the return (or reservoir in the case of the radiator cap). the higher pressure raises the boiling point of the fluid and reduces water pump cavitation.
quoted for it's nutritious content.
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Old 06-18-07, 05:27 PM   #22
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Default Re: Dual STi radiator cap?

hmm, I just placed it on the tank next to the intercooler/turbo since the cap there was the same shape O_o... am I too simple?

This is only picture I found so far:

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Old 03-26-14, 01:32 PM   #23
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Default Re: Dual STi radiator cap?

Gentlemen, I am appalled. I thought sure someone would provide the correct information on the cooling system, but after reading all posts it's not there.

Here is how your turbo Subaru's cooling system works:

The two caps are not only rated at two different pressures, they are constructed differently to serve two different functions.

The cap on the upper reservoir above the turbo is of a lower pressure rating AND, very importantly, has a reverse pressure valve. This cap's lower pressure rating allows it to vent pressure into the overflow hose that then goes into the overflow container. The reverse pressure valve comes into play when the system cools down, the coolant shrinks, and the negative pressure in the cooling system sucks coolant back into the coolant system from the overflow container.

The upper tank's cap is responsible for system pressurization, therefore, as well as ensuring the back and forth movement of coolant from the overflow reservoir.

NOTE: Failure of the negative pressure valve is an invisible condition... and results in the lack of a pressurized coolant system, higher and uncontrolled coolant temperature, and possible engine (head gasket) failures. Always inspect the negative pressure valve, and clean as needed. Also, periodic replacement of the cap with the correct OEM cap is advised.

The cap on the radiator itself has a higher pressure rating, it's job is to maintain system pressure, and virtually never comes into play because the upper cap's lower pressure rating means it vents to the overflow container before the lower cap's threshold is reached.

Do NOT mix the caps up. Use ONLY OEM caps.
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Old 06-02-14, 04:30 PM   #24
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Default Re: Dual STi radiator cap?

Quote:
Originally Posted by SeeeeeYa View Post
Gentlemen, I am appalled. I thought sure someone would provide the correct information on the cooling system, but after reading all posts it's not there.

Here is how your turbo Subaru's cooling system works:

The two caps are not only rated at two different pressures, they are constructed differently to serve two different functions.

The cap on the upper reservoir above the turbo is of a lower pressure rating AND, very importantly, has a reverse pressure valve. This cap's lower pressure rating allows it to vent pressure into the overflow hose that then goes into the overflow container. The reverse pressure valve comes into play when the system cools down, the coolant shrinks, and the negative pressure in the cooling system sucks coolant back into the coolant system from the overflow container.

The upper tank's cap is responsible for system pressurization, therefore, as well as ensuring the back and forth movement of coolant from the overflow reservoir.

NOTE: Failure of the negative pressure valve is an invisible condition... and results in the lack of a pressurized coolant system, higher and uncontrolled coolant temperature, and possible engine (head gasket) failures. Always inspect the negative pressure valve, and clean as needed. Also, periodic replacement of the cap with the correct OEM cap is advised.

The cap on the radiator itself has a higher pressure rating, it's job is to maintain system pressure, and virtually never comes into play because the upper cap's lower pressure rating means it vents to the overflow container before the lower cap's threshold is reached.

Do NOT mix the caps up. Use ONLY OEM caps.
so I got it wrong as many others..
What is of the role of the hose that goes from radiator cap to overflow (joining the hose from the 'filer reservoir')?
May be then I'll find and answer to the problem that I have.
My radiator cap (137Kps) is not holding labeled pressure, it leaks down to103.4kps(15psi), checked with a STANT tester. The upper cap is holding 108psi OK.
What made me check it was a bubbling in overflow tank at fully warm engine after a 100km drive. I wasn't overflowing, just bubbles.
I did a sniffer test, but came up negative.
So it looks like air coming from somewhere, radiator cap?, cannot envision though how hydraulically it happens..
I bought the car at the auction and initially I thought I have a HG problem, oil in coolant, but I run compression, leakdown, cooling system and sniffer test, and all came up negative.
Did all fluids flush and change, plus some other things and apart from slipping clutch car runs fine. Coolant looks a bit discolored (darker), but I think it still could be some original oil residue, although I flushed the cooling system 15 or more times.
I'd appreciate some input.

Last edited by Kazo; 06-02-14 at 06:51 PM.
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Old 06-02-14, 04:49 PM   #25
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Default Dual STi radiator cap?

Start off by getting a new OEM rad cap
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Old 06-02-14, 05:12 PM   #26
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Default Re: Dual STi radiator cap?

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Originally Posted by BlueScooby View Post
Start off by getting a new OEM rad cap
sure I will, just wondering if there is theoretical explanation to the occurrence..
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Old 06-02-14, 05:23 PM   #27
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Default Re: Dual STi radiator cap?

Quote:
Originally Posted by Kazo View Post
sure I will, just wondering if there is theoretical explanation to the occurrence..
As far as the hose from the Rad cap to the over fill goes, that is there because as the rad fluid heats up it expands. Once expands enough that it exceeds the pressure of the rad cap, it starts to dump into the overflow reserviour. As the system cools, it will draw some of that fluid back in due to lower temp fluid contracting.

If your rad cap is not holding proper pressure, it is dumping a bunch of hot rad fluid into your overflow...

Assuming I understood your problem correctly, a rad cap holding proper pressure should solve this.
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Old 06-02-14, 06:40 PM   #28
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Default Re: Dual STi radiator cap?

Quote:
Originally Posted by BlueScooby View Post
As far as the hose from the Rad cap to the over fill goes, that is there because as the rad fluid heats up it expands. Once expands enough that it exceeds the pressure of the rad cap, it starts to dump into the overflow reserviour. As the system cools, it will draw some of that fluid back in due to lower temp fluid contracting.

If your rad cap is not holding proper pressure, it is dumping a bunch of hot rad fluid into your overflow...

Assuming I understood your problem correctly, a rad cap holding proper pressure should solve this.
It's a bit academic I know, and hopefully a new cap will resolve(?) the problem, but as far as I understand the cooling system the upper cap is a two-way, the rad cap is one-way, and the coolant 'traffic' both ways is between upper reservoir and overflow tank.

When I look at the cooling system diagram in the WRX STi manual, to me looks like both caps 'see' the same pressure, no matter what, so I don't understand why the caps are differently rated.
The only answer that comes to mind is, that if the top caps fails to open, the radiator one comes to the rescue.
Still don't understand the air(?) bubbles in the overflow..

Last edited by Kazo; 06-02-14 at 06:46 PM.
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Old 06-02-14, 06:53 PM   #29
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Default Re: Dual STi radiator cap?

i would imagine bubbles are simply boiling fluid... sudden lowering of pressure is a good way for water to flash to steam
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Old 06-02-14, 07:08 PM   #30
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Default Re: Dual STi radiator cap?

Quote:
Originally Posted by BlueScooby View Post
i would imagine bubbles are simply boiling fluid... sudden lowering of pressure is a good way for water to flash to steam
that maybe as simple as that. Good thinking..
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