After doing some research I've read that the cap next to the intercooler is the one to replace with the STi high pressure cap. I've come in possesion with two of these, so would it be beneficial/hurtful/ect to run two caps, one next to the intercooler and one on the radiator itself?
Dual STi radiator cap?
Gouki
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... am I too simple?
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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.
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.
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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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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..
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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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Bump revival!
Hey folks, so just to clarify, both OEM caps are rated differently or the same?? Need to know because I'm planning to change them out soon for maintenance purposes. No no, not getting any flashy STI caps, just the ones from the dealership with the yellorange label hehe. Just need to make sure that I'm putting the right cap where they need to be. I noticed that there are 2 different looking caps for the cooling system. One looks like it has wings on both sides, and the other is just plain round. Are they the same/different? Just FYI purposes to anyone, I do have a Mishimoto rad installed. Got rid of the provided cap that came with it because of all the horrific leaking stories, did not want to chance that. So yes OEM for the win. Thanks in advance guys.
Hey folks, so just to clarify, both OEM caps are rated differently or the same?? Need to know because I'm planning to change them out soon for maintenance purposes. No no, not getting any flashy STI caps, just the ones from the dealership with the yellorange label hehe. Just need to make sure that I'm putting the right cap where they need to be. I noticed that there are 2 different looking caps for the cooling system. One looks like it has wings on both sides, and the other is just plain round. Are they the same/different? Just FYI purposes to anyone, I do have a Mishimoto rad installed. Got rid of the provided cap that came with it because of all the horrific leaking stories, did not want to chance that. So yes OEM for the win. Thanks in advance guys.
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the two caps are different. they have different rated pressures, different functions, and the lids have a different look.
Be sure not to mix them up...
Be sure not to mix them up...
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Thanks for the reply Blue.
Yea ok just noticed that the upper res tank cap says 108kpa, and the one on my Mishimoto rad says 1.1... So I just googled the 108kpa is actually 1.08bar. can that be considered 1.1 since .08 can be rounded to .1? So that both caps would be similar? I don't believe I would benefit from a 1.3 on my rad would I? I read on some website that increasing the pressure would create a new weak point in my cooling system. My radiator hosed are practically still new. Lower hose is a year old, upper house is 1.5months old. I average 180-190F coolant temps monitored via AP. 1.1 should fair enough considering the hot SoCal weather I endure? Thanks in advance!
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137kpa (roughly 1.3 bar) is the OEM radiator cap pressure on the STI. The 108kpa is the upper res cap.
Not sure if the pressures are the same for the WRX.
That said, i would not stray from OEM.
Not sure if the pressures are the same for the WRX.
That said, i would not stray from OEM.
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Oh I did not know that, so my 1.1 on my rad cap should he 1.3? I'm estranged to wonder how a 1.1 cap ended up on my car in the first place. I did buy the car from someone last year but I did the rad replacement (and of course ditched the 1.3 Mishimoto cap) and used the cap that was already on the car (1.1)... I'm not overheating or anything but I did notice little bubbles burping every few odd seconds into the plastic overflow tank next to the rad(like every 5seconds it would bump a little bubble), could I be boiling over a tad bit? I will go to Subaru stealership tomorrow and get a 1.3 OEM cap. Now I am getting somewhere with this. Thanks again in advance.
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That is odd...
FWIW, id grab a new OEM cap for both just to have new/fresh ones... I recently had to replace a cap that wasn't functioning properly. Im in favor of replacing cheap stuff before it breaks...
FWIW, id grab a new OEM cap for both just to have new/fresh ones... I recently had to replace a cap that wasn't functioning properly. Im in favor of replacing cheap stuff before it breaks...
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Right, am I the same... Preventive measures... So just to clarify, I should replace both rad with the OEM 1.3 and get a new 108kpa(1.1) for the upper res...yes? Just for safe measure...
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You got it :tup:
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Thanks Blue, you've been helpful.
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No prob!
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