Previous attempts at catch cans failed because of Air/Fuel ratio problems created by the venting. It may be wise to plot A/F ratios before and after installation to ensure you haven't fixed one problem but created another. It may be significant that despite all the expensive AOS problems for all these years there is no commonly accepted ,simple,economical fix yet. On other engines the catch can fix is successful and has been for widely used for many years. There has to be a reason it has not been successfully implemented and widely adopted mod for our engine.Let's hope the proposed fix works.It will be a very welcome discovery for many of us.
For these reasons it may be better to fit a catch can in addition to the stock AOS .Previous failures were trying to replace the AOS with a catch can.

 

Sound advice!

 

Just for clarification Schell, were previous attempts done with a catch can that vents to atmosphere, or vented to intake vacuum?

 

The whole project with drawings should be in Search under the author "Stelan"

 

Got a link? I searched and found nothing.

 

the previous attempts being referred to are from 986forum where a member was trying to put an aftermarket aos/catch can in place of the oem unit. jake noted in the thread that his efforts to do the same thing were unsuccessful because of fuel trim issues. i don't think you can extrapolate that to 'catch cans cause fuel trim issues'; gotta compare apples to apples.

the oem aos vents the crankcase through an air/fluid separator into the intake. it goes to the intake for emissions purposes - to try and combust the nasty vaopurs coming out of the crank case. further, porsche uses the vacuum present at the intake to help evacuate these vapours. speculating, but presume that the vacuum also helps maintain airflow to ensure that any remaining fluids in the crank case vapours remain suspended in air so as to not deposit on the intake tract and combust better. there may also be the consideration that the low tension piston rings used by porsche require a vacuum to function properly, and that a vacuum helps limit aeration of the oil in the sump. otherwise, porsche could have introduced the aos output at a number of locations that didn't have vacuum; the charcoal cannister for fuel vapours immediately comes to mind. stop for a second and think about the pcv valve on the crank case; without vacuum facilitating flow through this valve, will crank case pressure be adequate to keep the oil and gas-contaminated vapours moving through that thing; is that sufficient to keep it from gumming up?

a running motor can produce a lot of vacuum, and porsche wanted to limit the amount of vacuum applied to the crank case so the aos has a diaphragm - as vacuum increases, the diaphragm closes so as to reduce flow. this diaphragm is the failure point in our aos and why a failed aos is identified as a very high vacuum on the sump - so high that it is difficult to open the oil fill cap. again, porsche could have avoided this failure mode by venting the aos someplace different but they didn't, which again leads me to believe that vacuum is required. they could have also removed the diaphragm all together but they didn't, which is an acknowledgement that the amount of vacuum applied to the crank case has to be managed in some manner.

replacing the oem aos with an aftermarket aos, a catch can, or an aos with a higher flow diaphragm will cause fueling issues. situation: you come off wot, throttle closes, engine doesn't want more fuel but is still spinning quickly and crankcase pressure is high. further, vacuum at idle is high and fuel/oil vapours get dumped into the intake without a diaphragm to regulate flow. fuel trims go rich.

this is not the situation being discussed here. in this instance the aos is not being replaced, but a catch can/aftermarket aos is added after the oem aos as backup. so, diaphragm still in place, and second catch can serving as a 'canary in a coal mine' to aos failure. the concern with this approach (as jake notes in the previously mentioned thread) is catch can volume; too small and it is useless.

further, some consideration as to where you vent the secondary catch can; i'd suggest back to the original location so as to maintain the vacuum as dr ing intended. concern here is getting a big enough catch can to be of any use. you could also vent to the air box as is being discussed here - here you have enough volume in the air box to mitigate any disasterous aos failure and might not even need the secondary catch can, but concerns are (a) no vacuum, and (b) oily vapours fouling your mass air flow sensor. if an oiled k&n filter can foul a maf (it did mine) then crank case vapours certainly can even with secondary separation. hey, those who have done this successfully; perhaps you are not fouling your maf because you pcv valves are plugged!

ultimately, porsche is aware of the issue and has been slowly and quietly upgrading the aos over the years (like the oil pump piston and chain tensioners for example). get the latest part number (ie, not nos from 2001) make sure it is an oem part and replace it. perhaps consider it a maintenance item that porsche didn't identify in their original service manuals, like water pump and ims. and speaking about posche upgrades, look what they did for the aos on the m97.2 (dfi) engines; same as on m96 and m97.1. that's a tough place to try and engineer an improvement (serviceable diaphragm would be nice ...).

or i could be wrong. you get what you pay for on the internet.

 

post #9 in this thread.

 

just google "Stelan AOS"

 

The damn "rabbit hole" strikes again!

 

If you vent the aos rerouted line under the air filter oily fumes are less of an issue as the filter takes most of it out. But sure, keep into account that you might have to change the MAF at some point, probably a bit earlier. But i consider a MAF as a wear item. It slowly degrades.

You are right about the right amount of vacuum. Not only for the cranck-case and rings. But also to get proper flow for the cyclonal functioning/separation of the AOS. The idea of the rerouting with proper big enough diameter tubes is to get as much necessary vacuum to the AOS as possible. I did not design my current setup. But I know at least some sort of research has been done towards the issue of pressure and vacuum.

At least we agree that there is a purpose for the vacuum. But the diaphragm is also indicating that the position at the intake is not the ideal. Too much vacuum. Even Porsche did not like it. And I can understand that they did not want a lot of extra tubing in the engine bay at odd locations being prone to
wear and failure. For Us as enthousiasts we take care and baby our cars. Much different then the first time owners of our cars!

 

and i think that is a great solution. i would watch my air filter like a hawk, as once it saturates, even in one spot, fluids will pass right through it. are you comfortable that you are getting enough vacuum there? i think you'll see pressure drop there (air constricts at the entrance to the air box, expands within the air box and pressure drops, constricts again as it enters intake pipe). you note in your initial post that the longer hose acts to cool the intake charge; i note that porsche actually heats the intake charge (that's the two other connections at the diaphragm). oddly the boxster doesn't. are you worried about condensing oil/fuel if you cool it too much?

 

The only challenge with this is all the reported failures of brand new, OEM AOS units shortly after installation. I seem to remember (and I may be mistaken) even Jake mentioning experiencing these units failing from brand new himself. I'd gladly replace mine with new, but presently it is working and I worry about actually be worse off with new.

 

Yup, I actually considered pulling the one off of my other motor (only 15K on the AOS) and using it. But ordered OEM and hope to have it by weeks end.
I really need to get this fixed and behind me...

 

Running already two years and a lot lot of miles and going strong. Also more then 250 modified cars driving around here. The long term effects however are not predictable. That is true. But once again it saved my ***, euhhh engine, already once (at least that i am aware of).

You are right, dont forget that at the intake you have also the venturi effect, which I think due to the perpendicular fitting on the intake and with throttle body opened is the main vacuum exponent working on the AOS. I did not do any measurements. Maybe someone can do it to see what kind of magnitude of vacuum we have there under the air filter. At least we agree that too big vacuum is not good.

Is it confirmed that under driving conditions the engine is under such a significant vacuum? It sounds really strange to me. A healthy +3.0L engine has a considerable blow by. Would the tiny diameter of the AOS line be enough to compensate for this, and on top of that create a vacuum? My intuition says it is just to create a sufficient flow get the crankcase fumes out and get enough airspeed for the cyclonal separation the be efficient. And yes to lower the average crankcase pressure caused by the natural blow by.

Okokok ... related to the statement right above ... I admit that my practical technical knowledge about engines is rather limited ... and I did not research all the previous relevant threads here on this forum. But for me there remain a lot of questions unanswered. How long does the AOS stays open? Does the diaphragm closes also under other driving circumstances. What level of vacuum is required to close the AOS? The diaphragm will also close under (wide) open throttle and in high rpm range when the air speeds are at max?

Heating the AOS serves another purpose. With really cold weather the AOS will not heat up enough due to its location exposed to driving winds. If it stays too cold it loses its cyclonal filtration capacity as fumes (water / fuel?) will condensate too fast and disturb the effective functioning of the AOS. The Boxster engine doesn't need that active heating as its AOS is located at a spot which is not exposed to the (cold) driving wind. It will come up to temperature by itself due to the surrounding engine heat.

I checked my fuel trims, they are perfectly in line with what to be expected.

 

Silk, so you didn't actually develop and install your "system", but rather someone over there is doing these mods for people? Is this correct?