19 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHQuestions about AOS [air/oil separator]
Leading reported evidence
Replace every 7 years or so
Performed · outcome unconfirmed
No successful repair reported for this finding.
0
Reported fixes
0
Unsuccessful attempts
1
Discussions
Read evidence and context
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Discussions that performed work without a confirmed outcome
1
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Performed · outcome unconfirmed
IMSB: can be upgraded with the LN Solution which is a permanent solution or the LN Retro which will need to be replaced every 7 years or so.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
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Suggestion
The two-part answer is that one, you can test pressure vacuum to confirm intact function and secondly, regardless of function you should consider replacing the AOS with an ultimate AOS to avoid catastrophic failure.
Leading evidence: 'Replace every 7 years or so' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Replace air oil separator' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
Source reports, not a diagnosis or a success probability. Counts refer to discussions and can overlap.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that contain suggestions
1
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Suggestion
The two-part answer is that one, you can test pressure vacuum to confirm intact function and secondly, regardless of function you should consider replacing the AOS with an ultimate AOS to avoid catastrophic failure.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that contain suggestions
1
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Suggestion
Easy fix by replacing the water pump every 5 years or so with a new one.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that contain suggestions
1
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Suggestion
Easily mitigated by running high quality high viscosity oil with lots of moly.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that contain suggestions
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Suggestion
Change it 2x a year, keep your fuel injectors, oxy sensors and MAF fresh and running properly and you should be in good shape.
These are separate views of the same evidence. Frequency alone does not establish a fix, and these counts must not be added together.
Replace air oil separator
No successful repair reported for this finding.
0
Reported fixes
0
Unsuccessful attempts
1
Discussions
Read evidence and context
Discussions that contain suggestions
1
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Suggestion
The two-part answer is that one, you can test pressure vacuum to confirm intact function and secondly, regardless of function you should consider replacing the AOS with an ultimate AOS to avoid catastrophic failure.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Suggestion
The two-part answer is that one, you can test pressure vacuum to confirm intact function and secondly, regardless of function you should consider replacing the AOS with an ultimate AOS to avoid catastrophic failure.
Counts describe captured discussions, not unique people or a diagnosis. Author identities are not reliably stored. Exact duplicate evidence is counted once across matching discussions; one discussion counts at most once per action/state. Multiple repairs reported together remain a combination. Suggestions and failed attempts do not establish a fix. Matches cover whole discussions and can include related faults, model variants and incomplete threads.
19 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
0 evidence records excluded for query context · 0 excluded by outcome qualification · 0 adjusted by outcome qualification.
0 acknowledgements have no verified action link; they do not establish a repair result.
Generation: 20261026T024951Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources
Hi All, I'm liking this forum a lot.. many good suggestions from my first post...As an aside I am not a completely clueless novice to Porsches having owned multiple air cooled versions ; [ 56 4-cam speedster... 356 "C" coupe...several 67 911's [one an "S"..hence the user name]...85 911SC Targa...several 914 cars [restoring a 74 1.8 now].. and other water cooled versions 924 and my wife's [I like It too] 16 Cayenne......Enough background information, This 99 996 coupe is my first water-cooled 911, so I am going to depend upon the collective [resistance is futile.. :-)] wisdom to help me through the birthing pain ....Coupe [68k on the clock] is now having all four struts replaced, total brake flush, engine transmission [TIP] flush and filters, spark plugs... plus cabin filter...NOW for the good part...The AOS system [to my untrained eyes] appears to be functioning ; BUT are there any
in situ
tests that I can perform to verify it's function... NO smoke on startup, NO leaks, NO smells of unburnt fumes..BUT still inquiring minds want to know , is it OK for now... car has had limited miles on it for about 5 years....AFTER everything else is sorted...ITALIAN TUNEUP?...Please give me the wisdom of your experiences... THIS will be a KEEPER for me....Thanks/Cheers....JON
Hey Jon, Looks good. there are several good threads on your aos. The two-part answer is that one, you can test pressure vacuum to confirm intact function and secondly, regardless of function you should consider replacing the AOS with an ultimate AOS to avoid catastrophic failure. So search ultimate AOS here and you will get the picture in a hurry. Cheers.
Congrats Jon.
There are 3 main catastrophic failure points on the 996 that are very easily addressed thru upgraded parts.
1. AOS: Otto had mentioned the AOS, which can be upgraded to the UAOS to prevent smoking, AOS intermix and worst case hydrolock which will destroy your cylinders.
2. Water Pump: Failure of the bearings in the water pump leads to a wobble which breaks off the fins and they lodge in the heads creating cracking. Easy fix by replacing the water pump every 5 years or so with a new one.
3. IMSB: can be upgraded with the LN Solution which is a permanent solution or the LN Retro which will need to be replaced every 7 years or so.
The fourth failure point is bore scoring, which is a slow deterioration in your cylinders. Easily mitigated by running high quality high viscosity oil with lots of moly. Change it 2x a year, keep your fuel injectors, oxy sensors and MAF fresh and running properly and you should be in good shape.
Enjoy your 996. They are incredible cars that are easy to keep running strong.
If the AOS is original (likely given the mileage) it's a good idea to have it replaced. The rubber diaphragm would be aged out by now and not worth taking a risk on. Same goes for the water pump... it's not worth taking a chance for the cost of replacement.
OTTO...By "pressure" do you mean positive or negative....my understanding is that this is a PCV system to scavenge crankcase fumes.... so my money is on positive pressure at the oil filler cap...
Absolutely correct.... negative pressure on the system.... I did read and was informed by the AOS thread... -5.0 to 7.0
negative
pressure is optimal... will be testing this soon... meanwhile new water pump, and belt is in order....[IMS has been addressed]....full speed ahead....Thanks and CHEERS, Jon
Add a new thermostat and upgraded metal AOS coolant line adapter to the list as well while you have the system drained:
https://www.rauchandspiegel.com/prod...oil-separator/
Also, the 3-way hose from the coolant tank is usually crispy from the exhaust manifold heat, you can see the hose here in my documented coolant tank replacment thread:
https://rennlist.com/forums/996-foru...it-begins.html
I recently had to have a major rebuild of my 2003 C4/CAB after the AOS failed, Oil level was correct, the AOS failed and I didnt realize it, went through a hard 180 right turn and the engine sucked raw oil in to the drivers bank cylinders. Wasn't even a lot of oil, cracked the cylinders (Major D-chunk) luckily no rod or crank damage and I was not hard on the throttle when I did it. FUn part is the stock AOS can and has failed days after being installed.. Your ileage May vary as they say.. (My car has about 116K on it when it happened and it had been serviced before with new factory Porsche parts. )
GOod Luck.
Basically, oil/mist goes thru the factory AOS into the intake and accumulates in the cylinders and creates excessive pressure that cracks the cylinders.
The factory AOS is a weak link in the engine because its unable to process the oil/mist and the oil flows to the cylinders. The UAOS is a just a better design. It has a bunch of screens/filters that catches the oil so it cant get into the intake amd instead drains it back to the sump. As we have talked about many times on Rennlist, the UAOS os so we'll designed that you can overfill the engine with oil, park it on a hill or even do high G turns at the track, and it collects the oil without even a hick-up.
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