Help needed with recently acquired 996 C4S

So i picked up a 996 recently, im normally a transaxle Porsche guy (924S/944/968) but i came across a 2003 C4S Exclusive that was in really great condition (95 000km mileage), full Porsche dealership service history for all its life, and had the RMS/IMS done, as well as a resleeved rebuilt block by a Porsche specialist. Before purchase the car was put through its paces by a friend of mine who is a trained Mercedes engineer. Literally everything worked, and the engine had been run in for about 5000KM after the engine rebuild no issues at all. So i paid the high'ish price and bought it, and I drove it for not even a day before it started spewing out white smoke like crazy. Happened while going up a hill and turning right gently at about 100Kmh/60mph. Had to stop and get the car towed it was smoking so bad but no loss of power or engine light or anything, though the fuel consumption seemed really bad before that. I very quickly learnt about the Air Oil Separator (AOS) issues these cars have (what a crappy part) and had it changed at the seller/dealerships cost since its covered for a few months after purchase by my countries equivalent of the US lemon laws. Anyway after a long wait since its a lot of labour to change this part (entire intake cleaned,new spark plugs), i get the car back. Fires up perfectly again absolutely zero smoke even from initial start. Great, so i take it from the Porsche dealership, and use it an entire day making sure to test it properly, start it stop it, do some freeway and city driving, grocery runs etc and no smoke at all, not when idling or under acceleration or startup. Im happy, so i park it off (my driveway is on a slight slope). Later that evening i start it up again because my dad came over and he wanted to hear the engine and it immediately starts pouring out white smoke from both exhausts...not as bad as the first time, but still pretty bad, it eases up a bit but its constantly smoking at idle now but otherwise its revving smoothly and sounds normal. I dont think theres anything wrong with the engine, because if the rings or valve stem seals failed it would smoke constantly and I would not have had those periods where i had zero visible smoke coming out the exhausts ? What the hell is going on here ? Maybe ive been a bit spoilt by 924S ownership but i really didnt expect this car to be so problematic with such good history. Its worrying me a bit, this would be its third AOS according to the history but what else could be wrong here ? I did a fair bit of research before buying this car, but clearly i still dont know enough about them to know why its doing this.

Whats your oil level. Its probably too high and it loaded up oil in the intake.

These engines hold a lot of oil in the heads , and it takes quite a while to drain back to the sump . Get the car on level ground and leave it overnight . Check the oil level in the morning . If your dash gauge is accurate you want it about half way up the gauge . It is possible but not likely that you got a bad AOS . Counterfeit parts are a problem along with low quality vendors . If the dealer knows these cars a simple test can rule out the AOS . Good luck

Get a UAOS and never worry about it again but let another specialist check the car

Okay so i did a bit of testing based on what you guys said. The digital oil reading is max, all bars lit up. When i checked the dipstick in the morning the oil level was about 3mm above the top most dot. According to the manual the max is the notch or line below that dot ? I checked it three times to make sure. So that to me seems its very high. Anyway i used it pretty much all day. It smoked for about 45 seconds this morning and stopped. Big puff and then cleared. Then no smoke for the rest of the day when i started and stopped multiple times even after a hard run. Though just before i went back home i stopped at a friends shop where the parking is sloped slightly downhill and when i restarted the car, huge puff of smoke which cleared after about 30 seconds. A bit embarrasing, everytime that happens random people nearby start telling me theres a problem with the engine, ergh lol... I also noticed two drops of oil under the car where it was parked in the garage, if looking from the back its on the left dripping onto the rear anti roll bar then onto the ground. That wasnt there before the Porsche dealership changed the AOS and theres no oil line there that i can see so im hoping its just spillage from when they cleaned the oil out the intake system. So whats the verdict here ? Is this smoking on startup issue something i just have to deal with as a 996 owner ? I do plan to get the Ultimate AOS if the AOS fails again in future but considering they just put a brand new one in i want to give it a chance.

Two things. 1. Probably makes sense to take it back to the porsche dealer who installed the AOS and filled up the oil and make sure you don't have another issue. 2. Get rid of the oem AOS and get the UAOS Track version. You can run an extra quart of oil without any problems. This means you can fill it to full and not worry if youvare over or not. Oem AOS is a poorly designed and brittle part. and a 3rd. 3. Park it on a flat surface if you can. Dont know if the direction of parking face up or rear up on an angle will help at all. Maybe someone will comment with experience with hills. Good luck and keep us posted on what you find.

If the oil is above the max, that could be why it�s smoking. Until you pull the filter and drain some of that oil, it may continue. As for the drips of oil, just keep a lookout. If it doesn�t decrease, you have a leak. If it�s coming from the rear main seal, and you haven�t been driving the car a lot, driving it more might slow or stop the leak there. Some engines just smoke occasionally on startup. Often it�s because the engine didn�t warm up properly. For example, don�t start the car just to move it, and the shut it off. Once started, the car should be driven right away, gingerly with revs between 2000-3000 RPM until its warm. Then the car can be turned off. This is usually 15 mins. Other engines smoke on startup due to ring gap alignment or worn valve seats. They �all� can do this, any year, the old ones, the new ones. It�s not always easy to figure out why. Sometimes it�s that the car is not in a level surface. Other times, it�s not clear, and those owners just live with it. My 88 Targa did that for 90,000 miles.

I have no idea if this is related, but I hear GT3s smoke on start up. I have a 4.5L DeMan engine and it also smokes a bit on start-up - much more than the OEM 3.8L. The only experience I have with thick white smoke is when a fuel injector went bad on a Buick Enclave. The white smoke was so thick and there was so much of it, that it looked cartoonish - I looked like an airshow airplane with smoke turned on.

Yeah, about 20 years ago, Porsche came out with a a service bulletin to run a half quart less of oil to help mitigate the inability of the aos to handle the oil. You could also solve the issue by replacing your stock aos with the motorsports aos, which could handle the oil for street driving and track driving. Today, a better solution is the UAOS track version which can handle an extra quart above full. The motorsports aos could not. Been through my share of two failed aos' in 4 years back in the mid 2000s. Went to the motorsports aos and then earlier this year the uaos track version. Just a better engineered product that solves alot of silly problems on the street and on the track. But again, get it in for the Porsche dealership to look it over before your lemon law window ends just to be certain.

Once the car cooled i went underneath it to try see where this oil was dripping from. I managed to find a bolt on what appears to be the lower left side of the cam cover ? I cleaned it up and will check it again in the morning. Strange it started leaking after the AOS was changed, probably just an unlucky coincidence. Im not sure how bad that would be for these 996s, but ive had cam/timing cover leaks on my other cars before and they were easy fixes, is that the case here too ?

The 996 engine doesn�t leak the same way the old air-cooled cars did. Which is to say, you can actually fix 996 oil leaks and they stay fixed, lol. The cam covers are not a common leak area, but when they are fixed, stay that way. In general the 996 engine is pretty dry, should stay dry, and any oil leak should be fixed. I had a cam solenoid cover leaking and it was an easy fix.

Running too much oil in these cars is not good. I once took my 4s back to my indy after an oil change when they overfilled it. I made them drain some out.

Did more testing today, really enjoyed the driving today the car goes really well so its a shame its got these issues. Anyway so I definately think 996C438's theory about the oil being held in the head is a factor here, because the car smokes on first startup in the morning and it doesnt seem to smoke again for the rest of the day. Once the initial smoke is gone the engine starts up clean everytime (so far at least) theres not even a hint of smoke. I can get a fuel smell at idle though, as if the car is running rich, but it has been decat so maybe thats why. I decided to cut open the "old" AOS which apparently failed after 3 years and a couple months, and the diaphragm was completely fine, no holes or anything, theres a couple springs inside, the smaller one inside the plastic cylinder which was not moving super smoothly (felt as if the plastic cylinder was rubbing too much) and then that rides on top of a larger spring which was totally fine. Im not convinced the AOS alone was to blame here for the initial smoke show, since it seemed to be in functioning condition with no obvious problems. But after the dealership replaced it, its pretty obvious the car smokes less so WTF. Makes me wonder if the AOS wasnt getting strong enough vaccuum or whatever it needs to operate that diaphragm. Anyway i gotta reduce the startup smoking as much as possible since my car is parked in a communal estate and the smoke is going to blow into the apartments of people nearby and im sure they will complain about it eventually.

Have you removed some oil to get it down to correct level ? If not you are chasing your tail . As others have stated running it too full will cause issues .

Any way you can get some manometer readings once the engine is hot? I'd be curious to see how many inH2O you're getting. I'd also start by draining some oil via filter.

Thanks guys im going to drain some oil tomorrow. Will let you know how it goes.

I had about 0.5L or half a quart of oil drained. The digital oil gauge shows about 2 or 3 notches down from max. Starting the car up now the smoke is significantly reduced, and once it's been running a bit theres almost no smoke. As for the oil on the cam cover bolts, after putting it on the ramp a Porsche mechanic was able to confirm that the oil isnt coming from there but rather was spilt, probably when they removed the intake system to change the AOS, so there's no leak Thanks for the advice guys, i seem to be in the clear now so hopefully i can get a few days good running and enjoy the car.

Get the engine up to temp then run her hard . You want to get heat into the cats and mufflers to burn off residual oil that may be in them . Don't be surprised if you throw a CEL with a code for the O2 sensors . Just clear the code and keep driving.

Best habit to get into is to park on a flat surface and check the digital gauge prior to the first cold start each day.

^^ He�s right ^^ Although the manual might say otherwise, a lot of 996 owners do it this way because it�s the most consistent way to do it.

Awesome, looking good!

I'm convinced 7 out of 10 dealership techs overfill these engines and the AOS blows shortly thereafter. It happened to my car on the drive home after purchase. If the digital gauge is showing all bars as full, the engine is over-filled with oil.

It�s pretty simple:

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This is interesting, because when i cut open the old AOS (which was around 3 years old) i didnt see the typical broken diaphragm like what i expected. The piston inside that operates the diaphragm seemed fine too. When the AOS suddenly gets flooded with oil, what are its mode of failure ? What actually goes wrong with it ? Because im starting to think what happened was when i was going uphill the oil got sucked up into the AOS and basically filled up that cavity/body and stopped it from functioning correctly. Once its loaded up with oil like that it, its basically deadlocked itself and is continously in a state of sucking in more oil while being unable to flush the existing oil out. I suspect if that old AOS was drained, cleaned and put back in it probably would have been fine. That is unless the seal around the diaphragm was somehow broken by oil being forced into it. AOS Internal.

Which is better to trust, the dipstick or the dash gauge? On mine they both indicate full at the same time. Sounds prudent to fill up with 8 quarts and then measure out 8 oz at a time. Also remember that the flex pipe on the oil fill is their to unclip and raise up to assist on filling. First time I used it though I found it was cracked where the flex met the straight pipe, what a mess.

From what I�ve learned the gauge is best when the car is cold and sitting on flat ground. If the car has been running I believe the dipstick will give an accurate reading, assuming the car has been allowed to sit for enough time to allow the oil to fully settle back to the sump.

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I'm outta my element on the technical side, but my gut tells me you are onto something. My experience was similar- the failure mode didn't add up in terms of the AOS itself. I'm skeptical by nature, but I really don't think most dealership techs have any understanding at all about the subtle nuances to servicing these cars. IMHO, they are much more used to seeing 2012+ VAG-era cars. My car will never see the dealership again after that initial sale This thread could turn into another oil thread of differing opinions, as there are countless 996 owners out there filling up with 9 quarts of oil regularly. Even back to my air-cooled days, I am personally convinced these engines do much better with slightly less oil than initially thought or recommended.

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Skip should be able this question if he sees this.

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Best not to touch that fill tube after all these years. Look at it strange and it breaks 😂

I hear you, but if it’s cracked you need to know so you eliminate a potential vacuum leak. If it’s an old one, good thing to do preventatively if you got to replace any upper items like an alternator.

So today i went up the same hill that initially "broke" my AOS. Its a fast uphill banked highway that winds right for about 500m. When i started the car before the drive, oil level was 3 bars below max, no smoke on startup. After going up the hill then switching off when back home, if i check the oil level again its only 1 bar from max ! If i start the car then theres a reasonably large puff of smoke and then gradual smoking for a couple minutes until it eventually disappears. If i drive the car the smoking goes away quicker. This engine doesnt like uphills at all it seems, how very odd. Never experienced anything like this with another car before. Though at least its mostly fine now....though its quite a odd behaviour.

You might have some residual oil in the intake that still needs to drain over time. Either that or oil is still being pulled into the intake, or oil is getting past your rings or valves. The only way to know is to check the intake and connections and see if they are wet. If your vacuum levels are too high, maybe that�s a clue. I can�t think of other ways oil can get in there and get burned�you are sure smoke is coming out of the tailpipe and not around the area, right? Just in case you have an oil leak that is dripping on the exhaust manifold�

A quick and easy test to see if you may have an aos issue, keep your car running and try to remove your oil cap. If it's suction gripped to the oil filler pipe, you may have an issue. The guys gave you a few other things to test as well. You can always do the smoke test as well as get a manometer to test as well. But if you have alot of suction with the oil cap, you may want to explore the other tests. Or just take it to a good indy for their opinion.

Maybe you need new fuel injectors?

Your problem is that the factory AOS system was poorly designed in the first place!. Then the motor was overfilled with oil . Next thing was that oil was being introduced in excess into the intake ( your lucky to not have hydrolocked the engine) .. The oil will pool in the intake upper chamber as seen in this photo below. Then in a sharp turn or and uphill like you described, the oil that has pooled in the intake will spill into #4 cylinder ( and sometimes others as well) and a big cloud of smoke ensues, if your lucky ( if your not lucky and 60cc of oil enters the cylinder in one stroke, the cylinder will hydrolock and bend/break the rod, piston, or cylinder wall, and will be probably be misdiagnosed as the root cause of failure) .. 100cc oil laying in he pool ( cream color ) Shows the path from AOS to manifold Bent rods from AOS failure 2 D-chunks on same engine at same time from hydrolock Yes, your diaphragm probably didn't fail, just the same symptoms as a failed diaphragm , and is due to the root cause being a poorly designed AOS system.. As mentioned the UAOS not only has a diaphragm that is twice as thick as the factory, but also is modified not to let oil get in there to start with....Then has numerous other advantages, like a 10 minute change of the diaphragm is it every is necessary, and a warning light is if it is ever necessary, and being able to "over-fill" by 1qt (track Version only)...Over-built ? Not a chance !! Nothing is too good for our 996 M96 engines......

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Though surely it would smoke consistent if oil is getting past the rings or valves ? Like it would always smoke when cold, or always smoke when at temperature ? Currently its fine 90% of the time theres no visible smoke, then smoke shows up the rest of the time of startups only but usually after driving up a hill. I think two bits of new advice here im going to look into: 1. Check with a good independent Porsche mechanic. 2. Try and get that Ultimate AOS (not easy to source for me, shipping is really slow and restrictive here).

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Retro, you may not know it, but Porschetek3 is the inventor of the UAOS and a guy you want to listen too when he responds to a post with ideas, due to his background as a Porsche tech.

See post #35. Sounds like a dead ringer for your issue.

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Erm..Ive literally said im going to try what he recommended ? The main issue is that it going to take a while to get the part to my country.

I've sent quite a few UAOS kits to South Africa , how big a hurry are you in ??

The first step now is to look inside your intake manifold to see if you have oil pooled up in there, if so, that confirms what @Porschetech3 said. Just remove that oil, keep your oil level halfway, and you should be fine with the factory AOS as long as you don't track your car.

Overfill the oil by a 1/4 quart and you can/ will have problems. Underfill by 1 quart and it will typically be fine. Obviously keeping it on the middle of the gauge is ideal however the engine can take being low WAY better than being high, which goes against conventional wisdom. I have the UAOS so I am not worried however run mine in the middle of the gauge / stick. Just to give a point of reference with the car and what they can handle with the stock AOS and my car filled correctly. I used to go up Mad river glen in Vermont NH which is a mountain very steep with hairpins all the way. We have tires squealing around every corner aggressively driving and I never had I never had a smoke issue. Point being I am confident that overfilling the oil was probably what caused all 3 of those AOS's to be replaced. Of course all that being said the stock AOS system sucks and i am sure I would have eventually have issues hence the UAOS purchase. CHEAP insurance.

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Oh okay thats good news. Not in a hurry now that the AOS has been replaced, so which is the official site to order from ?