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Old 07-13-2019, 10:53 AM   #1
GravyRobber
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Default AOS Line Routing Queston

*****SEE VIDEO IN POST #3*****

I've had an IAG Street AOS on my car since the week I bought it new. I recently built my engine with lots of other goodies, but I wanted to retain my recirculating AOS vs VTA. While I was re-installing it, I noticed that the PCV line was clogged. I replaced the PCV itself with a metal check valve and went on my merry way.

Well yesterday I as doing a boost leak test, and noticed that air was coming OUT of my PCV hose in the direction the check valve is supposed to prevent. The only thing I can think is that perhaps that check valve wasn't rated for boost and failed, allowing air to flow both directions. This would be bad as it could allow boost to go INTO the AOS, and potentially reach parts of the engine that shouldn't see boost. I will order a metal Supra PCV and install that, but in the mean time, I had a thought:

Would it be okay to cap the nipple under the throttle body, and cap the PCV nipple on the AOS, deleting that line entirely? It seems to me the rest of the lines vent from the same spots, and recirculate back into the intake. I can't think of any reason this would be unsafe for the engine. Obviously, this would affect the tune, but that can be compensated for. I'm just wondering if I can eliminate some complexity yet retain my recirculating system verses going to the competition series and dumping the vapors under the car.

Thanks.
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Last edited by GravyRobber; 07-13-2019 at 02:00 PM.
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Old 07-13-2019, 11:24 AM   #2
T-37
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When doing your boost leak check, I assume you connected your pressure source to the turbo inlet hose upstream of the turbo. The turbo inlet will never see boost as it's in front of the turbo (this should the only place where the PCV & AOS route back into the intake tract). Pressure in that hose would only read 0-vacuum on a boost gauge. So you'll never see a boost leak into your AOS through the PCV as you're doing something in your boost leak test that the car won't ever recreate in real world conditions.
Normally in a boost leak test you would cap or block all of the connections to the inlet.

*I'm making the assumption that there are no connections to the intake manifold and that that port is capped. This is how the crawford system is set up (with no PCV at all), I haven't personally used the IAG

That said, your check valve may be a problem that may cause you issues with crankcase ventilation. The OEM subaru pcv is perfectly suitable (and it's also metal)
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Old 07-13-2019, 01:45 PM   #3
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Quote:
Originally Posted by T-37 View Post
When doing your boost leak check, I assume you connected your pressure source to the turbo inlet hose upstream of the turbo. The turbo inlet will never see boost as it's in front of the turbo (this should the only place where the PCV & AOS route back into the intake tract). Pressure in that hose would only read 0-vacuum on a boost gauge. So you'll never see a boost leak into your AOS through the PCV as you're doing something in your boost leak test that the car won't ever recreate in real world conditions.
Normally in a boost leak test you would cap or block all of the connections to the inlet.

*I'm making the assumption that there are no connections to the intake manifold and that that port is capped. This is how the crawford system is set up (with no PCV at all), I haven't personally used the IAG

That said, your check valve may be a problem that may cause you issues with crankcase ventilation. The OEM subaru pcv is perfectly suitable (and it's also metal)
I think that just answered my question. The IAG, aside from all the other connections has a "PCV" line which contains a check valve. That line goes:
(AOS)-----(-->Check valve -->)-----(Nipple under throttle body)

When I did my boost leak test, pressure from that nipple on the manifold flowed backwards from the nipple, through (against) the check valve, and back into the AOS, which tells me that check valve failed.

I think what I have in my head is to delete that "PCV" line, which sounds like it will mimic the Crawford setup if I understand you correctly. I'd cap that port at the AOS, and cap the nipple under the throttle body. I would have no PCV at all, and a free flowing, fully recirculating AOS.

EDIT: Added a video to help 'splain:


Last edited by GravyRobber; 07-13-2019 at 01:59 PM.
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Old 07-13-2019, 06:28 PM   #4
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Well, I deleted that hose and capped the associated ports. Car idles much better now (whole other story). Previously, when removing the oil cap while idling, it would be under a significant vacuum, and removing it would cause the idle to swing drastically and the AF Correction would peg at 30 something. Now with the line deleted, removing the oil cap has no effect on idle and rather than a vacuum, its a neutral "puff puff" as it should be.
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Old 07-17-2019, 07:49 PM   #5
Haevn
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I like the crawford aos, it has you remove the pcv system altogether ><
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Old 07-24-2019, 11:27 AM   #6
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Yeah after talking to 2 different tuners and engine builders, they both said its extremely common to simply delete that PCV hose and just route the lines back into the turbo inlet, or VTA.

I do have a theory though:

The IAG Street AOS worked perfectly on my stock engine, no tune required. When I rebuilt my engine and cleaned my AOS, I found the PCV check valve in that line to be clogged. Rather than buying a new line from IAG, I ordered a new check valve from Amazon. Well apparently the check valve in the IAG line also has a restrictor in it, because the new check valve allowed too much air to flow through, which was causing my car to idle high despite the throttle plate being completely closed. So my thought is that the IAG PCV valve meters air differently than just a generic check valve.

Also, apparently not all check valves are rated for boost, because the new (metal) check valve was found to have failed while I was troubleshooting, allowing boost to go back into the AOS.

So lesson learned, either replace with IAG PCV line/valve, or simply delete entirely.
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Old 11-30-2020, 06:40 PM   #7
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Man thank you for this thread. I've had this issue for about 3 weeks now and this was exactly what the problem was.

IAG street AOS on my 2018 WRX (not STI).

Previously I took out the MAF and MAP sensors and cleaned them, cleaned the intake filter, did a smoke test and found a small leak, and did a full idle relearn procedure - no change at all.

I stumbled across this thread and went out there and stuck a clamp on the PCV line from the AOS to the PCV valve, boom idle immediately dropped to normal.

Thank you thank you thank you. This probably saved me hundreds of dollars at a dealership when I finally gave up.

(throwing in some keywords so others can find this when they google the issue)

IAG street AOS high idle 2015 2016 2017 2018 2019 2020 wrx FA20dit P0507 idle higher than expected
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Old 12-01-2020, 06:32 AM   #8
Seangraz
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Donít cap off the port on the aos. Put a vent filter on it. Still needs the be able to vent crank pressure.
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Old 12-02-2020, 09:35 AM   #9
biggin215
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Seems like that plastic check valve has a high probability of failure.



I took mine apart after replacing the line/check and the rubber diaphragm in the old one is punched right through.



Hard to tell if it failed from one event or has just fatigued over time, but my guess is the rubber probably fatigued over time due to thermal cycling. IAG should really source a better check valve with a thicker diaphragm or a material more suited to the thermal/mechanical cycling.



My AOS isn't that old.....



https://scontent-dfw5-2.xx.fbcdn.net...33&oe=5FED91CE


https://scontent-dfw5-2.xx.fbcdn.net...5b&oe=5FEB66E0


https://scontent-dfw5-1.xx.fbcdn.net...9c&oe=5FED69D8
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Old 11-03-2022, 11:11 AM   #10
chet24
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In case anyone still comes across this thread, I replaced my failed check valve with pn 47245K118 from McMaster-Carr at 1.53/piece. It looks identical to the IAG part, and probably where they source theirs if I were to guess.

It's a polycarbonate body with silicone rubber diaphram. Supposedly rated to 50psi @ 65ļF, with a overall temp range up to 275. Theoretically it should be fine, but obviously it isn't.
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Old 11-04-2022, 08:21 AM   #11
KillerBMotorsport
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Quote:
Originally Posted by chet24 View Post
Theoretically it should be fine, but obviously it isn't.
For a while it will. Polycarbonate should not be used in this application IMO. It just will not last long.
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