HELP - diagnosing AOS. Is it the issue?

I have a 2002 C4S I bought with 50K miles and began tracking it this year. GREAT CAR. The car began to smoke at the 2nd event this summer. It was great on track. I would come off track and shut down the car. When the car was re-started after it cooled a bit (20-30 Minutes), it would puff blue smoke like it was on fire. The smoke was BAD, but would only last for maybe 10-15 seconds, then would stop. I was told by other 996 owners that I had simply too much oil in the engine, and I should run it with about � quart+ less oil. (sounds bogus) The smoke has only happened once when not at the track, by the way. It is almost an exclusive �TRACK� issue. Related or Unrelated? - - About a month ago, I began getting CEL�s. I had them read and it said that the O2 sensors in the left bank were on their last legs. It did not say FAILED. They cleared the code and it was repeated a few days later. (I also had a new Oil Pressure Sensor installed around the same time. But I continued to get the same O2 CEL even after that.) I also got a catalytic converter CEL in there as well. The challenge is that the shop I took it to had no idea what was throwing the codes, cleared them and told me to keep an eye on it. NOW, the CEL is constant. I have been reading about the AOS failures online, and some of it sounds a bit too familiar. Can you help me to figure out if this is really my issue? I have not yet looked into the throttle body or AOS area to see if there is residue. Questions: 1. Can a failed AOS cause a Cat or O2 sensor failure? (This may explain some things.) 2. IF I replace the AOS, should I also replace the O2 sensors? 3. Do you REALLY need to drop the engine to replace the AOS?** **I have not yet found a shop I trust around me. The local dealer thinks they can simply fix, replace, do anything to my car (without my authorization) and then hand me a bill. They spend more time looking for ways to bulk up the bill, than fixing the issue. Thanks for any help you can provide.

I have had this issue at the track as well. First time it happened i nearly sh@t myself. Over the course of the day though, i saw a 996 GT3 and a 993TT do the same thing. Asked a few questions, and also called my friend who races these things for a living. Apparently this is one of those "they all do that" things. Given the boxer layout, the oil can get past the rings sometimes and then smoke on startup (kind of weird that a dry-sumped car would do it though...). Not actually sure if it's a G force issue or an oil temperature issue, although I'm guessing the latter, as it only happens if I turn the engine off soon after leaving the track. If you let the car idle for a few minutes in the paddock and cool down, you probably won't smoke on startup. Now, I've never investigated the issue farther to see if I have an AOS issue, but the car has been in for service a couple times since this first happened, and nobody has mentioned any AOS problems. It may turn out that my AOS is shot and I just gave you crappy advice. But I hope not.

Some here on the forum have replaced their AOS without removing the motor, and many have had severe smoking especially after tracking their Porsches. Using the search button will give you plenty of extra reading about both subjects. Can't help you with your questions concerning the O2 sensors. Knowing what CEL codes your Porsche is throwing would be helpful to help pinpoint your other issues, before they are cleared.GL

Regarding the smoke after track use, I had the same experience. The service manager at Pfaff Porsche (Toronto) who also runs Apex track days at Mosport said all was well. His advice was to let the car idle for a few minutes after each session. Works perfectly.

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+1

Idling can help, but it's not a cure. My new 3.4 motor has been experiencing the same white smoke after startup and then last weekend a competitor behind me said it blew white smoke in a few turns. Apparently I didn't get a new AOS when I replaced the motor. So I took the motor back to the builder Monday and had him replace the damn thing (on his nickel). If you're tracking the car you don't want to mess with it as it can fail completely and cause a hydro-lockup. The parts cheap at around $110., but it takes 4-5 hours of to change the POS. It's like a toothache, it won't get better by itself...

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+1 on what it takes to replace it... toothache, while eating tinfoil at the same time.

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A failed AOS could possibly cause a converter or O2 sensors to fail if the AOS is ignored long enough and the excessive passing of oil allowed to continue. However, the AOS even ignoring the oil it can subject the engine too can cause O2 sensor and converter sensor errors. The AOS can be a big old intake air leak. Replace the AOS. Ensure the oil level is ok. Not too high, but not too low. Running at the track can up the engine's oil budget in that more oil is in suspension -- not in the oil sump/tank -- due to the higher rpms and g-forces. Clear the error codes of course. Run a bottle or two of Techron as per instructions then after the gas with the last bottle of Techron gets low enough to say 1/4 tank fill up the gas tank and change the oil/filter. Drive the car. If the sensors/converter(s) are bad the check engine light will come on and when (if) it does, pull the codes and post for help. But right now I'd concentrate on getting the AOS fixed. You might find the sensors/converters are ok. My 02 Boxster has had 2 AOS's now and while one sensor failed shortly after the first AOS failure (at nearly 80K miles) AOS failure I believe this sensor failure to be a coincidence. The other sensors lasted thousands of miles. Even after the 2nd AOS failed ( at around 180K miles or around 100K miles after it was installed when the 1st AOS failed) and the engine smoked like crazy upon engine start, the sensors/converters suffered no harm. However, when I started the engine and spotted the smoke I let the engine idle only long enough to snap a few pics and then shut off the engine. The engine ran maybe a minute, no more. Anyhow, I had the car towed -- flat bedded -- over 90 miles to the nearest dealer where I had the AOS replaced. Afterwards I drove 2K miles home here on the west coast and and then have driven the car thousands of miles since (2nd AOS replaced around the 180K mile mark and car now has over 236K miles on it) and while I've replaced over the last 8 years and over 236K miles every sensor once clearly in my car's case the AOS failures did not take out the sensors or converters. Original converters but one is growing weak (wearing out) cause in cold weather it kicks off a P0430 error. Oh, I don't know what's involved with an AOS R&R in a 996. For my Boxster the AOS is accessible without having to drop the engine. Sincerely, Macster.

Wow. You guys are great. Thanks for the feedback. I found online that a failed AOS should create suction on the oil filler cap while running. I just tested it. There was very minor suction. Nothing I would call at all unusual. Next: I plan on getting into the Throttle body this weekend to inspect for oil, dirt. I also ordered some MAF cleaner. Let's see what this does to solve my problem. Overall, I am feeling much better since the AOS failure is not at all a certainty.

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My AOS went out and I really didn't have an exceptional suction on my tube ("That's what she said"), so you can't rule out AOS. Are there any sqeaks that sound like a bad pulley? FYI you can replace your own AOS in 6 hours start to finish. If you're me you'll take longer because I screw things up sometimes...most times. Me replacing the AOS

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There is a failure mode that results in the engine seeing a huge amount of vacuum at idle, but absence of the vacuum is not sufficient to eliminate the AOS. When my car's first AOS started acting up and the engine started manifesting various symptoms -- unbeknownst to me at the time what was actually causing the symptoms -- I stumbled upon this oil filler tube cap thing without at the time realizing what it meant. When the second AOS proved to be bad I didn't even bother checking. The billowing cloud of oil smoke from the exhaust pipe told me all I needed to know. Sincerely, Macster.

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You're not listening. It's the AOS and it needs to be replaced. Nothing else will throw those big white/blue clouds of smoke from your exhaust on startup and then disappear moments later. Put your finger on the walls of the plenum. If there's oil residue on the walls it's the AOS...

Just chucking more wrench's into the works but....... Any chance your plugs are fouled or too old? Have you checked the oil for parts of a piston ring or piston head? Got a valve leaking oil or warped or cracked valve? Any memory of engine temp doing some weird too hot then lower? Check your coolant pressure after sitting after shut down a little bit to see if you still have pressure. Could be slight crack in........ewwwwwww Unburned fuel especially diesel will smoke a lot so if its water that got in from ultra thin crack it might screw things around at start up and throw more trash down the line onto the O2 sensors. Did I mention just throwing wrenches in there...... You mention looks like smoke. What does it smell like? Oh yah get 2 pretty white pieces of paper for the mechanics best tool and place each at the two different exhausts. You want to sense water, fuel and oil being in the exhaust. Touch and sniff test the stuff.

all the variables are the culprit....not idling after tracking, high G forces that forces oil up the AOS column, to much oil... The bad cat and O2 sensors are just the tail end of the problem. You have address all of the stated above.... there is a lot at play here. If you have a bad cat... get a duramteric scanner and look at the pre/post O2 sensors.... a failed cat looks like this Bank1, Bank 2 is good.

Smoke at start up usually isn't the AOS.. At start up the IAC is open to provide a higher idle speed, which reduces the vacuum the AOS sees, which reduces the tendency for AOS smoke. Smoke on start up usually is a sign that the rings are losing tension, or that the engine is firing up too rich, creating reduced ring seal characteristics. "Rebuilds" that include using original lokisil cylinders almost never seal up as well after rings are replaced or engines are disassembled, thats why I don't play that game and Nikisil EVERY bore on an updated engine.. been there, done that! Blowing smoke in turns can be the AOS, but it'll usually only do this in right handers if its the AOS since the G forces stack lots of oil on the side of the engine where the AOS is located. I see lots of engines that smoke in corners that have compromised intake valve stem seals. Lots of people are making the mistake of using OEM stem seals in rebuilds, which don't hold up very well at all. Bad stem seals allows the oil in the heads to be pulled by the intake valve guides and then enter the port and then the chamber where its burned. Sneaking a boroscope down the intake to see if the valves have oil on the back sides of them is a good way to locate this issue.

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early cast rings only, or late versions too? We figured the Boxster we just did to have collapsed rings - early 97' motor been sitting under the bench for a couple years - Jason tore into it for "fun" the other day now that we understand more to see if he could find the smoking gun, that seemed the highest suspect part (was a smoker when it ran last)

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Jake this particular type of smoke, which is mostly white and billowing out like a fire extingusher, is always the AOS when it presents itself after a track session. I can't believe I'm disagreeing with Jake Ruby, A man with more knowledge about the internals of these motors than anyone I've met. But....., I've personally replaced too many of these POS's on 2.5 Boxsters and 3.4 996'. It always cures the smoke, always.....

Morbid curiosity, did you get it diagnosed and repaired?

I can only address your CEL problem and I had a similar one saying left bank sensor at end of life. It was my MAF and after cleaning it it was good for a while.Repacing the MAF cured it for good. When the indy reads your codes you need to get the number and post it here. We can help you look up the code. That being said you should buy your own OBDII code reader just get a cheap one ($50) keep it in the trunk and you too can read and erase these persistent codes till you get to the problem.It will save you the time of going to your mechanic ,waiting, paying etc.