Bench test an AOS? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Duquette5 · 03-13-2015, 01:04 AM

So my resurrection project is nearing completion and I will be driving it to Hershey. I still have a funky idle - it drops to under 500 and hunts a little and then revs and drops revs and drops... I do see some smoke on startup and an leaning towards the AOS. I have 2 spare AOS's that were in the trunk when I picked it up. One has a cracked lower bellow still attached to it the other looks fine. Is there a way to bench test them? I searched and couldn't anything...

Thanks!
Mike

Timco · 03-13-2015, 04:28 AM

I would pour oil into it. If oil comes out the bottom, and the air gets out the side vents, it's separating and it's GTG.

(Actually, any sign of oil on the vent tube means bad.)

KRAM36 · 03-13-2015, 04:39 AM

Quote:

Originally Posted by Timco

I would pour oil into it. If oil comes out the bottom, and the air gets out the side vents, it's separating and it's GTG.

(Actually, any sign of oil on the vent tube means bad.)

Sign of oil on the vent tube means bad means the AOS is bad or bad in another way?

Timco · 03-13-2015, 05:36 AM

Quote:

Originally Posted by KRAM36

Sign of oil on the vent tube means bad means the AOS is bad or bad in another way?

I've read in a few places, and mine was the same, that oil on that top vent tube means oil has passed the AOS and its bad. I had puddles of oil in my intake manifolds when I pulled the throttle intake tube. AOS upper tube was oily.

Jager · 03-13-2015, 08:27 AM

Quote:

Originally Posted by Duquette5

So my resurrection project is nearing completion and I will be driving it to Hershey. I still have a funky idle - it drops to under 500 and hunts a little and then revs and drops revs and drops... I do see some smoke on startup and an leaning towards the AOS. I have 2 spare AOS's that were in the trunk when I picked it up. One has a cracked lower bellow still attached to it the other looks fine. Is there a way to bench test them? I searched and couldn't anything...

Thanks!
Mike

The unstable idle could be a dirty throttle body.

Fintro11 · 03-13-2015, 08:42 AM

While engine is running unscrew the oil fill cap, if you can feel a vacuum time to change the aos.

My moms 06 boxster sucked the cap down when the aos went.

Gelbster · 03-13-2015, 03:56 PM

Mike,
A defective AOS creates an abnormally high vacuum in part of the crankcase ventilation system. This may cause a vacuum leak in tubes/connectors/pipes that may look perfect but have concealed defects. Try the old WD-40 test over the whole AOS system ?

patssle · 03-13-2015, 04:17 PM

Some vacuum is normal on the oil fill cap. If you can test the pressure - normal is 5 inches. They say if AOS is bad then there is a ton of vacuum.

Also don't run the engine much if you can - they say too much oil coming from a failed AOS can ruin it.

Gelbster · 03-13-2015, 04:40 PM

In theory the vacuum is measured with a " slack tube manometer".
Perhaps someone has a diy? I found this but be careful you don't suck all the water into the engine!!!!
Homemade u-tube manometer - EcoRenovator
Here is the Pelican info:
Pelican Porsche Tech Article - Porsche 911 Air Oil Separator - Porsche 996 Carrera (1998-2005), Porsche 997 Carrera (2005-2008)

patssle · 03-13-2015, 04:45 PM

Duquette5 · 03-13-2015, 06:31 PM

Ive done the WD40 test and didn't find any other leaks. I didn't even think to try the oil fill cap. Was busy recovering the interior trim pieces today. Tomorrows to do list just got longer...

_Oil fill cap
_static vacuum test
_pull and clean throttle body
_Help fellow member wasateener pickup his newly acquired 2001 in CT

Gelbster · 03-13-2015, 06:32 PM

Nice photo but without a brand/model, ........
Would this work?
PYLE Meters PDMM01 Digital Manometer with 11 Units of Measure - - Amazon.com

Duquette5 · 03-14-2015, 03:08 AM

Forgot to mention that it is also popping through the intake while idling. Im afraid to really let it idle to long to see if it clears up after warming up good. Once I do the oil filler cap test and pull the static vacuum if they are fine I may just let it idle for a bit.

Duquette5 · 03-14-2015, 03:13 AM

Oh and the trim work I've been cleaning up. All the parts that were covered in the peeling rubberized crap are getting redone:

wogin · 08-09-2023, 02:23 PM

I realize this is a super old post, but I just cut my old AOS apart to see how it works and there is a way you could non-destructively bench test it. The AOS looks to be basically a vacuum actuated diaphragm valve between the throttle body/intake tube connection (i.e. the connection closest to the round diaphragm body) and the other three connections on the device. One side of the diaphragm is vented to atmosphere and the other is common with the 4 AOS connections. The diaphragm is spring loaded so when there is no vacuum on the intake the spring holds the valve completely open for open flow between the three other connections and the intake tube connection. As the intake tube connection draws heavier vacuum, the pressure differential across the diaphragm increases in the diaphragm valve begins to shut. From posts that I’ve read, it seems the air oil separator stops working when there is a hole or a leak in the diaphragm ( a leak in the diaphragm does not allow pressure differential to counteract the spring, so the valve does not actuate and flow path to the intake tube connection is wide-open). To leak test the diaphragm all you should need to do is plug all but one connection, then blow air at !LOW! pressure into the third. Apply soapy water to the outside of the round diaphragm valve portion while pressure is applied and a leak across the diaphragm should be visible as bubbles around the diaphragm valve casing.

This will not test how well balanced the spring and diaphragm are together, which is probably also important to a properly functioning valve, but it would definitely detect a crack/tear in the diaphragm which seems like the main way these things die.

Posting this because hopefully it will help someone out!