Air Oil Separator Replacement

A bad Air Oil Separator (AOS) can make your car look like this because oil is being sucked from the crankcase into the throttle body and then into the engine in amounts the engine can't burn off in its normal combustion process. The normal cause for this is a hole in the Air Oil Separator which is an external to the engine plastic part part of the pollution control equipment.  The AOS is intended to reduce the pressure in the crankcase in an acceptable way by allowing some pressure created by piston ring blow-by to be vented into the engine air intake for disposal through the normal combustion process. When there is a hole IN the AOS, the airflow increases and more oil is sucked out of the crankcase in amounts that overwhelm the AOS's ability to separate the air/vapors from the oil droplets. The result is more oil through the AOS into the air intake, through the throttle body and into the combustion chambers. Of course while this is going on the Mass Air Flow Sensor (MAF) is totally confused because there is a source of air that is entering the airstream after the MAF has measured the airflow so the ECU (computer that controls the air-fuel mixture) is now totally confused. The result is a bad running engine.


A quick puff of lite blue smoke occasionally on startup is not at all unusual nor is it a sign of the AOS failing as long as it is only at startup. It is a characteristic of the flat-six engine and the way oil sometimes drains down to the cylinders. 

You should check inside the AOS tubing where it goes into the air intake for heavy oil in the tube before removing/replacing the AOS. A light coating in that tube is normal. Here is a picture of the throttle body, the gray metal part of the intake near the engine and after the plastic portion of the air intake into which the AOS dumps its vapors.

Large amounts of blue oil smoke out the tailpipe while running is a "stop right now" sign as you could do damage if too much oil is sucked into the combustion chamber and then the piston tries to compress it.

White smoke while running is a sign anti-freeze got into somewhere it shouldn't and you should stop lest you do further damage.

The AOS performs a similar function to a PVC valve, by trying to vent crankcase fumes (hot oil vapors, minor blowby the rings, etc.) back into the intake, rather than polluting the air. If the AOS starts failing, excessive vacuum can build up in crankcase, and sloshing oil can sometime be sucked into the intake, creating the smoke/smell. 


This picture shows the rear of the M96 Boxster engine removed from the car and on an engine stand. The AOS is circled in red, the bellows,  a frequent point of failure, is indicated by the blue arrow. (the engine was removed because of IMS bearing failure.

One possible clue is to try and remove the oil fill cap while the engine is running. If the AOS is normal, you should be able to easily remove the cap. If the AOS is failing, the cap will be under very noticeable vacuum and harder to remove.


Credit for suppling these R&R (Remove and Replace) instructions goes to Roy (Eddie) Turner. He says he isn't sure who he got them from though they probably began with some input from Doug DeVetter. I have amplified them using material from the Porsche Repair Manual and comments from Winnie The Pooh. Differing years may have slightly different looks but the basics are the same.

A legal disclaimer is necessary here. I am not a professionally trained mechanic. Use these instructions at your own risk. Follow the warnings items in bold particularly carefully please.

I'll begin to quote from the email I received here. The "I" may be Eddie or perhaps the original source Doug, I'm not sure.

After getting codes P1124 & P1126 from my 99' I did some research. The codes indicated an air leak, now I have a starting point. 

I opened the engine compartment using the instructions in the Owners Manual.

After checking all connections and inspecting the engine bay area I notice a "misty" build up on several areas surrounding the oil separator. Upon thorough inspection of the oil separator I could not find a leak or loose connection anywhere. I removed the "J" tube connecting the oil separator to the throttle body 


and I found a small build up of oil?! More than a slight dampness is bad. This can't be good as you don't want large amounts of oil getting into the throttle body and into the combustion chamber!
After making a call to Sunset and $108.50 (Shipped) later the parts arrived. 


I ordered a oil separator(w/ bellow),part # 996-107-023-04 and the upper "J" tube, part #996-107-145-06. Those parts are actually updates to the ones shown in the picture. Porsche shows updates/improvements by changing the last few digits in the part number up by 1.

My old oil separator was the original design, part #996-107-023-01,which means it has been updated 3 times, something for all 2.5L owners to consider as our cars are older. 

You'll also need a tube of Kluber Syntheso Glep (Porsche part# 

I found two difference on the oil separators just from looking at it, the bellows was redesigned and the connecting "bays" for the tubes have been redesigned as well. I did not even attempt to take it apart so who knows how it has been redesigned internally.

Now for the install, put aside 2-4 hours to make sure you have enough time. I took 3 hours and I'm a "weekend mechanic" if that. Winnie took 4 hours.  

E-gas models (2000-2004) will look like this:

The AOS itself is the same, the J-tube is not needed and a Y-tube is. I haven't been able to find the part number yet. Perhaps 996 107 146 00? Ask the parts department.


Use something like an old towel to protect the fenders when you lean over them. 

Take off any belt that has a sharp edge. 

And wear gloves, this is a messy job.


Jack(w/ hockeypuck), jack stand, tools to remove the rear wheel,10mm wrench & hose clamp pliers. I found the "Powerbuilt" hose clamp pliers from Kragen worked perfectly.($15.99)

It was recommended that I buy a 90 cent 1 inch screw type clamp for the bellows, it is much easier to remove in case the bellows ever cracks again. If you do this you will need a flat head screwdriver to tighten the clamp.

How to remove the oil separator

- Disconnect the "J" tube from the throttle body, and then from the oil separator. Because you can see this, it is obvious so I won't explain it.

- Disconnect the tube from the middle of the oil separator pointing towards the outside of the car.(Mine didn't come loose easily, I wiggled it a lot and eventually it came off)

- Here is where the "fun" begins, "break loose" the rear passenger wheels, jack up the car, remove the right rear wheel. Jack the car up high enough for you to fit under the wheel well.
(You must be able to access and see the bellow and bolts holding the oil separator.) 

- Place a jack stand in place just in case something happens to your jack as your head will be inside the rear wheel well and if the car falls off the jack.....

Here is what you'll see. The red circle indicates one of the bolts in the picture below this one.



- Look inside and to the upper right area of the engine bay from the rear wheel well. See the picture above. Find the two 10mm bolts and the "accordion" style bellows. Remove the two 10mm bolts from the base, save the bolts. You may need to use a socket wrench with a flexible shaft to do this or perhaps a wrench with a head that pivots.

 - Read this entire paragraph before you proceed with this step as there are several ways of doing this described. This step can be a BIG pain if you do not have the hose clamp pliers. While on the ground looking through the wheel well, loosen the bottom clamp and push it up up so it is above where it would clamp onto the connection inside the tube. You don't have to remove it, just make it so that it isn't creating pressure by sliding it up so you can lift the bellows off of its connection. If you don't have the hose clamp pliers, try using two screwdrivers and pushing them towards the center. This is a clamp you'll want to replace with a screw type clamp to make replacement the next time easier. The bellows connection seems to last about 50k miles from all reports. Winnie suggests bypassing all this difficulty and just cutting the accordion shaped portion with a utility knife. Once you do this, you remove the AOS from above and now can reach down and have easy access to the clamp which you remove as you remove the  remainder of the  accordion shaped portion.

- Now release some pressure from the jack enough to lower the car some, so you can give a good grip on the oil separator. Now from the top of the engine bay pull the oil separator outwards to release the last connection.

- Remove the oil separator from the engine bay and here is what is left will look like. The arrows indicate where the AOS attaches to the engine.

Congrats now the hard part is over!

Installing the new Air/Oil Separator

- Lightly grease the 2 O rings with Kluber Syntheso Glep (Porsche part# 

- Position the new oil separator in its home, it may take some turning and wiggling but it can be done.

- Connect the "elbow" shaped tube first, don't be afraid to push it in a little.

- Now re-raise the car up high enough w/ the jack so you can sneak under. Put the lower part of the "accordion" shaped bellow on its home and tighten.

- Put the two bolts back on that hold the oil separator in place and tighten.

- Get out from under the car and put your rear wheel back on. Remove the jack stand and lower the car.

- Now from on top of the engine bay connect the middle hose that you removed.

- Connect your new "J" tube to the oil separator and then to the throttle body. The new parts will look like this.

- If you had a CEL its time to clear it and see if this was the problem. Fire up your car and watch for anything abnormal. It may be that there is oil in the throttle body and you may need to clean that out or wait till it works itself out so the clean exhaust may not be immediate.

- If all is well put all the tops on the engine bay and reconnect your top. Torque the wheel studs to 96lbs. and take it for a spin.

The attached pictures are of the hose pliers clamps I bought and a comparison of the older and newer style oil separators.

Hope this helps in the future.

A special thanks to Doug DeVetter for input and advice. 

Anover good description with lots of labeled pictures specific to the 986S is at