Can a leaking AOS breather tube cause a vacuum leak?

[gsmiller]

I've never really quite understood the whole AOS system so please excuse my ignorance in this question.

I have a breather hose related to the AOS that is leaking oil onto the right side of my engine. My mechanic said that as the leak worsens, it will likely cause a CEL due to vacuum leak. Can someone explain how this would be? My basic understanding is that clean air flows into the AOS device and into the crankcase (from somewhere?) and the polluted air in the case flows out, has the oil separated out and is sent back into the intake. I'd like to better understand the system but can't find any information out there.

My assumption here is that the leaking hose is pulling air from the crankcase into the intake, and that the leak would allow excess air in creating an overly lean mixture and triggering a CEL... but that's just a guess. Understanding what all the connections involved actually are would be a big help for me.

Thanks everyone.

[JFP in PA]

Your assumptions are incorrect. The AOS is a glorified PCV device that pulls a controlled 5 inches of water vacuum on the crankcase using intake vacuum as its source, to help the low tension piston rings maintain a proper seal. It is not a "flow through" device, it is designed to hold the low vacuum while preventing oil from being sucked into the engine, which could cause hydro locking and total engine failure.

If you have leaking hoses running from the AOS, you will eventually get codes. Change the hoses, they are not expensive.

[gsmiller]

Got it, thanks JFP. Any insight on the various connections from the AOS and where they go? At least one connection must go back into the intake. Are there connections to flow the separated oil back somewhere?

[JFP in PA]

Originally Posted by gsmiller

Got it, thanks JFP. Any insight on the various connections from the AOS and where they go? At least one connection must go back into the intake. Are there connections to flow the separated oil back somewhere?

One line goes to the intake (vacuum source), one each goes to each cylinder head (where the vacuum is applied), and a line for coolant on a 996 (Boxster does not use these). Entrained oil drains back into the side of the engine case and into the sump.

[gsmiller]

Super helpful, thanks! I've had this confused for years and finally understand. Cheers.

[bradmd]

Trying to understand this myself on an 08 C4S. Installed new AOS primary last yr, seemed to get better but recurred. Oil in intake again. Will replace secondarys too this time I guess.
I did find a loose AOS hose where it returns to the intake. Low vacuum from a leak like that wouldnt cause the oil to be sucked into the intake, would it?

[JFP in PA]

Originally Posted by bradmd

Trying to understand this myself on an 08 C4S. Installed new AOS primary last yr, seemed to get better but recurred. Oil in intake again. Will replace secondarys too this time I guess.
I did find a loose AOS hose where it returns to the intake. Low vacuum from a leak like that wouldnt cause the oil to be sucked into the intake, would it?

No, but a cheap after market AOS rather than the factory unit will fail prematurely and cause you to have to do it all over again............

[bradmd]

I guess that is what happened then, as it was aftermarket