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2002 Subaru Outback LL Bean H6
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Discussion Starter #1
Noticed the vacuum hoses I replaced on my EZ30D were collapsing and spraying a little oil onto the power steering line. Ordered the OEM fitted hoses today that should hold the vacuum pressure better than the soft rubber hoses I currently have.

Question is, I tried searching and couldn’t find an answer. What does the Air Assist Vacuum line do? I see it connects the two valve covers together but can’t figure its purpose.

Anybody by chance familiar with its function and could explain it?




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06 OBW 2.5, 05 Forester, had 03 H6 OBW
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Never noticed that line on mine.
I would imagine if it is fairly large in diameter the purpose is to ensure the right valve cover area 'sees' some of the PCV valve draw from the left valve cover.
 

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2002 Subaru Outback LL Bean H6
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Discussion Starter #4
About the same as the hard fuel lines... didn’t think of that. Was thinking that maybe it was to vent any built up pressure in the valve areas, but that may be it.


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Discussion Starter #5
Was just curious if that collapsed line could cause my slightly irregular Fuel trims.


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If that line is collapsed without something physically squeezing it that would indicate the crankcase is under vacuum, which would be caused by the vent(s) being plugged somehow, and the PCV valve then evacuating the crankcase.

Which would be odd and could cause who knows what weirdness.
 

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2002 Subaru Outback LL Bean H6
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Discussion Starter #7
Well the first one collapsed for two reasons I believe. One just like the one I have now it’s just fuel hose, not meant for vacuum and two I had it too long so the extra bending created a weak spot in the hose letting it collapse.

Ordered the OEM hoses which chould solve that problem.

Out of curiosity, know where I could find the vent/breather so I could make sure it’s not clogged somehow? I searched the forums with no luck, and can’t seem to find it in the FSM’s either.


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01 Outback LL Bean
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The breather is the hose that goes to the right valve cover. Its possible that there is a clog in the line or the valve cover plumbing.
 

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06 OBW 2.5, 05 Forester, had 03 H6 OBW
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The crossover hose should really see zero vacuum, so I'm interested to see what checking the breather turns up.
 

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I once read of a guy with an STI that had odd oil pressure issues. The breather/PCV hoses were chaotically misrouted and he had enough vacuum that his oil pressure light came on!
 

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2002 Subaru Outback LL Bean H6
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Discussion Starter #11
Still trying to figure out the location of the breather. Can’t seem to see any hoses or tubes on the right side other than the EGR and the fuel lines.

Replaced the two hoses with the correct OEM hoses. Perhaps I was wrong on the hole vacuum thing though. Had the PCV line disconnected earlier and there was no vacuum coming from the left side.


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Discussion Starter #12
Hey back again, so I was working on the power steering lines and had to remove the intake box and noticed clean fresh oil in the box. So on inspection, it’s coming from the PCV valve. The PCV valve is new OEM, less than 6 months old. I just ordered new hoses to see if that fixes the issue.

While researching realized the air assist lines I replaced are connected to the AICV. Could that be where it breaths from? If so, was thinking maybe the AICV is dirty and not letting the venting take place.

Your take?


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I was looking at this the other day and noticed the same thing. Seems the IACV gets its air from these lines. If the hoses are collapsing, I would think there is blockage on the engine side of the line and not the IACV side. The IACV is pulling vacuum. If it can't pull air then the hose collapses. Does that make sense?

I am really curious to know where those lines go inside the head.
 

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2002 Subaru Outback LL Bean H6
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Discussion Starter #14
Well, all I know is it starts at the front corners but I’d be afraid to try to feed a wire or pipe cleaner through it. A) pushing the blockage back into the heads and B) if I break something off in there.


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Discussion Starter #15
Wish I never scrapped the old engine’s heads so I could check it out...


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Discussion Starter #17
Started a new thread on there. Sucks it apparently doesn’t link with Tapatalk...


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Okay, I think I have this figured out.

That is your breather line coming off the IAC Valve. I'm not sure why the designed it like this, but that line is ported to the fresh air portion of the intake through the IAC Valve on throttle body port where the IAC connects.

I guess if there was a clog on the IAC fresh air port side then the engine would start trying to suck air from the breather line and create a vacuum in the crankcase. Pull the IAC and check. Make sure to have a new gasket on hand as the old one is generally swelled and not able to be reused. They are available from rockauto or dealership, maybe local parts stores.

If there is no clog in the fresh air port, which I think would lead to some engine idling issues, then maybe your PCV valve is stuck open? Try disconnecting the PCV hose and see if the vacuum clears.
 

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Discussion Starter #19
Alright I’ll look into it and I’ll let you know what I find. Thanks for the help.


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Discussion Starter #20
Well on start up tonight on my way to work, I just had some up and down throttle issues. Once I revved it a bit it settled down. So might as well clean the IACV while I’m at it. Could be the culprit with the added symptom. Just need to order the gasket and tackle that Thursday along with the new hoses for the PCV valve and breather lines


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