Air Oil Separator Replacement (AOS)

[gdv]

I replaced my AOS, drove the car 8.5 miles without a flaw. Then it started pouring smoke out of the exhaust again and running rough. Any ideas?

Yes, common occurrence, your old defective AOS probably deposited a bit of oil inside your intake system and it is starting to come out. You can either let it burn off (may take quite a bit of time, depending upon how much oil is in there), or pull the intake system off an clean it.

I cleaned the air distributors out while i was changing the AOS and on start up I didn't have much smoke, which then went to none very quickly. Then after about 8.5 miles it started smoking and air distributors have oil in them again. Same symptoms as before, with pouring smoke and rough running. Pulled the throttle body off and air distributors are full with oil again.

[JFP in PA]

I replaced my AOS, drove the car 8.5 miles without a flaw. Then it started pouring smoke out of the exhaust again and running rough. Any ideas?

Yes, common occurrence, your old defective AOS probably deposited a bit of oil inside your intake system and it is starting to come out. You can either let it burn off (may take quite a bit of time, depending upon how much oil is in there), or pull the intake system off an clean it.

I cleaned the air distributors out while i was changing the AOS and on start up I didn't have much smoke, which then went to none very quickly. Then after about 8.5 miles it started smoking and air distributors have oil in them again. Same symptoms as before, with pouring smoke and rough running. Pulled the throttle body off and air distributors are full with oil again.

Check the new AOS by trying to remove the oil fill cap with the car idling, it should come off with little effort. If it is hard to get off, the new AOS is a dud.

[krazyk]

No bellows on the 2003. The part is very different. Having recently done this on my 2003, I dropped the engine and trans. I also had a few other things to do but I and others will tell you its so much easier to replace if you remove the engine on a 3.6 car. Its also a good time to replace the coolant tank, hoses, and several other things.

[rich69]

My car has a full milkshake syndrome

  It still runs smooth except for some smoking from the exhaust and disappearing coolant

  Also it looks like the breather hose going to the right side head is cracked and leaking milkshake mix ( a lot)

 

  Does this sound like a bad AOS  ?  OR am I back to the cracked head theory  ??

[JFP in PA]

My car has a full milkshake syndrome

  It still runs smooth except for some smoking from the exhaust and disappearing coolant

  Also it looks like the breather hose going to the right side head is cracked and leaking milkshake mix ( a lot)

 

  Does this sound like a bad AOS  ?  OR am I back to the cracked head theory  ??

 

While the AOS can fail and leak coolant into the oil system, it is not a particularly common type of failure.  More common on your model car is a cracked cylinder head (and before you ask, these engine's never blow head gaskets, the MLS head gasket is actually stronger than the head itself).

 

In order to confirm what is going on, you need to pull the spark plugs and look for any signs of unusual plug coloration (cracked heads often lead to "steam cleaning" of the plug in the bad cylinder).  You should then run a leak down test on that cylinder to confirm the problem.  You can also perform a pressure test on the cooling system, but that would be a bit redundant as it is already obvious it is leaking.

 

I also would not be driving, or even running the engine in this condition; coolant is Hell on bearings and other critical surfaces, and could result in totally losing the unit.  You need to plan on and budget for dropping the engine (while the AOS could be replaced with the engine in the car, getting at it is a bit ugly, so most shops lower the engine to save both time and money).  Assuming you do have a head issue, it is also far easier to pull the heads with the engine on a stand.

[rich69]

AOS  Follow Up :

 

  OK  No.  1       Does the fact that I have a coolant and oil mix coming out of a cracked breather

                         hose on the right side cylinder head indicate a possible bad  AOS  ?

                           My theory here is that the bad AOS  is allowing fluid to get into the breather pipe

                           thus causing it to burst...If so that would explain a lot...

 

   and  no.  2       If the problem described above is correct  then would I still have high vacuum in the case

                            or could it be a normal vacuum but still failed in the way that I decribed ?

                           In other words  does a bad  AOS  always involve high crankcase vacuum  

                           Or can it fail only on the water cooling curcuit  ??

 

   and  3.          I will repace the  AOS  while in the car because I am very good in tight places :king:

                  

[JFP in PA]

AOS  Follow Up :

 

  OK  No.  1       Does the fact that I have a coolant and oil mix coming out of a cracked breather

                         hose on the right side cylinder head indicate a possible bad  AOS  ?

                           My theory here is that the bad AOS  is allowing fluid to get into the breather pipe

                           thus causing it to burst...If so that would explain a lot...

 

   and  no.  2       If the problem described above is correct  then would I still have high vacuum in the case

                            or could it be a normal vacuum but still failed in the way that I decribed ?

                           In other words  does a bad  AOS  always involve high crankcase vacuum  

                           Or can it fail only on the water cooling curcuit  ??

 

   and  3.          I will repace the  AOS  while in the car because I am very good in tight places :king:

 

1. No, it simply indicates a failed breather, which often become brittle with age and crack.
2. You should be reading a vacuum level of 5 inches of water (not Hg) at the oil filler cap.  If you are reading higher than eight to ten inches of water, the vacuum diaphragm section of the AOS has failed.  These fail all the time without any intermix problems whatsoever.
3. That is your choice, but going through that exercise you may then still have to face dropping the engine anyway.

[Y2K911]

JfP:

What do shops prefer to do to r&r an AOS?

1. Access from the top removing left intake manifold, etc & from the bottom; or

2. removing the Xmission to get easier access?

Merci bien!

Martin

[JFP in PA]

JfP:

What do shops prefer to do to r&r an AOS?

1. Access from the top removing left intake manifold, etc & from the bottom; or

2. removing the Xmission to get easier access?

Merci bien!

Martin

 

Depends upon the model and how it is equipped.  In general, lowering the engine is the quickest and therefore the cheapest for a shop; but not everyone is equipped to readily handle that.  Most DIY's are done from above (there is a DIY tutorial in that section at the top of the page).

[Y2K911]

Belated thank you, Jean-F.

Merci encore une fois.

[Ahsai]

For MkII AOS specific steps, please check this thread http://www.renntech.org/forums/topic/47759-mkii-aos-diy-sanity-check/?p=262962

With the engine lowered as much as it could, it's not that bad.

[tac27]

Here's another "I did it!" success post!

Many thanks to: Hobbes for the original DIY AOS post; to Ahsai for his post ('MkII AOS DIY Sanity Check') regarding the procedural differences for the MkII engine; and the many other users who have posted about their AOS replacement experience .

Especially thanks to Loren et al who created and maintian this resource!

If it hasn't already been done, I would suggest that the link to Ahsai's post be added to the Hobbes original DIY post. The search function didnt return Ahsai's MkII post; it was just 'blind-luck' that I found it while wandering through other post.

Edited September 14, 2015 by tac27

[Haiyan]

My AOS on the 2001 996 failed just before couple of days and replaced it. I was able to identify the failure at the beginning when the engine started smoking a bit I turned it off immediately. However, I decided to change the internal units too (inside the oil pan) when I removed the oil pan I saw some broken plastic pieces around the pickup tube so I had to clean it off and replaced the two oil separators (99610708054).

 

The job was a bit easy but the clean up toke a good amount of time because a significant amount of oil was found in the intake, when sealing the oil pan back you should let the sealant cure properly before adding engine oil. Also it is a good idea to change the oil filter at the same time.

 

After the work is done make sure that everything is fitted properly and the oil level is at the max mark (if replaced the oil filter fill the oil filter cap with oil before installation) then start the engine and let it idle for some time to circulate the oil in the engine. you might experience some noises and smoke during idling this will be present as the intake got some good amount of oil in it.

 

Edited September 16, 2015 by Hayyan Al-Raisi

[Hobbes]

Here's another "I did it!" success post!

Many thanks to: Hobbes for the original DIY AOS post; to Ahsai for his post ('MkII AOS DIY Sanity Check') regarding the procedural differences for the MkII engine; and the many other users who have posted about their AOS replacement experience .

Especially thanks to Loren et al who created and maintian this resource!

If it hasn't already been done, I would suggest that the link to Ahsai's post be added to the Hobbes original DIY post. The search function didnt return Ahsai's MkII post; it was just 'blind-luck' that I found it while wandering through other post.

 

 

Thanks.  Glad the AOS DIY worked out for you. 

 

Hobbes

[Youri Ko]

I did mine last week, for nothing, as my old one did not have a broken diaphragm , but still I can probably bring few points to this DIY 

 

This is for 2001 C2

 

1: Lower the engine with a jack underneath as much as it goes, eventually it will sit on the crossmember and the jack won't bare any weight. I still used bunch of jack stands by precaution. I did not unplug anything prior to lowering.

 

2: Work from top most of the time, Keep alternator in place. Use 1/4" air ratchet for most of the work

 

3: and probably the most helpful - to put the provided clamp back, first use Clamp pliers, put them around the new clamp, squeeze them and use a tie-wrap to hold the pliers squeezed. Slip the clamp/pliers over the bellow and use the pliers to help you orient and slip the bellow over it's receptacle on the engine's part. Once it place, cut the tie-wrap.

This helped me a lot, otherwise I would curse for hours and probably ended up putting a generic clamp over.

Edited October 20, 2015 by Youri Ko

[Akbar Tokhi]

Great DIY thanks. Now i'm a bit stuck here and need some help please.  2004 996 C2 6spd. I have finally managed to take the last intake bolt out but I'm having a hard time detaching the lower AOS hose that runs through the upper intake and across the engine. There is simply no room to apply pressure and pop the hose out. Any help is greatly appreciated.

 

Thanks

[royp4]

I have a 2003 996 cab. AOS found to be cracked. Due to time constraints and lack of faith in my ability to do replacement, I chose to let dealer do the job. $1800 was the cost of the job. I feel ashamed that I did not tackle this myself, and weep for my depleted bank account. Bravo to any and all who did this job themselves. I'm sure they are both wiser and wealthier than me.

[JFP in PA]

5 hours ago, royp4 said:

I have a 2003 996 cab. AOSfound to be cracked. Due to time constraints and lack of faith in my ability to do replacement, I chose to let dealer do the job. $1800 was the cost of the job. I feel ashamed that I did not tackle this myself, and weep for my depleted bank account. Bravo to any and all who did this job themselves. I'm sure they are both wiser and wealthier than me.

 

Don't feel bad, the AOS replacement on a 996 is not for everyone and can easily turn into a knuckle busting, "wish I hadn't started this" affair; particularly if they do not have the necessary facilities at their disposal.

[wyovino]

Every month or so I re-read the DIY and about halfway through it realize that this is way beyond my comfort level.  I'm impressed when someone reports that they've done it.

[royp4]

Thank you for your kind comments. Glad to see that I'm not the only Porsche owner who is unwilling on unable to tackle a repair job on my ride. To soothe my hurt feelings I fixed my dad's lawnmower today. Lol

[BillyRay]

A recent AOS failure pitched me up here. Fairly handy under a car but this is easily the toughest task I've attempted & wouldn't have had a cat in hell's chance without your outstanding tutorial, Mr Hobbes. Probably took me a good 10hrs all told but the DIY was bang on the money - a thousand thanks!

 

For what it's worth, here are the tricky bits I encountered:

 

Alternator removal -  no amount of prying was shifting mine so used a 1/2" drive extension piece, a hammer and a lot of tapping on the right side mount to slowly rotate the alternator clockwise. This requires A LOT of patience (I almost threw in the towel at this early stage) as you don't want to use too much force but it works.

 

Fuel lines - when disconnecting each of these use 2 spanners, one to hold & brace the base of the connector while you use the other to turn the actual connector. They take a fair bit of torque to release and the thought of all that force transferring across made me sweat. Also worth draping a rag over each one as you disconnect to catch the little spurt of fuel when they pop apart.

 

Coolant tank - had to remove the sensor underneath (1/4 turn anticlockwise, be gentle!) with the tank in situ as there was no way the tank was going past the fuel line connection with it in place. Also dropping the engine down to the limit of the motor mounts made tank removal a breeze. Spent a good hour trying to tease the tank out before this occurred to me.

 

Throttle body and upper intake were straightforward but for the love of god, as soon as you take that intake off get those lower intake holes taped over with duct tape. Dropping anything down one of those doesn't bare thinking about! 

 

AOS - getting enough purchase on those **** squeeze fit hose connections was a true test of faith but they do come apart. The bottom one was the hardest and I could only do it reaching up from underneath the car. Took a while lining up the new AOS (found I needed to manoeuvre it both from the engine bay and underneath the car) but when you get it in the correct position I found it kinda dropped into place and felt 'right'.

 

Things I broke - one of the little plastic vacuum line pipes (part no. 000.043.205.01 420mm) and a plastic coolant connector (part no. 996.106.226.52). This connector is bolted to the block directly above the tensioner pulley and connects the coolant hose from the AOS to another coolant hose and had become incredibly fragile. Breaking this screwed up my plan of just siphoning out the coolant tank and meant I had to drain the engine block instead.

 

It's a hard job but doable. Just take your time, go slow. and most importantly be patient, patient and more patient. Ideally leave 2 clear days so you don't feel rushed by time.

 

Only specialist tool I used that was a genuine help, bordering on invaluable, was a pair of flexible long reach hose clamp pliers.

[Michael Laity]

Just replaced mine this past weekend. Took 7 hours start to finish. I have an 03 996.2. I found loosening the AOS first was easier before trying to get to the last manifold bolt. Assembly didnt go as well because some bolts weren't lining up. But I got it done.

Sent from my ONEPLUS A3000 using Tapatalk

[Amtaylor]

I read this post and found it helpful replaced AOS on 99 3.4 cab didn’t need to lower engine just removed the vacuum canister easy access for rear inlet bolt 

 

[Amtaylor]

For the bellows clip there is a tool that. Compresses the clip and its articulated, took me 5 hr to do only need small tools except for belt removal 

[rhkwon]

Here is my AOS replacement experience.

AOS DIY

1999 Porsche 996 Cabriolet C4 Manual Transmission

Before I get into the nitty gritty of the actual AOS replacement, let me give you some background on my car.  I bought this car about a year ago from California with about 127,000 miles on it and it now has 133,000 miles on it.  Car drove perfectly all the way to Texas but we all know that perfection never lasts.  After a couple of days, the code for a faulty spark plug coil pack came on and the engine ran rough.  So, I changed all 6 coils with the updated 997 Beru coils and new Bosch spark plugs.  I also changed the motor mounts.  Ran great for a few thousand miles.  I noticed every once in a while, that the temperature gauge would fluctuate up just a tad but nothing to worry about.  Then last September my daughter stole my car while I was away on vacation in Cancun with the wife (Risky Business) and drove it to Tyler TX and the lower 3-way splitter hose failed.  So, got the car towed to a pretty good European mechanic and he replaced the splitter hose and the coolant hard line pipe and flushed the coolant.  After a couple of months of enjoyable and care free driving I noticed the smell of coolant and find out that the top of the coolant expansion tank is leaking.  I figured it was the bleed valve and so I tightened the bleed valve bolts (after much cursing to get to the 2 bolts covered by the side of the engine).  This did not work so decided to go cheap and replace with a Uro brand bleeder valve.  The bleeder valve would not seal correctly so I decided what the hell, rather than chance it and try to buy an OEM bleeder valve and then the real cause being a cracked expansion tank, why not replace the entire cooling system?  I bought every single coolant hose from front to back, a new water pump, new thermostat/housing, gaskets and new bolts.  After cleaning the front radiators of debris (which I highly recommend every few years), I decided that I would leave the front end of the cooling system alone for another time.  This will be Phase II when I decide to replace the radiators.   Although the hoses are 20 years old, I honestly think that the main front and the 2 heater hoses under the middle of the car could last quite a few more years.  

Since I already had everything out and apart, I decided I might as well change out the AOS.  Hearing horror stories about this procedure did not inspire any confidence in me.  I have never had the white smoke symptom but an occasional faint lumpy idle (which could mean a million things) caused me to think the AOS could be the culprit.  The clincher for me was not having any documentation that the AOS was ever replaced (which I later found out that it had indeed been replaced) along with not wanting to have to remove everything again in order to replace it if it did actually fail. I don’t know about you guys but I am lazy as hell and I hate doing things twice. Kill 2 birds with one stone.  Measure twice cut once, etc. I think I went overboard on this project but I can never leave well enough alone.  Most of the parts I replaced could easily have lasted much longer but again I am very lazy and I’d rather just do it and not have to worry about should I have replaced that part while I had the chance or when it was much easier to do?  Besides, the labor I saved doing this by myself basically made all the new parts and beer free!

The DIY guide on Renntech https://www.renntech.org/topic/37442-air-oil-separator-replacement-aos/ proved invaluable and if you have a 1999 – 2001 manual transmission this should be your guide and you should be fine.  This is the ultimate guide and my details are just ancillary with a little bit more detail.  I did notice that there are some minor vacuum hose differences between 1999 and 2000-2001. Pelican also has a good DIY. I did not have to lower the engine.  There were no special tricks or unusual surprises where I got stuck.  Take lots of pictures and bag each set of bolts for each particular part that is removed.  Human nature is to forget things and you will forget how a part went and a picture is invaluable.  When putting back everything, I did get lost a couple of times. 

After having completed this job, I have come to the conclusion that the AOS replacement is not difficult at all. Keep in mind the later model year 996 may much more difficult but for the 1999 it is very easy.  What is makes this job appear so difficult is the totality of having to methodically remove so many different parts to get to the AOS makes it look so daunting. Removal of each separate part is not difficult at all.  Don’t be intimidated. Be confident in your abilities.  I’m an idiot.  If I can do it, you surely can. Take your time.  Go slow.  Take lots of pictures and/or videos.  Write down notes.  Bag your bolts and parts and label them.  Buy your parts before hand.  Even if you buy all the parts you think you will need, you will probably order more parts later on because something broke or looked old.  

Here is a summary of the steps in the order I used to get to the AOS.  Note that these steps are after having jacked up the car, draining the coolant and replacing the water pump/thermostat:

1.    Remove air box.  Disconnect the electrical connector.

2.    Remove engine coolant hoses.  You do not have to do this but I had every single coolant hose removed as these were all being replaced anyway. I would rather replace a hose on any car when given the opportunity as opposed to being stranded out in the middle of nowhere. Gives you a ton more room to work and prevents snagging.  I even replaced the 2 heater hoses that are located below in the middle of the car fore of the engine.  In order to keep track of which hose went were, I took a lot of photos and even left a few dangling so that I would know how to put them back together.

3.    Remove air pump, associated hoses, and disconnect/loosen the air check valve of which I believe there are a total of 3.  The first is just to the left top of the alternator which I left unbolted from its bracket and later replaced. There is one under the driver side intake manifold which I also replaced and the last one under the passenger side intake manifold which I left alone for obvious reasons.  The part number is the same for all 3. I replaced mine with Pierburg brand 7PP906270.

4.    Remove fuel filter along with the rectangular shaped fuel line.  Use a 19mm crow foot and a 17mm wrench.  Although my fuel filter had only 12,000 miles on it, I decided to replace it along with a new fuel line as some of the fuel line rubber had cracks in it maybe from me pulling on it when I removed it.

5.    Remove coolant expansion tank – easily removed once the fuel filter and fuel line are removed.  Slides right out from the holding bracket to the right towards the engine.  Just be careful to disconnect the coolant level sensor electrical connector first.  Now is also a great time to buy a brand new expansion tank with a new sensor, cap, bleed valve and overflow hose.  These stupid tanks are so badly designed that any time is probably a good time to replace these.  My Toyota Land Cruiser has 300K on it and the coolant tank has never been replaced.  Why anyone would engineer a split tank somehow glued together and capable of splitting at these very seams is beyond me.  Well, to make money on replacing of course.

6.    Disconnect all the electrical connectors, vacuum hoses, throttle body valve and hose connectors that go to the AOS. 

7.    Remove the alternator.  Removing the alternator gives you a ton more room to work under the intake manifold when accessing that last intake manifold bolt in the back.  I also noticed that the pulley was making a slight grinding sound and with no documentation that the alternator was ever replaced, I decided to replace the alternator anyway.  Secret tip: rather than pounding on that god **** bushing for hours and putting hammer marks and dents all over your car and weakening your tensioner bolt and the lower bolt, just remove the engine block cap 997-101-021-00 that is held on with 6 10mm bolts.  This cap attaches to the coolant hose that goes to the joint support connector that cools the AOS and attaches to the 3-way splitter hose.  Removing this made alternator removal a breeze.  And make sure to replace the gasket with a new one 996-101-331-50.  Gasket only costs $5 which is well worth it as opposed to hours of pounding.  Another reason to go this method is that it looks like significant pounding of the long tensioner bolt could cause strain on the hole of the cap where the lower bolt goes through which could then cause a coolant leak. Torque back the bolts to 7.5ft/lbs.  Now would also be a great time to replace the coolant hose and the joint support bracket.  Once the alternator is removed, you can see the “sunflower face” of the AOS in the back!  Be patient, you are getting really close now.

8.    Remove the 6 upper intake manifold bolts.  The first 5 are easy as hell.  The 6^th one is also fairly easy.  I used a ¼” ratchet with a 4-inch wobbly extension and 10mm socket.  There is a large dome shaped plastic 993-110-140-03 vacuum reservoir that can get in the way if you have X-large hands like me and can make getting your right hand into the right side of the manifold for ratchet guidance difficult.  Secret tip: remove the Cut Off Valve for Air Injection (part# 993-113-245-01-M100) and the 2 associated air hoses connected to it (the longer hose attaches to the air pump and the shorter elbow hose attaches to another valve that is attached to the engine block).  This will give you tons more room to get your right hand in there.  Now is also the perfect time to replace both of these valves, the 2 air hoses and the dome shaped vacuum reservoir.  Once the alternator is out, removing the last bolt is really easy.  For me, removing just the longer air hose and the cut off valve gave me enough room.  Don’t forget to undo the thin vacuum hose that is connected to the dome shaped vacuum reservoir.  Removing the cut off valve gives you much more space to get your right hand to hold the bolt and guide the socket onto the bolt.  You can also use a magnetic bolt pickup tool/stick as extra insurance so that you don’t lose or drop the bolts.  For bolt number 6, I used a 4-inch extension and a ¼ socket wrench and went straight down from the top.  On the 1999, there are 2 fuel lines that appear to run across and over the bolt but this is not the case.  This just appears that way due to the angle that you are looking at it from. You will have a perfect straight shot down to the bolt.  You can use your right hand to guide the socket on.  Whatever feels comfortable for you.  When I had the bolt loosened just enough, I used my left and right fingers to turn the bolt undone and carefully cradled the bolt out.  You can also use a magnetic pickup tool/stick attached to the bolt as extra insurance so that it does not drop and get lost.  I did notice that all the bolts had blue thread Loctite on them.

9.    Remove the throttle body.  Remember to disconnect the electrical connector and the AOS hose. Remove the 4 large hose clamps (use either a screwdriver or 5mm socket) of which there are 2 on each side to remove/slide off the rubber sleeve. Then remove 10mm hexagon nut from the throttle body housing mount (basically a rubber grommet that slides over a screw that bolts to a bracket that attaches to the engine block) located on the bottom left corner (right above the power steering pump pulley) that gives the throttle body extra bracing via a bracket.  After 20 years, the rubber on mine broke apart.  Just replace with a new one 999-703-265-00.  Be careful not to drop the nut or the grommet!  After the 2 left side clamps are undone slide the boot to the left and go on to the right side and undo the 2 right side clamps.  At this point you can remove the throttle body.  With the throttle body out, you will have lots of room to loosen/undo the 2 rear left sleeve clamps.  Sliding the left sleeve to the left onto the intake manifold will allow for extra room to slide the throttle body to the left and out.   Now is a good time to clean your throttle body and the rubber sleeves. The front left rubber sleeve has a thin plastic vacuum line that connects to it as well as the rear right rubber sleeve.  I noticed some obvious heavy oil residue in the lower intakes as well as a nice oily sheen in the intake manifold and the rubber sleeves. Wipe clean and/or replace the rubber sleeves making sure which vacuum line goes to which rubber sleeve. 

10.  Remove the upper intake manifold.  Just lift up the top hoses/electrical wires, etc. that are at the rear of the engine and the ones connected to the throttle body regeneration valve out of the way so that the manifold can slide right out towards you. Be careful not to snag on any wires or hoses and it should come out easily and cleanly.  Wipe clean the inside of the intake manifold as best you can.  Once the manifold is out, you will have a straight and easy shot/access to the AOS. That beautiful sunflower face stares right back at you.  With the manifold on the engine, the AOS looks so far away and hidden.  With the manifold off, the AOS is actually very close and access is easy!  At this point, I contemplated removing the passenger side manifold but this looks like a far more involved process including removing the AC compressor and the power steering pump so I left it alone.  I wiped clean the inside of the passenger manifold as much as I could. 

11.  Remove the AOS.  Jack up the car on all 4 sides so that you can have plenty of room to work.  Removal of the driver side rear tire is not necessary at all but doing so can give you some extra space. To give you a sense of direction, I slid underneath the car head first with my head directed towards the front of the car.  Once your head is slightly past the AOS, you can shine a light onto the AOS.  You should have decent access to the AOS with your left and right hand.  First remove the AOS hard plastic hose connector on the right side with your right hand.  I used my right hand to pinch down the 2 ribbed areas and was able to pull the connector off.  The trick here is to slide yourself to the left a little bit and with your head facing towards the passenger side of the car you can manipulate your right hand by feel and disconnect the connector.  I tried using my left hand at first but the angle and available torque was not good. There are a bunch of miscellaneous unrelated electrical wires right below the connector that look like they will get in the way but don’t worry about these.  No need to undo or move them out o fthe way.  Now you can either cut the bellow in half or just undo the bottom clamp. Whoever previously replaced the AOS on my car used a black zip tie!  If you have an Oetiker clamp on the bottom of the bellow, use a plier cutter and then remove the final part of the bellow.  If you bellow is attached via a spring clamp, I highly recommend using a coolant hose spring plier.  Then unbolt the 2 10mm bolts that hold the AOS to the engine block.  I used my right hand and a ¼” ratchet with a long 10mm socket.  Take your time and the bolts will eventually come out.  It got a little tiring because the bolts are fairly long.   They also had blue Loctite on them.  Use a big long flat tipped screwdriver to carefully pry the AOS to the drivers side out away from the engine block and then you will be able to pull the AOS out from above the engine.  There was only a bit of oil that dripped out of the AOS on mine.  Most of the inner passages of the various hoses and intake manifold/boots had a thin coat of oil on them.  Not sure if this is normal but I would think that a thin mist will eventually turn into a heavy mist which in turn will become a heavy coating of oil.  Some of the hoses had drops of oil running out/down so I think it was a good time to replace the AOS.

The following are optional items that are readily and easily replaced now that you are in this deep:

12.  Replace oil filler tube and cap. Driver side bolt easily removed. Easier to tackle the passenger side bolt from the right using your right hand.  Use a 4” extension to get the 10mm bolts out.

13.  Replace knock sensor.  Using a 13mm wrench, undo the bolt and the connector and replace with Bosch 996 606 125 00.

14.  Replace starter.  First undo the ground and electrical connector. Then, using an extra-long extension or a combination of several long extensions, undo the 2 15mm bolts that attach the starter.

15.  Replace Secondary Air Injection Check valve 964-110-950-02. 

16.  Replace all vacuum lines/hoses.  Although you could leave well enough alone, I highly recommend replacing as many if not all the rubber vacuum lines as possible.  This is the best chance with the easiest access to do this.  Some of the lines are very thin plastic lines, standard 3.5mm rubber lines and several semi u (L) shaped rubber connectors known as L fittings 928-574-717-07 which help connect several of the valves’ hard nipples to the thin plastic lines.   

17.  Replace the regeneration valve from throttle body for fuel vapor system and the associated rubber vacuum hose.  OEM Porsche costs $220 and the exact replacement from Bosch is $40.  The only difference is the 5” hard plastic hose fitting attached to the valve.  You can try to reuse or buy a similar type hose and heat it with a heat gun so that it seals.  I went all out and just bought the OEM Porsche part.  I believe this part was on its way out as I had begun to hear whistling sounds from the back when in 6^th gear cruising on the highway I would tap the accelerator to gain speed.

18.  Replace the power steering upper tank and/or O-ring.  My tank was weeping so I replaced the O-ring and tightened the screw connector down.  Siphon out the fluid first and wrap a towel around the connector to avoid spills.

19.  Replace the engine mounts.  Very easy and simple to do.  Just jack up/support the engine and replacement is easy and cheap.  Makes your muffler pipes look better too.

20.  Replace both idler pulleys and the tensioner pulley.  Al 3 pulleys were making a grinding sound when I spun them freely so I had the idler pulleys replaced with Dayco brand and the tensioner OEM.  Good time to replace those bolts also as there is a lot of stress on these.

21.  Replace plastic water line support (2) for large driver and passenger side coolant pipes 996-106-421-51. Easy to take off and replace, especially with all the hoses removed.  And although not really necessary, I am very anal so I replaced mine just so that my engine bay looks cleaner. 

22.  Replace serpentine belt.  Self-explanatory.

23.  Replace air temperature sensor and rubber grommet.  Again, not totally necessary and not made any easier as this is located on the passenger side intake manifold.  But with the driver side manifold removed, replacement is a breeze.  First disconnect the electrical connector and then remove the sensor and the grommet.  (My grommet was a bit hardened from age and heat and the male part of the sensor was just loosely inserted into the hole and not seated fully into the grommet).  Then remove the sensor and the grommet.  Install the grommet first. The foundation for the grommet is a circlip shape and so if you first put the sensor into the grommet and then try to install these two together as one piece onto the manifold, it will not work.  It took me forever to figure this out but you need to install the grommet first and then using some soapy water onto the grommet, push the sensor in to the grommet with your right hand while at the same time using your left hand to support the grommet from underneath (and made so much easier now that the driver side manifold has been removed).  I had tried to install this prior to removing the driver side intake manifold and it took me forever to figure this out.  I even lost the sensor and grommet down into the engine bay but was able to retrieve it after the driver side manifold was removed.   

24.  Replace the intake manifold gaskets.  You could reuse the old ones but why?

25.  Replace oil and filter.

26.  Replace rear differential fluid.

27.  Replace elbow vent valve and O-ring that connects to AOS air hose on the passenger side.  See https://rennlist.com/forums/996-forum/1141866-what-is-this-part-on-the-passenger-side.html for details.