squawbum

Hello all. I think I'm going to have to replace the AOS. The bummer is that it was replaced 3 years ago at around 70K miles. The car now has 103K miles on it. It is a 2002 C4 Cab.

There is a lot of vacuum on the oil fill tube when the car is running and I take the oil filler cap off. You can also hear a distinct hiss of air being pulled in and the engine starts to idle rough. When I cover the oil tube with my hand, the rough idle goes away. I started wondering about this because I was getting LOTS of white smoke on start up, but only sometimes. It seemed random actually and I thought it was only when I started the engine cold and turned it off cold that I would get this. I tested this and got a little white smoke consistently doing this so I thought this is the normal behavior.

Then I took a nice drive and parked in my usual spot at my golf club and after playing golf, started the car to go home and noticed smoke like it was coming from James Bond's car on start up. It went away quickly and didn't come back. The next day the same thing happened. My theory was that oil was seeping into the intake or cylinder because of the angle my car was parked. It was on an incline nose down.

So the next day I went to play, I parked in a different spot where the nose was inclined up and after my round of golf, started the car and zero white smoke. I was thrilled actually. The oil wasn't able to accumulate with the nose up on the incline when parked. Then I did some more research and just decided to check the vacuum at the oil filler tube when the engine was running and it was like described above.

So, I think I need to change the Air Oil Separator. My question is this:

Is there an updated part number that is an updated part that makes these things more reliable than previous versions? It seems that these AOS parts, genuine or not are not very reliable. Jake Raby mentioned in one post that he has had genuine AOS's that are bad out of the box! Has this been addressed and if so, what is the new part number?

If they are still infamously unreliable and won't last that long, what about installing it in a location that makes it much easier to replace?

I know it goes into the block, but its really just pipes that connect to it. Can't we just in concept, extend the pipes to the AOS so that it can be placed in a place that would be a no brainer to replace? As far as I can tell, there are 5 ports on the thing, 4 of which are just connected with pipe clamps. What about making those pipes longer? The part that connected to the block, is there an adapter that could be installed to the block and a pipe run from that to the AOS? Has anyone done this?

Is there a way to bench test the AOS so that I know that I'm putting in a good one considering that people on here have experienced them bad out of the box and it seems to be a royal pain in the rear to R and R?

Thanks in advance!!

RennPart

.. sometimes they don't last long, if you have a vacuum leak this shortens the life of them. If you drive/track your car a lot they don't last as long either. We just replaced our AOS in the spec Boxster after 10 races and it was bad.

You can cut open the end to view the diaphragm. The diaphragm is the part that fails, its very soft/flexible and can get small tears in it.

No upgrades other then the MOTORSPORT AOS, but it is will cost about 6x the amount as the standard unit.

Extending the tubes would require some kind of bracketry and custom made soft lines, the factory lines are hard plastic with flex hoses. This would not be something easy to do.

Of course we sell all this stuff on our website with free shipping

Motorsport AOS: https://rennpart.com/product/motorsport-aos/

Standard AOS: https://rennpart.com/product/air-oil...carrera-02-08/

Motorsport picture:

Schnell Gelb

A useful tool to diagnose a failing AOS is a digital manometer. It has been mentioned and linked many times. It is only $30 from Amazon and is a quick and easy diagnostic method. Use an old oil filler cap and adapter to make it easy .JFP & Ahsai have shown photos of this in the past

5CHN3LL

Is the same crankcase pressure/vacuum present at the dipstick tube? If so, it would be fairly easy to make a silicone and fuel line plug adapter to fit the dipstick tube...

squawbum

Thanks for the answers! I've been thinking about it and I guess the main issue with the AOS is the rubber diaphragm being compromised and letting manifold vacuum to the crankcase. I guess to bench test a new AOS unit one just needs to pull vacuum on the proper port of the AOS and see if it holds. If it does it is fine but if it doesn't then it's bad. Would this work as a bench test?

which port on the AOS pulls the manifold vacuum?

another thought. What if one just disconnects the manifold vacuum connection and just plugs the tubes. Any negatives to doing so besides emissions related issues. Without the vacuum to the AOS, there would be no vacuum to the crank case. Would love thoughts on this.

Thanks!

squawbum

Another thought would be to extend the vacuum line to the AOS and put a valve on it that is easily accessible. If it goes bad just turn off the vacuum and when you need a smog check just turn it on temporarily? Once the AOS goes bad just turn off vacuum and go on your way?

squawbum

Actually. I think the two hoses, small rubber hose and larger plastic hose connected to the front plenum is the vacuum line for the AOS. I've attached a photo. Don't know how to add arrows to photo on my phone so sorry if it's not clear.

Can anyone one confirm that the large plastic pipe is the vacuum line for the AOS?

Also what is the smaller rubber vacuum line for? It is connected to a valve that is controlled by the ECU, I presume, because the valve is connected to an electrical connector.

Id like to disconnect it and plug up the hole on manifold side and see if vacuum on oil filler goes away. Then drive around with it disconnected and see if there are any negative side effects to having the AOS vacuum disconnected, but there's that darn smaller rubber vacuum line that I don't know what it does. Please help!

Thanks in advance!

dan_189

Yes that large rubber hose connected to the intake plenum goes to the AOS.

I applaud you thinking outside of the box but in all honesty if your AOS is bad or going bad it needs to be replaced.
Also a manometer on the oil filler is the best test as if you test anywhere else you may get different data that is potentially incomparable.

Don't disconnect an​​​​​y vacuum lines they are there and connected for a reason.

NJS_04_996

If you can't find a spare oil filler cap to sacrifice, here's my $2 adapter for measuring the vacuum via the oil filler tube....
(Its one of those sink stoppers) Was pulling almost 14 in Hg with the old AOS!

squawbum

Thanks for the reply! I'll get one of those meters. It will be a nice addition to my toolbox!

Thanks for the info on the big tube. I found in my Bentley manual, by dumb luck, that the smaller line is the fuel vent line. Essentially this vacuum port on the front plenum is an INPUT for the crankcase vent system (the AOS) and the EVAP system. I disconnected them and plugged the hole and have been driving around and the car runs great. Also the excessive vacuum from the oil fill tube is now gone and there is no studder of the idle when the oil fill cap is off.

Im going to drive it around like this for a while and see if the white smoke disappears and see if my oil consumption gets better. I've ordered a new AOS from the dealer but really don't want to do any work on the car right now. I just want to enjoy it as I've done lots of projects on it. Most recently R and R of the tip and I just got the car in Jan. I'll probably get around to changing it out later this summer.

Here re is a pic of what I did:

squawbum

NJS. That's brilliant. I'm going to rig one of those up. Thanks!

Macster

Be aware if you have removed vacuum from the crankcase the engine oil accumulate water at an impressive rate. Unburned fuel too. This can be dangerous. If fuel builds up in the crankcase oil it can form an explosive mixture and if it ignites... Say goodbye to some engine seals.

The behavior you posted regarding removing the cap and even putting your hand on the open tube is normal.

In one case with my Boxster the vacuum to the crankcase because of what proved to be a bad AOS I was unable to remove the cap with the engine running. Had I managed no way would I have covered that opening with my hand given how much pressure difference there was.

However, I have to say an AOS can go bad at any time. Usually not after just 30K miles but there have been a few cases of what would have to be considered premature failure.

It is possible the new AOS has suffered a case of infant death, so to speak.

If the vacuum at the crankcase is excessive this is a good sign the AOS is bad. If so you should replace the AOS and afterwards be sure all vacuum lines are connected properly.

If the crankcase vacuum is not excessive be sure the oil filler tube cap is not leaking. These can over time develop some "slop" where the fit to the tube and the cap fails to seal properly even though it is screwed down tight.

With the engine up to temperature and idling see if you can move the cap around and if the engine reacts.

Be sure the oil is not overfilled and the oil doesn't have too many miles on it. As the miles accumulate on the oil of my Porsche cars both engines are more likely to smoke upon cold start. And be sure you are running an approved oil. The wrong oil can have a foaming problem -- these engines generate a lot of oil vapor/foam -- and this can overload an already marginal but otherwise working OK AOS.

dan_189

Same as my car, start to remove the oil filler cap and the car begins to change behaviour, can't remember what it did but I got nervous and did the cap back up.

As Macster mentions I'd be very weary of blocking off things that usually aren't...

squawbum

Hey Macster. Thanks for the input. I've been researching PCV's and the vacuum is need to flow fresh air into the crankcase and vent out the bad stuff.

So so simply removing the vacuum connection is just causing the flow to not happen and like you said, bad accumulation of bad stuff.

So so looks like I'm going to have to change the darn AOS out assuming its bad. I need to get an actual reading of the vacuum from the oil fill tube so I need to get the meter and rig it up.

Also is it possible to test by pulling vacuum on the AOS vacuum port that connects to the intake and see if it holds. Not sure if that would work because simply pulling a vacuum might not close the valve?

Thanks again!

squawbum

I wanted it test the vacuum tonight but couldn't get to the store to get a drain stopper so instead Inised this:



A little meatsause for dinner then oil filler cap. If somehow one day your oil filler cap goes missing then...

I could feel the slightest vacuum on the cap but not enough to pull the freshness seal.

I ised the oring ring from the original cap to help seal it up. Removing this and the real oil cap resulted in same behavior. There is very little vacuum which leads me to believe the AOS isn't bad.

Ill redo the setup with a real oil filler that I'll pick up tomorrow when I pick up my new AOS to double check.

Original oil filler cap
O ring removed
O ring filler tube
Ragu on oring