Hurdigurdiman

How do we know when the AOS is going bad and needs to be changed. Also, where is the AOS situated on the 996? Photo please if possible.

LexVan

One of the BIG clues with a failing/failed AOS, is constant smoke on start-up. Not just the puff of smoke on cold start-up (normal) that clears in a couple seconds, but one that lasts and persists.

ditto

If your oil cap is hard to remove when the engine is running, you may have an AOS problem.

Sneaky Pete

^^^^Nope......it is on the front side (driver's side) of the motor. There is one in the oil sump but this is not what Hurdi is referring too.

Take a peek.

http://www.pelicanparts.com/techarti..._Separator.htm

Ubermensch

When it ultimately fails, or just before, it makes a very distinctive noise. Many describe it as " a whale falling from 40,000 feet".

Macster

The 996 AOS failure must be different then for I've had two fail in my Boxster and they never made a sound.

Macster

Based on my experience there are several AOS failure modes.

In the 1st case the engine developed a mild hesitation off idle. Later the idle speed varied, going up then down then up again over and over but not by very much, and with at least a one second frequency, but before it didn't do this at all. Before the idle was always stable.

At some point after trying various things with no real success, although there was some initial success, improvement, in using Techron; the hesitation went away, but came back along with other behavior in practically no time. How quickly? Well, I went through about 1/8th of a tank of gas with the Techron in it and the engine perked up some. But the next engine start or two had the symptoms back and that was that. Short lived improvement.

At some point deeper into my investigation I tried to remove the oil filler tube cap with the engine idling. While I could unscrew the cap I could not remove it against the pressure difference.

Then the constant smoking started.

I do not recall any CEL through this.

Might mention the above was in Jan./Feb. of 2004 with my 2002 Boxster with just under 80K miles and at least then AOS was a rather unknown issue.

When the smoking started I called the SM at the dealer and he said "AOS" and do not drive the car and bring it in. I called a flat bed tow truck and had the car hauled 40+ miles to the dealer for the AOS replacement.

The 2nd AOS failure happened over several days of a 2000 mile drive. That pattern developed that at the end of the day when pulling off the freeway for the last time for the day the engine would experience a very mild roughness, short lived and very mild, but because the engine so smooth at all other times noticeable. Concurrently the CEL would come on.

I do not remember the error codes but based on them I diagnosed the problem as a MAF and figured in a pinch I could disconnect it. (The error codes probably were related to fuel trim thresholds.)

This happened one night in Flagstaff, another night in Albuquerque, and one more night somewhere else I have forgotten.

But I made it to my destination with no real drama thinking on the way back home I'd stop at the Porsche dealer in Merriam KS (where I bought the car) and pick up a new MAF just in case. The next morning I start the engine and the smoke just billowed from the exhaust. As it kept coming I realized what the behavior of the previous couple of days really meant and managed to get the car flat bedded to the dealer (90 miles away). The car got a new AOS in a day or two and that was that.

Based on the above, I can say some of the symptoms are a mild hesitation, a varying idle speed, excessive low pressure in the crankcase, a CEL with maybe helpful maybe not helpful error codes.

The only real hard symptom is the constant smoking. The engine may or may not run rough. But no matter constant smoking is bad.

Now in the case of the 996 with its AOS that gets fed coolant there is another aspect and this can be a coolant leak either external to the engine or possibly internal into the engine.

An explicable drop in the coolant level, although there are so many other possible places a plain old fashioned leak can be this drop can be hard to pin on an AOS, but of course if you see coolant in the area of the AOS...

If the coolant leak is internal then another possible bad AOS sign is excessive water vapor production at the exhaust. Water vapor is distinguished from oil smoke by the fact water vapor dissipates while smoke lingers/holds together. If the vapor has an anti-freeze smell...

Even if the AOS is given a clean bill of health this is not a good symptom and its cause must be id'd and corrected pronto.

Might mention last but not least if at any time the engine starts running rough, has a varying idle speed, or exhibits any behavior out of the ordinary one should seriously consider stopping the car and turning off the engine and resisting the urge to attempt a restart of the engine, instead getting the car towed to someplace that can deal with possibly the worst of the possible issues.

With the engine acting up this is not the time to try force the car that last mile home or hope the symptom was a "glitch" and a restart will find everything just fine.

Hurdigurdiman

That link only refers to the 986 Boxster (1997-04)
987 Boxster (2005-08)and no mention a Carrera 996. Does the same procedure and parts apply?

BTW my engine is running brilliantly and the only reason I was asking is as a PM measure if changing the AOS was a DIY I could manage. Plus would it be a wise DIY or should I hang on. There is no hint of rough idle or smoke coming from the exhaust either running or on start up. I will however try to take the oil cap off when the engine is running. What dyu think Mactser? Worth the effort or leave well alone for now.?

LexVan

Hurd, I'd wait. If you want to change something early, if you haven't already, do your waterpump instead. That's a good one to replace before it fails.

sjfehr

AOS can fail with different symptoms. When mine failed, there was no smoke at all, just a godawful screech from a vacuum being pulled on the crankcase; in my case, it sounded almost like a slipping belt, but took about a minute to build after turning the car on, got quieter under accelleration, and stopped completely when I managed to finally get the oil cap off. (Taking off the oil cap resulted in the extra airflow sucking oil into the intake and removing all doubt... I don't recommend leaving it off that long.)

TomF

First off, the AOS is a wear item. It will fail somewhere between 60 and 120K. Personally, I would replace it before it fails. It is a PITA to do- one Rennlister described his arms afterwards as if he had been in a fight with a herd of cats...

I did mine on my C4 Tip Cab and I had been told that it was impossible without dropping the motor and transmission. It can be done, but it was pretty darn near impossible. For those with a manual transmission, you have it much easier. There are two great write-ups on Rennlist and Renntech for the DIY. Do search and you should find one or both of them.

And, oh yeah. Change your darn water pump if you haven't. That can most certainly be an engine killer.

Cheers,
TomF

996_North

+2 on this, check the propellers

Sneaky Pete

Hurdi, it is the same (I think). Here is a link to renntech http://www.renntech.org/forums/tutor...placement-aos/

perryinva

It is much easier on the MarkI cars tan on the MarkII also. Totally different part and installation.

mcipseric

I have an AOS for sale if you need one.