So I replaced my Air/Oil separator

Last week, I noticed that my 112,000 mile 996 was starting to idle a little erratically - not too bad, but definitely not normal either. Then the Check Engine light came on, and stayed on. I went to a friend's shop and they plugged in a code reader. While I can't remember the code, the end result was that the system had detected a big vacuum leak; in fact, we could actually hear it sucking air. I was told it was likely the AOS. I went home, researched it a bit, and found that a good test for that was try to pull off the oil cap while the engine was idling (normally it comes off easily). As predicted, it was sucked on there tight - so tight, I could barely get it off. So, it seems I had a AOS failure. After some consideration, I decided to tackle this myself, but there's so little real DIY info out there. It's piecemeal, at best. I put together a basic plan, figured I'd take my time, and ask questions when needed. In fact, I'm lucky enough to have a friend who lives down the street who happens to be a Porsche dealership mechanic - and he's really good. He offered to answer any questions I might have and come on by to take a look and offer advice. Perfect. The back of the car was put on jack stands and the left rear wheel removed, in case I'd need to gain access (in fact, this helped me to retrieve the sockets dropped down onto the case all too regularly ). The air filter box was removed and put aside. The tasks ahead were to remove the throttle body, disconnect some electrical connections and hoses, remove the left side intake manifold bolts, then remove the intake plenum and varioram plenum, and finally pull out the intake manifold. Only then would I have access to the AOS. More to come...

I'll be reading this one in the morning!

So, first I removed the throttle body. I unplugged the electrical connection up on the upper left corner. It's a bit of a bugger (as they all are), but just squeeze the top and bottom of the connector while pulling gently and it should pop out. move the connector aside and then remove the four 10mm bolts that attach the throttle body to the plenum. Pull it out and store it away in a clean place. I'll re-post the shot above to make it clear, along with another one. In the second shot, you'll see a couple of hoses that come together into a union at the upper left corner of the intake plenum. The clip that attaches them to the plenum is a mystery to me, but I fiddled with it, twisting and squeezing while I pulled and it finally popped off. I never figured out how it worked, but it was off, so I didn't investigate further. That big hose on the left of the picture that ends in a pointed contraption needs to be unbolted where you see that bracket just behind and above the alternator in that shot. Unbolt it and move it aside. In the last picture, you can see that I lowered the engine as far as it would go before hitting the suspension sub-frame (before I did all of the disconnecting of hoses and electrical lines I just described). The timing of the engine lowering is probably not too important - other than you want to have it down BEFORE you try to remove the intake manifold bolts. I can't imagine trying to do that with the engine tucked up high in the engine bay. You can see that it adds about three inches at least to the working space above the engine. To lower the engine, place your floor jack and a large block of wood under the engine and support it. Then climb under the car and look for the two engine mount nuts just behind the rear bumper. Remove them. You'll need an 18mm deep socket for that. Now, lower the engine carefully with the floor jack until the engine is just about on top of the suspension cross member - and leave it there for the duration. I placed a couple of jack stands and wood under the oil pan in case the floor jack deflated over time. It didn't.

Now that the engine has been lowered and the throttle body is off and some basic connections are disconnected, it's time to dig in deeper. The first task should be to loosen all eight of the big hose clamps connecting the plenums to the intake manifolds. I used a 7mm socket with extension to make quick work of that job. Now leave it. Don't try to remove the plenums yet, you will likely just waste a lot of time trying. Next, loosen and remove ALL of the intake manifold bolts. Trust me on this (I learned through trial and error throughout this job) - if you remove the manifold bolts first, you'll then be able to move the manifold outboard a bit to enable easy removal of the the two plenums. The hardest job yet is finally upon you, your poor arms, and your patience. Trying to get the 10mm intake manifold bolts off is not easy - especially the last one in the very front (my favorite expression always began with an F. My neighbors probably think I'm crazy now - muttering to myself while hugging the engine of my beloved 996 ). The bolts aren't tight; they're just hard to get to. Actually, the rear five aren't too bad. It's the previously mentioned last one. Number six. Here's what I finally figured out: Do NOT try to come at it at any angle with a swivel socket. It won't happen. If you use a 1/4" drive socket extension about the height of the manifold itself - maybe 7 or 8 inches, and leave the wrench off for the time being, you can come around the left side with your left arm and then, as you feel your way past the edge of the manifold, come straight down - STRAIGHT DOWN - and feel for the bolt head down there. If you've gotten your socket near to where it needs to be, it should slip right onto the bolt head with only a little wiggling to find it. I HIGHLY recommend that you go in bare handed at first - with no wrench. Try to "learn" the layout of the manifold down there with your fingers, Find the bolt head. Feel where it is in relation to other landmarks. Get familiar with it. Then, when you're comfortable finding it every time with your left hand (basically you index finger only), try it with the socket and extension. You should be able to find the bolt head if you come absolutely straight down on it. It's a tight fit in there, but don't give up. Once you're on it, bring the wrench in, carefully attach it to the extension and slowly loosen the bolt. Make sure you have a magnetic pickup stick. You WILL drop sockets and bolts onto the case. Now you can see the AOS at the front left. As my Porsche mechanic friend recommended to me, don't pull any water hoses off of it just yet. Pull off the big hose on the right (with that funny connector again) and that's all. Now, head down underneath the car with a light and get into position to see the AOS from below. You'll see the bellows and it's clamps. My Porsche mechanic friend told me to just take some pliers, squeeze the clamp, and pull it (the clamp) up and off the metal tube that the bellows attaches to. It's a bit ungainly and will deform the hell out of the rubber bellows, but no worries - you have a new one. Now, take your trusty 10mm 1/4" drive socket and about 15 inches of extensions and come in from the side and remove the two bolts that hold the AOS onto the case. Get a screwdriver or something similar and start wedging it outboard. Once it's clear of the case, climb back topside, grab the AOS and pull it up and back toward the rear. Once you have it in front of you, it's a simple matter to pull off the two water hoses, but be aware that you will spill a few ounces of coolant, so have a small container ready for the overflow. Woohoo!! The AOS is out!

Now, that it's out, the installation is straightforward. It REALLY IS the reverse of removal. Find some good grease and lube up the O-ring that goes into the case. Climb back under the car, and slide it back into place. Do NOT be dismayed to see a gap between the case and the AOS when fully tight. That's how the mounting system seems to be designed. It does not sit flush. As for the bellows and it's lower clamp, I did what most here do - and my Porsche mechanic said is common - and didn't end up using the factory clamp. I tried, but it was a pain. Instead, I went with a simple hose clamp - works great. But first, I pushed the AOS in as close as I could, came in with the bolts, and drew them down carefully until the AOS was secured tightly. Then I came back to the bellows, made sure it was positioned properly onto it's down-tube, and then installed the hose clamp. Done. Climb back to the topside and re-install the two water hoses and the other larger hose. The easiest re-assembly (and I tried every incorrect method - trust me) involves slipping the intake manifold back into place first - but leave five of the the six bolts out. The sixth bolt, that blasted one in front, really can only be installed by putting it into its hole in the intake manifold first - BEFORE you slip the manifold into position. If you're worried about it falling out, then tape it loosely to the manifold first. Trust me - you WILL NOT get the bolt into the hole with the manifold already in place. I tried...and tried. Once the manifold is in place with the one forward most bolt sitting in place but not engaged - and all of the other five bolts NOT YET installed - reposition the intake plenums and their associated rubber sleeves back onto the right and left intake manifolds (noting their positions when they initially came out, by the way). Leave the hose clamps loose for now. Do not tighten them up yet. Now, position the left intake manifold back over its mounting holes and re-install those five remaining bolts. Tighten them down, but not too tight (the manifold is plastic). After you've tightened the intake manifold bolts, go back to the hose clamps holding the plenums in place and tighten them down, making sure that the two plenums are oriented the same way they were when you removed them. Now, raise the engine back up onto its engine mounts, re-install the engine mount lower nuts and tighten. Re-install the throttle body and plug its electrical connection back in. Re-attach the hoses and electrical connections you removed earlier. Re-install the air filter box, re-attach the wheel. Finally, lower the car to the ground. Start it and look for any leaks. You should be good to go.

nice diy.... welcome to the r&r aos club.

How many hours did this take and how many if you did it a second time knowing what you know now?

I'm impressed. Decent write up!

Excellent write up...Thank you.

very nice DIY, much appreciated considering a lot of people had to replace this part. Can you describe how much force is now required to lift the oil cap with a brand new AOS ?

A 5 year old could twist it off with ease.

As for time... I took my time, and was learning as I went, so it took me just a tad over two casual days. However, now that I've done it, I think it would take me about 7 hours total.

great writeup! Someone needs to put a sticky on this or post this to renntech too! Thanks for sharing.

Thanks.

Thanks for taking the time to do the write up and take pics. Posts like these add value to the site(s).

I finally got around to reading the instructions. Great write up. Couple things I would mention (due to my mistakes. Actually the first is informational, the hose clamp that fits to the plenum is like a medicine bottle where you sweeze the serated/ribbed parts and the clips perpendicular to the ribbed pieces come out and you can pull the hose off. Mention when you plug the intake holes so that when someone disconnects the coolant hoses they don't do what I did....dump some coolant into the intake. Take care in the plenums since the rubber bushings holding them in place have vacuum line connections that can break since they are now most likely brittle. When reinstalling the upper coolant line make sure you don't push the hose so far that it rides up onto the gusset which will then cause a leak when you tighten the clamp. I was able to get the last bolt in after the fact. I just made sure that all the others were loosely on to give the last one some adjustment. I also took a corner of the blue shop paper towels and pressed the bolt into the socket with the towel between them to make it "stick" as soon as it connected I puled the socket off and removed the paper towel. The only thing you need to make sure is out when lowering the motor is the air box. Good write up I hope this makes it easier for others.

Thanks for the additional input, Ed. Hopefully, this will help others who decide they may want to tackle this job. Also, prepare to have your left arm and hand scratched and cut beyond belief. I swear, I look like I got into a fight with a cat and lost.

Hi, thanks for the writeup. It gave me the confidence to tackle this involved job. Car now idles perfectly, there's no vacuum at the oil filler tube now (see before after pics) and hopefully no more P1128 error and no more startup smoke. My only suggestion to provide greater access and to save your hands and arms is to remove the generator. I had no problems removing any bolts or installing them since I could get both hands all the way to the front of the motor. Also used the porsche lower bellows clamp and tool - made light work of that too - just be sure to put the clamp with tool attached on the bellows, and lube the inside of the bellows before installing the AOS to the crankcase - then it was a simple job to fit the bellows and a quik flick remove the tool. I also took before and after AOS replacement measurements of the vacuum at oil filler tube using a modified oil filler cap and Mitivac. There is much debate in the AOS threads about whether there's a vacuum or not with a bad AOS. As you can see "before' there is a vacuum of -26kpa, and almost 0 kPa "after". I also replaced all the vacuum lines, switches, actuators, connectors etc while there and used Mitivac to test for vacuum hold. Found that the vacuum canister had failed - developed a hairline fracture at the seam where the bottom joins the sides. See the air bubbles in the picture. Who'd have thought it. So my other suggestion check all your vacuum lines and actuators are working and the whole system pulls and holds a vacuum. Its a no-brainer while the intake is off. I think this may have been the cause of the P1128 error. Torque specs from the factory manuals for the AOS bolts x 2; and the Intake bolts x 6 is 10nm (7.5 ftlb). I also replaced all of the o-rings (oil vent tubes, intakes, throttle body etc) since they're cheap, easy to do and I wanted no post job air or oil leaks. See my blog over on renntech

Nice work on the write up and notes gents. Sounds unpleasant, but if you gotta do it, you gotta do it.

Hey Scott, Did you ever get my responses to your plug removal. I was having issues with my phone it it kept saying that the messages were still waiting to send . I didn't want to keep sending over and over....

A little trick you can use to help with the squeeze type clamp on the bellow is to open the clamp prior to install and wrap the tab with a zip tie. This way the clamp will remain open during the install, after everything is installed cut off the zip tie.

great write up

I bought AOS from the porsche dealer (so it is VIN matached, correct year etc 996.107.026.51). My car is 2003 C4S and I have correct AOS but the dealer could not find accordeon pipe that connects to the bottom of AOS (number 10 on Pet picture). In fact when oerdered what is marked as number 10 comes as a small beded pipe 1/2 in. AOS also has a red ruber ring (see attached pic) so it is unlikekly that accordeon pipe fits it (in fact it doesnt as we tried with the dealer). I really dont want to take everything apart to find out that i need some other part. Does anybody know what the story is with this new design of AOS and what connects to the big pipe?

I ordered one this afternoon and saw this tonite ! Thank you.

does the mk2 aos use the bellows (item 10). I know for sure the Mk1 does... totally diff animal. Mk1 aos with bellows. IMG_0011.jpg

so doest it mean there is no bellow on my part? if that is the case doest it mean it is impossible to change without taking engine out?

any update to this. has anyone done AOS on 2003 C4s? do I need to take engine out or will I be able to wiggle this "no-billow" AOS out?

No there is still a bellows, it is just not in the picture. It's easiest to drop the transmission and then do some of the while I'm here other things (RMS, IMSR, clutch).

The 3.6 engine has a different type of AOS (the one pictured in sa321's post) I have hunted high and low on the internet to find any info on fitting the 3.6 type AOS but to no avail. Going to have a bash at fitting mine at the weekend, will keep you all updated on how it goes.

OK, job done! pretty much need to do the same amount of removal as you do on the 3.4 engine type, I removed the air box, dropped the engine by removing the 2 engine mounting nuts and removed the inlet manifold, did not need to remove the alternator or the coolant expansion bottle. The AOS is held in place with two 5mm Allen key headed bolts, and these are accessed from the top, not the bottom as in the 3.4 engine. Anyone needs to ask any more info on the removal of this type of AOS please feel free to pm me.

I recently bought an AOS for my car (04 C4S) from Pelican Parts...and my part doesn't look anything like these? :\ IMG130.jpg IMG131.jpg IMG132.jpg SIGH...what is this then?

You ordered the air-oil separator that goes in the bottom of the engine, not the one for the top of the engine (the one that most people replace when there is a problem). There are actually two of them in the bottom of the engine, and they are used to separate / remove air from the scavenge area of the lower sump. Give our Customer Service line a call at [phone removed] and they'll be able to setup an exchange for you. I've also already sent this thread to our catalog dept so that they can take a closer look at how to make sure this is crystal clear in our catalog. thx, Wayne

It doesn't appear that anybody cleared this up; The 3.6 air oil separator fits directly on the engine and does not use the bellows piece as on the 3.4 engine.

He ordered the air-oil separator that goes in the bottom of the engine, not the one for the top of the engine. The bottom one is shown in my photo here: -Wayne

Thanks Wayne!

Did you replace it as a preventative, or did yours fail/was leaking? I have an '02 as well, no signs od any issues on the AOS yet, but when I do the IMS, etc, I was going to hit it too.

I replaced mine as it failed (noise like a dry pulley bearing that disappeared after an hour or so but then i was getting the smoke after the car was sitting for a while) the engine light never came on with my car but did try to remove the oil cap when running and was not able to without a lot of effort. Defo worth doing if you are working in or around that area getting something else done, not an expensive item.

Wow, I've been as far as the plenum a few years ago which wasn't bad but that looks like a real pain in the ***. What's the charge for this work?

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wow, BIGchunk, many thanks. I have been waiting for this answer for a year! The fact that alternator and more importantly expansion tank can stay in place is a great news (that expansion tank was a major PIA for me). how many hours did it take and how difficult is it to get to those bolts? it looks like Porshe redesigned this part so it easier to swap. do i see it correct you still have to go for the clamp from the bottom? how much did you have to drop the engine? thank again

AOS is the same. Yes the clamps accessible from bottom. You can safely drop the engine on the mounts just so you don't see the threads below the nuts any longer. This will give you some breathing room. (but I would still support the engine SAFELY from underneath regardless). Any more drop and you are looking at disconnecting some stuff first.

Thanks for the great write up. Tell me will the engine drop those few inches in a tiptronic 996? Thanks again.

THAT, I don't know. Tip guys need to chime in for that one. Good luck to you.

Is it highly recommended / necessary to replace the intake manifold gasket (1x) and intake plenum gaskets (3x) after removal?

Question re: AOS replacement for my '03 C4S. Thinking of getting the Motorsports AOS vs the stock part; this is my second AOS failure (two diff 996's) and I'd like it to be my last. Any experiences to share? Is it worth the extra dough?

I had been through three of the standard variety and when Flat 6 did my rebuild, they told me told me I should plan on replacing it annually due to living and tracking so close to sea level. I did HMS on 9/8 and HLT shows a portion of the track being below sea level. They also said I should expect some smoking after it sat for a while. I took a flyer and bought the Motorsports AOS from Suncoast and, having zero wrenching skills, had to hire someone to install it. I got a couple "can't be done" responses but Orbit Racing in Riviera Beach fabricated something or other to make it fit in the limited space available and completed the install. Sorry I don't have any more details on how they did it. It's been great. No smoking at all and I'm confident it will last more than 12 months making it well worth the additional expense.

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if you track the car - i would use the motorsports part. i have a brand new one. i wanted to do a preventative replacement for peace of mind. hydro-lock fears.... unfortunatly, the motorsports AOS does not work on my X51 engine because of the the larger X51 intake manifold. i will sell my new motorsports AOS for $600 + shipping.

hello everybody, i just did this aos replacement. i thought everything went fine, but when we went to restart car we had a backfire and the car won't stay running. any ideas? i am thinking i might have a leak somewhere. maybe from the cross tube behind the throttle body, or the throttle body itself. does these sounds like the correct symptoms? thanks!

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just to follow up, yes that was it. the throttle body must not have been on right. i took it back off and spent a lot of time getting it lined up and after that it ran fine. also, i had difficulty removing the alternator. now that i have done it, to make it easier, i would do this: the alternator is held on by two bolts, the bolt closer to the passenger side screws into a bushing. loosen the bolt maybe a mm or two and hammer it inward to move that bushing towards the front of the car. if you move it a mm or two, it will make removal 100x easier. hope this helps.

Please, STILL can't find the answer to the question : On the 3.6L MKII engine, is there a bellows pipe that needs to be replaced? As stated, when you order the part number 996-107-175-50 as shown in the PET for part, "10", the bellows, you get a 90 degree bent oil hose pipe, instead. Pelican shows the part number for the bellows as 996-107-237-52, but lists it as '99 through '01 only. Yet the AOS is the same. Anyone that has done this, can you shed some light, please?!

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When I did mine I had no idea where to put the bellows. So it sits in my extra parts bin right now. Mine sits right on top of the engine without a bellows needed and then the other hoses attach in all of their respective areas. It's been about 2-3 months now and no leaks or problems even after two track events. So I guess I did it right.

So when you took off oyur old AOS, there was no bellows installed?

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No...Just directly secured on to the engine block.

Thank you!

Bigchunk....I know its been a while but about to do this job myself. Its a 2005 997 Carrera (3.6), how long did the job take? I like working on my car and do most of the jobs myself, my regular garage quoted me $450 labor but if this is straightforward once the manifold if off im keen to do it myself. Any tips? Thanks!

Very old thread. If your regular garage quoted you $450 to replace the AOS, take it. That is a bargain. 99C2...3 days...wrap your left arm in an Ace bandage because you will be using only your left hand, wounds and pride took 2 weeks to heal. Advil and beer (many) recommended..........

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Thanks for the write up. I�m halfway into this and so far and your posts have been very helpful, especially the tip on the 6th bolt which I was dreading. Manifold is now out and will be moving on to get the AOS out tonight or tomorrow!!

So I finally got the AOS out. Mine an 02 C4s, and the two 5mm hex key bolts secure the AOS to the block. Didn�t realize that and thought I had removed both mounting bolts when I actually only removed one and incorrectly removed a lower manifold bolt thinking it was for the AOS. Anyway, still have to get all back together. I was quoted 1250 to do it, and the car is stored now, so that�s why I figured I had the time to do it. It�s doable, but is a bit of pain. Had I been quoted 450, I would have dropped it off right away. I actually was hoping for an end of the year special and he�d offer to do it for $750, but he didn�t.