Dave!

So I've had some minor issues for a while that I believe to be the AOS. And unfortunately I've promised my wife that I'm going to sell the 996 this spring/summer ( Also, Cleveland's roads ) so I'd like to get it in perfect working order before I do.

Now for the symptoms that shout "AOS!" to me. The area around the TB on the driver side is oily and dirty and my idle often sits around 900-1000 when around town and idle after a hot start is about 600. Macster had a post about his AOS failing that had about the same symptoms, so I'm close to 100% sure. Thing is, car will run fine with the oil cap off (just a bit higher in idle), I've never seen any smoke and on a cold start I can smell a bit of gas in the exhaust.

The rich smell I'm really not sure about, if I do have a bad AOS could it be a result of that or am I looking at a second issue?

And of course, as always, no codes to help me.

So, should I just go ahead and replace it because it's obviously the AOS or is it unclear enough that I should have it diagnosed before working on it?

Hey, maybe I can get an Accord next!

Pac996

One of the AOS symptoms is too much oil going into the throttle body area. Try checking for oil there. If you don't have a lot of oil there then it doesn't sound that bad. Try cleaning the MAF. It could be thinking there is more air mass flowing by it and telling the injectors to add more fuel which is too much.

Barn996

Get someone to diagnose your problem before replacing your AOS, could be something else that is easier and cheaper to fix.

Macster

Sort of could be, sort of could not be, AOS.

My Boxster's first AOS's symptoms were a hunting idle that is the idle going up then coming down in a rather leisurely fashion. I don't recall the peak to trough numbers but the cycle time was a few seconds. The rpms never got real high nor so low as to make me believe the engine was going to stall.

Along with this the engine felt a little flat off idle. Enough that at first I attributed the symptom to the engine's miles (around 78K miles) and that I had reached the point the engine was wearing out. (This same engine just turned over 230K miles and is still running just fine so that wearing out thing was obviously not it...)

I don't know when the big vacuum at the oil filler tube first appeared but it wasn't early on. It was towards the end about the time I had decided the problem was beyond my ability to diagnose and was perhaps severe enough that I wanted the dealer's service department to "find" the symptom with the idea that there might be some secret warranty on the problem or some goodwill to be had. If the dealer found signs of me having been at the engine or some other shop at the engine this might have voided any secret warranty or goodwill chances. It helped me make up my mind when the service manager told me it sounded like AOS -- he had another customer with a car with same symptoms as mine -- and advised me not to drive the car, run the engine.

The throttle body was oily and in fact there was a drop of oil hanging from the bottom of the butterfly valve. I cleaned the throttle body up with the appropriate cleaner and installed it. Engine idle and perhaps even the flatness (can't remember for sure about this) gone but soon back after just a few miles of driving.

Is the car an early example that has a idle control valve? These can get dirty or even go bad and in either case affect the idle. Some have reported removing the ICV and cleaning it with right stuff makes idle problems a thing of the past.

I don't know the miles on the car or the condition of the car. Is the oil level good or have you been running it high, overfilled even?

If you want remove the throttle body and note its condition. Oily, wet with oil, that's sort of AOS-ish, unless you tell me the oil level's high, over the max. bar a bit. Then I'd suggest an oil/filter service and do not overfill with oil then clean the throttle body and drive the car. If the symptoms return, or if more AOS like symptoms appear then you know.

If the engine behaves itself, continues to behave itself, it was dirty TB and the high oil level and possibly the ICV if you tell me it had one and you cleaned it.

Oh, the rich smell is almost certainly nothing. Cold start will have some unburned gas being emitted from the exhaust and even a little bit of gas in the exhaust smells like a lot.

You won't see any error codes probably because the engine's cold and not in closed loop mode yet.

What you might see is if you looked at the short and long term fuel trims in real time some huge adaption numbers flying around. The AOS could be allowing some big amounts of air into the engine after the MAF and the fuel trim numbers would look chaotic.

Which reminds me: Check the oil filler tube cap for a good seal. With engine idlling give the car a wiggle and make sure even if you can move it a bit the engine doesn't react and you can't hear any air rushing around cap.

Have you tried disconnecting the MAF at the wiring harness and seeing if the idle symtpoms improve?

Bottom line is the symptoms are not conclusive the problem is an AOS. But it is not eliminated either.

My advice is you can with some care continue to monitor the engine's behavior and do various things to try to eliminate the AOS or confirm it is the AOS but you have to be careful that if the symptoms get worse, you shut the engine off first and ask questions later, questions that don't require the engine be run.

When the AOS does go totally bad the engine's at risk of serious damage.

My 02 Boxster has had 2 AOSs fail. In both cases I paid to have the car flat bedded to the dealer rather than risk driving it to the dealer.

The 1st at 78K miles the 2nd at probably 180K miles. 50K miles later the engine still runs just fine so if caught in time and dealt with appropriately an AOS is not that big of a deal.

Sincerely,

Macster.

Graufuchs

Macster where is the ICV, I have similiar symptoms, low idle when warm around 625-650, it doesnt jump around much though. I have 2000 C2 btw, tip trans. No engine light ever. 60k on car. No oil in TB.

Dave!

The ICV was a '99 thing, your '02 won't have it.

I've cleaned out the TB a couple of times over the past year and a half and each time it has had the grungy dirty oil gunk that you see a lot on the bottom of a leaky engine, but not much. Cleaning it out didn't help noticeably either time though. The outside of the TB and the plenum on the driver's side has a good bit of gunky oil buildup, nothing on the passenger's side.

The oil is about 4500 miles old and I keep it as close to the middle fill line as I can, I think I've added about a liter over that time keeping it at that level. The car is at 72k by the way.

I did try disconnecting the MAF one time, it kept idling but the hunting idle I have stops hunting once it's settled, i.e. I'll come to a stop and it will sit around 1000rpm for 5 or so seconds then dip a touch below 700 then come back up to 750-800. It doesn't consistently hunt so I can't tell if disconnecting the MAF helped.

And when you said the car felt "flat" at idle, did you mean it kind of hesitated off idle?

But all in all you're thinking take it to get diagnosed? Would a shop have a good way to ID a failing AOS or would it just be based on the mechanic's experience with AOS failure?

Edit: Will the car be driveable with the MAF disconnected so I could see if it helps or will it not go above idle?

Edit the second: Thanks a ton for the info Macster, it's a big help.

Tippy

Easiest method to for sure know if the AOS is dead is the test 99firehawk gave years back that has worked for most; pull the oil cap off while running and see if the motors changes RPM or like mine, dies.

If the RPM doesnt change much and there is a slight vacuum with your hand over the oil filler tube, AOS is good.

If it dies or sucks on your hand hard, the AOS has failed.

Dave!

There's enough negative pressure in my crankcase to make it fairly difficult to take the cap off, but the idle only changes a little bit when I do it.

Macster

My info is if you have a non E-Gas car the engine has an ICV (idle control valve). E-Gas appeared on the 2000 MY cars.

The engine felt flat as I accelerated the car off idle. In my Boxster it doesn't take many rpms above idle -- normally -- to get the car moving and once moving it gathers speed at a good clip. When AOS was acting up engine didn't move the car off as smartly as it did before.

A qualified shop would be able to eliminate the MAF and other things and then perhaps bring the AOS to the top of the list. Also, you would be dealing with a shop that has seen and worked on maybe hundreds of these cars and from the symptoms -- even if to me they're not conclusive -- would know the cause.

Based on my direct experience with my car I know the AOS can fail at least two different ways cause the symptoms so different.

Failure number 1 symptoms) A hunting idle, no apparent causes for this, MAF ok, lots of oil on the throttle body and butterfly valve, lots of oil at hose connection directly behind the throttle body, and at some point towards the end of my diagnosis on a whim I unscrewed the oil filler tube cap and was unable to remove it against the pressure difference at engine idle. Suspected something internal to the engine then and not MAF or fuel supply and decided time to call in the big gun. Related all this to the service advisor at the Porsche dealer near me who diagnosed the cause over the phone: AOS. At some point, very late, maybe after I had decided to bring the car to have the AOS replaced, smoking upon startup and rather heavy oil consumption. The oil level dropped a few bars in just a few minutes (accumulative) of engine running. At the dealer service manger told me oil was dripping from exhaust pipe exit.

Failure number 2 symptoms) Out of the blue check engine light after hundreds of miles of freeway driving when I was making my way along exit ramp to surface streets. Don't recall the error codes but they sort of made me suspect MAF. Cleared them and everything was fine, until the next day same thing after hundreds of miles of driving on the freeway.

Again cleared the codes. Oh, every time the CEL came on there was no signs of engine trouble. Engine running fine, getting good gas mileage -- on long trip and monitoring gas mileage -- and then CEL.

3rd day upon 1st startup after car sitting overnight in motel parking lot the engine smoking like crazy. No need to wonder where the CEL coming from now.

Based on my experience the engine will run just fine and the car will be driveable with the MAF disconnected from the wiring harness. There might be some bit of stumble if the engine controller suddenly has to deal with no MAF input -- assuming the MAF not bad all the time just some of the time -- but this should be brief. The check engine light might come on -- from the MAF being disconnected (when I tried this the CEL didn't come on) -- and you'll have to read the code(s) to know what they mean. When you reconnect the MAF you'll have to clear the code(s).

Now if you disconnect the MAF and the engine upon startup reacts extremely unfavorably or at any time it starts to act up that again I must stress may be the AOS going from bad to really bad, from bad to worse, and this is serious. It is serious enough as it is with the engine acting up and not knowing why.

I am leaning strongly towards AOS but I can not say with 100% confidence it is AOS. I would hate to say AOS and you have it replaced and the symptom is caused by bad oil filler tube cap seal, or a cracked oil filler tube for instance.

Also, I am always concerned as these diagnostic episodes drag out cause I know if an AOS going bad that at some point the AOS can get so bad the engine can suffer hydraulic lock with almost certainly fatal to the engine results.

If the AOS is the cause of your engine's symptoms you only have some much time -- and no one knows how much time -- until the AOS condition deteriorates to the point the engine is at immediate risk of suffering severe damage if the engine is started and run, even at idle.

Sincerely,

Macster.