Macster

Three AOS's have failed on my 2002 Boxster -- now with over 300K miles on it. I get around 80K to 120K miles per AOS. I never overfill the engine with oil.

The AOS is a wear item and one needs to be aware the failure modes and as soon as one shows itself get the AOS replaced pronto.

JuanK20

Out of curiosity, what is the average cost of having this replaced?

jchapura

What are the failure modes?

Macster

The Boxster AOS is simpler in that it doesn't have the coolant circulating through it. With this AOS then the rubber/cloth diaphragm fails. I cut one apart and the material had just cracked/split due to what I believe material fatigue.

For the 996 AOS and possibly the 997 AOS the diaphragm can fail or the housing can crack and there can be a coolant leak. It is possible for coolant to leak into the engine.

I typed "failure modes" when probably "failure symptoms" would have been more correct.

In my experience, with my first AOS failure, the engine can develop a slight hesitation off idle. After a while the idle speed can go up and down. Not by much and not in a dramatic way but the engine idle speed should be pretty stable.

In one case with the engine idling on a whim I tried to remove the oil tube filler cap. While I could unscrew it I couldn't remove it against the pressure difference.

'course, the latter stage is the smoking and the 1st AOS finally started showing that symptom. Copious amounts of oil smoke with no stopping. The smoking may not or may be accompanied by engine roughness. In this case at first it wasn't but as I was trying to get the car positioned for the tow truck -- the operator refused to back into the driveway and pick up the car -- the engine started running rough.

In the case of the 2nd AOS failure, things were fine except at the end of a days' worth of freeway driving at the freeway off ramp the CEL could come on. I forget the error codes now but they pointed to a lean condition that the DME was unable to compensate for. I distinctly remember thinking the problem was a problem with the MAF and I could press on.

This happened several days in a row -- I was driving east covering -- eventually -- 2K miles.

I made it to my destination but the next day at first start the oil smoke just billowed out and then I knew what the CEL/error codes actually meant.

In the 3rd case there were no symptoms. The engine was fine around town and on the freeway. I had completely forgot about the AOS. I stopped at a stop light and when I took off I glanced in the rear view mirror and spotted what proved to be smoke. Really at the time though I thought it was coolant vapor. But a check of the dash found no warning lights on and no signs of overheating. Then it dawned on me what it was. I drove down the street and made a U-turn and headed towards the dealer around 10 miles away. There was no smoke at this time and the engine was running just fine.

But not a mile down the road I got caught at a long light and the smoking started again. I hoped it would go away but it continued and the engine started running rough. I hadn't even covered a block after the long light and I knew then it was time to stop and call a tow truck.

I picked a handy grocery store parking lot and parked the car so the tow truck operator would have no problem picking up the car and dialed my auto club for a tow truck.


The problem with the motorsports AOS is it is very expensive. If it were a lifetime unit that might not be so bad but my info is it can also fail/wear out.

Para82

Mine with parts and labor from a top indy was just over 900 bucks + tax

jchapura

Thank you Macster for this insight.

Did you ever baseline (or at the time of the AOS incident) your crankcase vacuum with a manometer? If you did, can you share your values?

I recently baselined mine and I thought the value was low but there seems to be a dearth of folks who have baselined it.

Viper6

My buddy had the tool, and mine was pulling 10-11 inches of water... not sure if that is good or bad, but I had just finished a track session if that means anything. I have no visible smoke on startup. Not sure if I need to replace or not after reading a few threads now...

semicycler

Yes your AOS is failing. On the 9X6 and 9X7 Porsches, the engine crankcase vacuum reading is normally in the -4.0" to -6.0" inches W.C. range. Anything higher, especially significantly higher like you are reporting, can only happen from a failing/failed AOS. Time to get it swapped out.

gopirates

The 4-6 inches will be variance in your elevation above sea level. I know I'm in Raleigh, NC and I believe Raby said mine should be 5 - 5.5 if I recall correctly.

My symptoms were high consumption of oil and sooty pipes on both sides (both sides equally dirty). Mine did not smoke to my knowledge. Though light idling in the garage did mean there was even soot on my concrete below the tailpipes.

Macster

I've never measured the vacuum at any point on/in the engine.

The times the AOS has failed it was clear -- if not to me someone with more experience with these cars -- what the problem was: The AOS; and the first time when this was a new experience for me the SM told me what would be done -- replace the AOS -- and later after having one AOS failure under my belt when I dropped the car off at the dealer just asked the AOS be replaced.

Not saying there's anything wrong with wanting to check the vacuum but I have no numbers to offer as to what is normal and what a bad AOS would produce.

Petza914

Overfull is when the bar graph on the dash is completely full. That arrow to the left side of the bar graph that's 1/2 a line down is the Full mark. There's no way to discern how overfull the engine is, just that it's overfull. With these cars it's better to run 1 full line down than it is all the way full on the guauge, even if overfull is just a little overfull. Almost any overfull condition will compromise the AOS. This is why when the gauge reads 1 full line down, you should validate that same reading a couple mornings in a row before adding any oil, and if you do add oil, only do it 1/4 quart at a time and take readings for the next couple mornings before adding any more.

I got a deal on a Motorsports AOS that's sitting in my parts cabinet for whenever I need to replace mine - paid only slightly more for it than the standard one and figured I'd give it a shot. Currently I'm at 50,000 miles without any AOS issues.

Spiffyjiff, on the Suncoast site, it's listed under the 997.1 Carrera S & 4S categories (see attached screen grab with subset navigation above the photo), but maybe I'll have to adapt it when I go to install.