AOS upgrade?

Noticed the other day had a drop of oil in the garage, traced it back to the AOS bellows weeping. I also let the car idle and took the oil cap off, it had a slight stumble. So i'm thinking the AOS is about to go. I was going to wait for a while til I changed it out, but today I found a puddle of antifreeze under the driver's side exhaust the coolant tank is leaking again (changed it out new in August). Since I am gonna have to change it out, might as well do the AOS since I'll be going at it through the top of the engine. My question is there are two AOS the stock one and the motorsports one? I have a 99 C2, do I have to modify anything to get the motorsports one to fit? How long do the motorsports ones last? I did a PCA event a couple of months ago I think thats what started killing it as it looks like it has been replaced before? Has anyone installed a motorsports AOS before with the engine in place?

Motorsport AOS is $1,200+ & only fits 2002>3-chain engines

Replace the AOS with the factory stock unit. You and the car will be fine for a good number of miles. I've gone through 3 AOS's with my 02 Boxster -- and I've never done any track or PCA event -- and get anywhere from 80K to 100K miles out of the unit. The AOS is a wear item. As soon as I have one replaced I start saving for the next one.

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I've heard the AOS is a PITA to replace, I just want to do it once and they tend to have an early death if you track the car. That is why I am looking into the motorsport one.

I just traced my AOS leak down on my 99 c2. Mine had 3k miles on it and was dripping a little after 4 track days. It showed as "weeping" at the bellows, but the bellows is very unlikely leaking unless torn somehow. It is thick and anchored with a set of substantial hose clamps. Anyway, I have the motor out for winter stuff and one was to figure out the AOS. I've attached a photo of my culprit. I think that small return is for high volume situations--it looks like an overflow path. Anyway the o-ring was crushed--it's only pressed into the receiving port and held by the shape of the AOS. I replaced the o-ring and we will see. It is tough to get at from the engine compartment since basically you have to remove the intake manifold and clear out room to get to it. I wonder if the 99 can be accessed from below if the motor/transmission is lowered/tilted from the transmission end. It's right on that backside. Anyone know? (Looks like the photo attaching threw a server error. I will try to add by editing.)

I guess what I'm saying is I don't think that drip/leak at the track means the AOS is bad, rather that it's leaking a little at the track when run really hard. I don't know how many miles yours has, but it's worth replacing if you're not sure how well it's working--mine was virtually new and there weren't other AOS symptoms from what I could read and see online. There's a YouTube video that's pretty dramatic on a bad AOS and the vacuum created at the oil filler cap. (I had some roughness too from the IAC--showed up as codes.) Just an opinion and I'm quite the rookie so take that advice for what you paid for it.

As far as I know, the AOS does not go bad because of track use, it is rather a design flaw of the AOS that makes cars smoke during/after track use. So if you have replaced the AOS once, I don't see any point in replacing it again soon. On my 996 C2 2002 I replaced the AOS with a motorsport AOS, and that is the only (?) permanent solution if you want to avoid smoke during track use. I don't know if it exists any motorsport AOS for cars before 2002, but if you have problems with smoke during track days I would highly recommend to search for some motorsport solution to the AOS problem. When the AOS goes bad it is usually the membrane inside it that breaks. This can make the car smoke a lot even when driven on the street. On the 996 up to 2001 the bellow under the AOS can also start leaking. From 2002 and onwards, there is no bellow under the AOS. Robert

So who makes the Motorsport AOS? I'd like to speak with someone to understand the modifications for the 1999 c2 fitment. I tried to find it by searching for Motorsport Cup AOS and various combinations but got places to buy. Anyone know how to find out about modifications required?

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The Motorsport AOS is not compatible with the earlier 996's, as you already have been informed. However, I do recall coming across posts of owners -- a few anyhow -- who have claimed to have found a way to adapt/install the Motorsport AOS into an earlier model 996. I do not recall what is involved. You can do a google search "Porsche Motorsport AOS" and see what hits turn up and read the posts at the links to perhaps gain some insight as to what is required. Some owners use the stock AOS but fabricate or buy an off the shelf oil catch can that can I believe be installed in the middle of the hose that runs from the AOS to the intake manifold. If this catch can is designed properly it can well catch the oil that is passed by the AOS. This can requires periodic draining/emptying of course. The catch can doesn't do anything for the stock AOS that is making it any more robust but it is my opinion tracking is not an AOS killer. Since these things fail even without any tracking involved I suspect what happens is the owner shows up at the track with a marginal/borderline AOS that is an AOS that is on the edge of failure and the high temperature and excessive variation in intake pressure just takes the AOS over the edge. It fails at the track after maybe 60 miles of usage rather than after a couple of thousand miles of additional street usage.

Here is a pic I took while I had the car up on a lift. In theory you can get to a part of it through the bottom if you have a lift. I don't have a lift so I have to do it the hard way. Has anyone changed one out before without the engine or trans removed? I don't know when it was last changed out as I've only had the car since May, the only way I can tell its been changed out before is by the clamp it has on now.

Here's a how to by Wayne Dempsey of Pelican . Pics etc--well done. I have his 101 projects book he published from these articles which is a very well done list of ideas and important DIY how to's.

If you just replaced your expansion tank and it appears to be leaking check the cap first. My tank was replaced about 12 years ago while the car was still under warranty. Recently I started to ge a leak onto the drivers side exhaust. I assumed it was the tank so I pulled it out and pressure tested it (dang early model OEM tanks list for $375). It was not the tank but the cap, which was also replaced years ago. Put the tank back in with a new cap and no more leak! Was the tank you put in a Porsche tank or one of the aftermarket tanks? Just curious.

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The tank is aftermarket I believe its the German brand (bought it at pp) also replaced with a new cap when I changed it out. It looks to be leaking from under the area where the cap is there is a seam there. I am gonna double check it today just to make sure.

Seems like this would be the perfect application for a catch can setup... Manufacturers are required to build a completely closed system to deal with crankcase vapors, but it's not the ideal solution. Good catch cans are comparatively inexpensive and stop all that nasty crap from being re-ingested. Considering the cost of the AOS, difficulty in replacing the AOS, the MEGA cost of the track AOS, and the frequent failure rates, I would definitely be putting in a catch can system if (when?) I have a AOS failure on my current Cayman or future 996...

I changed my AOS in the car. I have an 04 though. I took off the rear wheel, and lowered the engine on the motor mount bolts as far down as I could get them to give me that extra little bit of space needed to get the bolts on and off, and to squeeze the new in. I just broke the old one to get it out. It was the other parts that broke when replacing that I wasnt anticipating, like the hose that goes to the other side of the motor, and the coolant hose plastic connector on the front of the motor.