What are the 996 oil separator problems?
Discussion
The diaphragm within the separator splits and allows full intake vacuum to reach into the crankcase, in minor cases the engine runs lean and the DME adaption facility alters the fuelling to compensate and the driver may not notice anything; in the worst case scenario the intake system takes in engine oil from the crankcase and the engine can hydraulic lock on the oil (doesn't much like compressing, liquid). In between cases usually result in smoke on start-up and at idle (burning off residual oil) and general fluffy running.
HTH
HTH
911Fiddler said:
Anybody know what happens when these operat badly or actually fail? Is it heavy oil use? Any idea why they fail precisely?
What Pope said. That last one I had replaced I cut it apart and found a tear/crack/split (from material fatigue would be my guess) in the canvas/rubber diaphram.Sincerely,
Rockster.
NBTBRV8 said:
How can you test for this?
One 'test' is to with the engine idling unscrew the oil filler tube cap and see if you can remove it from the oil filler tube. In some cases, my first hand experience, is one can't remove the cap against the huge pressure difference.But the AOS's failure may not be advanced far enough for this pressure difference to be present.
Sometimes you don't have to test for a failing AOS, it will, well the engine controller will let you know because it logs one or more DTCs (diagnostic trouble codes) and of course turns on the check engine light. The error codes generally arise from the engine controller reaching its enrichment limit. IOWs there is an intake air leak and the engine controller has gone as far as it can go adding more fuel.
The air is coming in via the AOS. I do not know the various ways this can happen, that is, the various failure modes. For instance, a hole in the diaphram may be an air leak, or the diaphram due to the leak may not react as much to engine vacuum and thus fail to dial back the amount of vacuum the engine crankcase sees. The air leak maybe air being pulled in past a few seals, or past a leaking oil filler tube cap, or even a crack in the oil filler tube, or a split in one of the hoses that connect to the engine. The engine crankcase is under very very low pressure and well where's there is a pressure difference...
Early signs of a possible AOS failure is a hunting idle, the idle goes up and down, oftentimes mildly. The engine may feel a bit flat off of idle, pulling away from a stop light say.
As the AOS failure progresses, the engine can smoke long after starting, or the engine can develop a rough idle. In real severe cases the engine exhaust smokes so much liquid oil can drip from the exhaust outlets.
At this stage the engine's at extreme risk from possible hydraulic lock. This is when the vacuum becomes so great engine oil is pulled from the engine's crankcase into the intake in sufficient amounts one or more cylinders can ingest enough oil that during the compression stroke the oil takes up what little space there is when the piston is at top dead center. Since oil can't be compressed any significant amount the rising piston meets an unmovable resistence. Severe engine damage is almost always the result.
In fact when the engine begins smoking for more than a brief time after engine start this is the time to stop running the engine and have the cause of the smoking id'd and corrected. Play time with the engine is over.
Sincerely,
Rockster.
Cheers great discussion. Re testing the integrity if the diaphragm, if the internal construction of the device is that manifold vacuum is the controlling factor upon the crankcase pressure, and the manifold vacuum is delivered by one or both of those two small pipes going in, surely a good test would be to pull off one or both of these pipes then test for leaks here? Engine off of course. Am thinking simply sucking myself somehow on it to see if it leaks - obviously need to be able do this without getting a mouthful of black stuff! I did a similar test once on the vacuum reservoir using the small bore pipe at the tank vent valve - was ok. I guess this hinges on how the device works but potentially it could be a way of finding the early or minor failures.
Found a discussion that advances somewhat the testing question....
Porsche Engine Oil/Air Separators 101
The Engine oil air separator directly affects the engine crankcase atmosphere because basically all Porsches from 1980 on including the current 986-987-996-997-Caymen & Cayenne engines all are positive crankcase type systems meaning the engine crankcase has vacuum supplied from the intake manifold to collect and burn the residual combustion by-products or blow-by in the crankcase by running it through the engine again, basically to lower the overall vehicle emissions.
On most occasions, when the oil separator fails, intake vacuum in the engine crankcase rises to an unusually high or abnormally high figure and more likely than not ends up drawing engine oil into the intake system usually fouling spark plugs and even possibly causing damage to the oxygen sensors (O2), catalytic convertors and even possibly various other sensitive fuel injection components.
Interestingly enough, the only way to test the oil separator is to periodically test the engine crankcase vacuum with a water filled Monometer AKA an ultrasensitive vacuum gauge. On BMW’s and Porsche’s, the engine crankcase vacuum is normally around 4-7” (Inches) of water. When these oil separators fail the crankcase vacuum can rise to 9-12” and even higher depending on the extent of the failure.
When the oil separator is starting to fail on Boxsters (986-987), generally you will start to see a substantial amount of smoke from the exhaust tailpipe. When the oil separator finally fails, the engine exhaust smoke can reach dangerous proportions that almost look like it was manufactured from an anti-mosquito smoke machine. Unusually enough, on the 999-997’s, when the oil separator fails, it hardly ever creates any exhaust tailpipe smoke, almost being like a silent killer or costly surprise.
On another note, it has been known on rare occasions, that when an oil separator fails that it is possible to have a slightly rich running condition that might turn on the check engine light or CEL. The only way to know what is happening is to check the fuel trim numbers on the OBD-II “Freeze Frame” data and crankcase vacuum to see exactly what’s going on, otherwise your just guessing.
Tony Callas
______________
Owner, Callas Rennsport
Phone: 310-370-7038
Located in the Greater Los Angeles Area
http://www.callasrennsport.com
Porsche Engine Oil/Air Separators 101
The Engine oil air separator directly affects the engine crankcase atmosphere because basically all Porsches from 1980 on including the current 986-987-996-997-Caymen & Cayenne engines all are positive crankcase type systems meaning the engine crankcase has vacuum supplied from the intake manifold to collect and burn the residual combustion by-products or blow-by in the crankcase by running it through the engine again, basically to lower the overall vehicle emissions.
On most occasions, when the oil separator fails, intake vacuum in the engine crankcase rises to an unusually high or abnormally high figure and more likely than not ends up drawing engine oil into the intake system usually fouling spark plugs and even possibly causing damage to the oxygen sensors (O2), catalytic convertors and even possibly various other sensitive fuel injection components.
Interestingly enough, the only way to test the oil separator is to periodically test the engine crankcase vacuum with a water filled Monometer AKA an ultrasensitive vacuum gauge. On BMW’s and Porsche’s, the engine crankcase vacuum is normally around 4-7” (Inches) of water. When these oil separators fail the crankcase vacuum can rise to 9-12” and even higher depending on the extent of the failure.
When the oil separator is starting to fail on Boxsters (986-987), generally you will start to see a substantial amount of smoke from the exhaust tailpipe. When the oil separator finally fails, the engine exhaust smoke can reach dangerous proportions that almost look like it was manufactured from an anti-mosquito smoke machine. Unusually enough, on the 999-997’s, when the oil separator fails, it hardly ever creates any exhaust tailpipe smoke, almost being like a silent killer or costly surprise.
On another note, it has been known on rare occasions, that when an oil separator fails that it is possible to have a slightly rich running condition that might turn on the check engine light or CEL. The only way to know what is happening is to check the fuel trim numbers on the OBD-II “Freeze Frame” data and crankcase vacuum to see exactly what’s going on, otherwise your just guessing.
Tony Callas
______________
Owner, Callas Rennsport
Phone: 310-370-7038
Located in the Greater Los Angeles Area
http://www.callasrennsport.com
Thread resurrection 
Edit to add: HERE is a link with a bit of how-to info if anyone is interested. I'll be putting this on my job list.
Rockster said:
Early signs of a possible AOS failure is a hunting idle, the idle goes up and down, oftentimes mildly. The engine may feel a bit flat off of idle, pulling away from a stop light say.
This is interesting. I've needed to replace the equivalent part on a couple of my BMWs, hadn't thought to check on this car, but I have noticed the idle hunting occasionally so I'll take a look.Edit to add: HERE is a link with a bit of how-to info if anyone is interested. I'll be putting this on my job list.
Edited by sundayjumper on Tuesday 17th April 15:18
Old thread revived but can AOS failure or imminent failure on a 987 cause a loud squealing sound on tick over? Mine has just started doing it and upon accelerating the noise subsides until you slow down then it gets louder . There is a short video on YouTube that demonstrates the exact same sound attributing it to the failed diaphragm !
Thanks
Thanks
Can't imagine how any noise could be attributed to it, the only vid I found on YT sounds like an issue with any of the fanbelt driven devices. He said it was "fixed" after the separator was replaced but I'd say that the belt tension was likely changed when the work was done and the noise just went away.
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