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AOS

Understanding the PCV and AOS Systems on Porsche Boxster, Cayman, and 911 Models

The AOS, or air oil separator, is a part of the PCV system on a modern Porsche sports car engine found in Boxster, Cayman, and 911 models. The AOS should be treated as a maintenance item and replaced using only Genuine Porsche parts, including all AOS vent hoses and lines, every 6 years or 75,000 miles, whichever comes first.

Generally speaking, all Porsches 1978 and newer are equipped with a Positive Crank-case Ventilation (PCV) type emissions device, meaning the engine crankcase has a vacuum source supplied to it from the intake manifold. This is to collect and burn the residual combustion by-products known as “blow-by”. “Blow-by” is combustion forced past the pistons and piston rings into the crankcase. Blow-by in the crankcase is then consumed by running it through the engine again, in an effort to lower the overall vehicle emissions.

On the Mezger based flat 6 air cooled engines built from 1964-1998, these dry sump type oiling systems normally do not require an Air Oil Separator (AOS), but all water cooled naturally aspirated 9X6, 9X7 and 9X1 Sports-Cars equipped with the water-cooled M96, M97 and MA101 engines (1997-2017) require this device for AOS diagnostics (Except for the GT2’s, GT3’s and Turbo models). The AOS directly affects the engine crankcase atmosphere by applying a very small vacuum to it by the nature of its job; it should be named a vacuum-oil separator. In most situations when the AOS fails, intake vacuum in the engine crankcase rises to an unusually high value. More often than not, this high vacuum ends up drawing engine oil into the intake system causing excessive exhaust tailpipe smoke. This heavily laden oily air mixture can foul spark plugs, contaminate oxygen sensors, catalytic convertors and even possibly ruin various other sensitive engine management components, e.g. the Mass Air Flow sensor (MAF). Even when the PCV system or AOS is functioning correctly, valve guide and piston ring wear, as well as cylinder ovality, all contribute to oil consumption caused by excessive blow-by. However, even a healthy engine might consume oil. Remember, Porsche states that one quart in approximately 600 miles is an acceptable level of oil consumption in a modern engine with low tension rings. There are several contributing factors including fuel dilution, however with modern oils volatility is the primary contributor to oil consumption and PCV/AOS failures.

Have you ever torn down a motor and seen oil in the intake manifold? How about an intake valve dirtier than an exhaust valve? Ever wondered why that is? The one word answer is “volatility.” This refers to how much vapor a motor oil releases when it gets hot. Just as how water vapors rise off a pot of water before it begins to boil, the same thing happens to motor oil inside your engine. As oil splashes onto the pistons and valve springs to keep them cool, the high temperature causes some of the oil to evaporate. Modern engines have a positive crankcase ventilation valve that vents these oil vapors into the intake manifold. The oil vapors condense in the cool air/fuel mixture, which leaves the oily deposits in the intake manifold and on the intake valve.

So what can you do? Using a lower volatility motor oil is the first line of defense. Fewer oil vapors mean not as many make it into the PCV system in the first place. More oil is then kept in the crankcase where it belongs. Secondly, not overfilling your engine and keeping the oil level closer to the min. fill level will further reduce windage in the crankcase, reducing load on the PCV system. In most cases, adding a deep sump to your Porsche engine allows you to run your engine half a quart low on oil, further reducing windage inside your engine, while increasing overall system capacity!

These two steps will reduce the amount of oil getting into the intake tract. Obviously a cleaner intake valve will flow better, providing more power and better fuel economy than a dirty valve. These steps also reduce oil consumption, which helps to protect O2 sensors and catalytic converters from damage due to excessive oil consumption. Specialty oils like Driven DT40, DI40, and DT50 feature a high quality base oil, reduce oil consumption AND contain more ZDDP for better engine protection. The O2 sensors and catalytic converters don’t know how much ZDDP is in the motor oil so it stays where it should – in the crankcase lubricating your engine. These oils can contain 50% more ZDDP since the volatility is 67% less than standard base oils. The higher quality base oil delivers better all around protection and performance.

Even with the right oil, faulty or dirty injectors can contribute to cylinder bore wash-down, further reducing ring seal, allowing for increased blow-by and contamination of the engine oil, all which have a negative effect on performance. Regular use of Top Tier fuels, such as Shell V-Power, and fuel system cleaners like Lubro-Moly Jectron or Driven Injector Defender will protect your port injected engine from intake and combustion chamber deposits.

Direct injected engines, like those found in 2009 and later Boxster, Cayman, and 911 models as well as Cayenne models since 2008 and all Panamera models, do not have the benefit of port injection to keep intake ports and the intake valve clean. As a result, the use of a low-volatility oil is even more important. Driven DI40 is a specially formulated oil that is designed to have very low volatility, meaning you end up with less intake port and valve deposits. Like with most modern Audi, BMW, Mercedes, and Volkswagen engines with direct injection, expect to have your intake ports and valves cleaned using a process done by the dealership and independent technicians, whereby walnut shell media is used to blast and vacuum these deposits away. But again, using the right oil will minimize the buildup and extend the performance of your direct injected engine and continued use of Top Tier fuels and fuel system cleaners will ensure combustion chamber and piston deposits are minimized.

For modern Porsche engines utilizing an AOS, measuring the crankcase vacuum is an excellent way to test for overall engine health as well as for the condition of the AOS. The only way to definitively test the AOS is to measure the engine crankcase vacuum with a slack tube tester or Manometer (an ultrasensitive vacuum gauge). On the 9X6 and 9X7 Porsches, the engine crankcase vacuum reading is normally in the region of -4.0” to -6.0” H2O (Inches of water). When an oil separator fails, the crankcase vacuum can rise to -10.0” to -15.0” H2O, or even much worse (-40.0” H2O) depending on the extent of AOS failure.

On the 9X7.2 and 9X1 Porsches, the engine crankcase vacuum reading is normally in the region of -14.0” to -16.0” H2O (Inches of water). When an oil separator fails, the crankcase vacuum can rise to -20.0” to -25.0” H2O, or even much worse (-30 to -40.0” H2O) depending on the extent of AOS failure.

Normally vacuum is measured in inches of Hg (Inches of Mercury) but in this case we measure it in H2O (Inches of water). Water is much lighter that Mercury.

When the air oil separator starts to fail on a Boxster (986-987), you will generally start to see a substantial amount of tailpipe smoke on cold engine start-up. When the oil separator finally fails, the exhaust tailpipe smoke can reach epic proportions; almost like it is manufactured from an anti-mosquito smoke machine. On the 996’s & 997’s, when the oil separator fails, it usually doesn’t create much exhaust tailpipe smoke, if any. It’s almost like a silent killer or costly surprise. It is a good idea to check your vacuum reading at each engine service to monitor the health of your AOS.

However, there are instances other than tailpipe smoke that can indicate your AOS is going bad. Two such symptoms caused by an early failing non-symptomatic AOS are ignition misfire and fuel trim faults. On 2005 and older models the DME fuel trim monitor does not operate immediately from cold start-up but the misfire monitor typically does. If the AOS is in its early stages of failure, it can produce some very high vacuum values i.e. -40.0“ H2O, but only for the first 3 minutes of cold engine operation, usually without any trace of tailpipe smoke. Hence the AOS can cause a lean running condition that can affect the fuel trim values but without any other symptoms. You can get misfire faults and the check engine light (CEL) will be illuminated but there are virtually no other symptoms. This is where having tools such as a Durametric along with the CR Tools AOS Analysis Tool can help you diagnose and catch an AOS failure before serious engine damage can occur.

Normally when an AOS fails, it can cause a lean running condition that will most likely illuminate the CEL. The only way to really know what’s happening is to measure the crankcase vacuum and then monitor the fuel trim numbers in the Actual Values section of the Digital Motor Electronics (DME) with a high quality scan tool/tester to see exactly what’s going on, otherwise you’re just guessing. So what do the readings actually mean?

 

How do you test the air/oil separator on a Porsche 996 or 997 with the M96 engine?

 

What is a normal AOS manometer vacuum reading on Porsche Boxster, Cayman, and 911 Models?
 

  • M96 & M97 Nominal crankcase vacuum is -5.0” (-4.0” to -6.0”) of H2O (not HG).
  • MA1 Nominal crankcase vacuum is -15.0” (-14.0” to -16.0”) of H2O (not HG).

Please note: These readings are at Sea-Level and at operating temperature. With elevation these readings should be lower in value due to thinner air.

 

Common Causes for Low Engine Vacuum Reading on Porsche M9X and Later Watercooled Porsche Engines
 

  • 1. Missing engine oil cap and or oil cap O-ring.
  • 2. Cracked or broken spark plug tubes, if applicable.
  • 3. Disconnected or broken dipstick tube, if applicable.
  • 4. Make sure the oil dipstick is fully seated, if applicable.
  • 5. Anything that can leak air can leak oil and vice versa.
  • 6. Torn AOS boot, broken or cracked oil filler tube, Dipstick and/or crankcase crossover accordion pipe.

 

Common Causes for High Engine Vacuum Reading on Porsche M9X and Later Watercooled Porsche Engines
 

  • The only thing that can cause an excessively high manometer vacuum reading is a faulty AOS and or sludge.

 

What are the symptoms of a failed AOS or air-oil separator on a Porsche engine?
 

  • The result of a faulty AOS (with high vacuum readings) will sometimes cause the engine to make a honking like sound; this is caused by vacuum/air being forced backwards past the crankshaft seal(s). Note: this does not have to be accompanied by tailpipe smoke. More often than not it is non-symptomatic. A high vacuum reading is an internal vacuum leak and this causes a lean running condition usually leading to an illuminated check engine light (CEL).
  • A failed AOS (with high vacuum readings) can cause the engine oil pressure warning light to illuminate. This is usually from too low of engine idle, a cause and effect condition created by a faulty AOS.

 

What happens if I have a catastrophic AOS failure and my intake fills up with oil?

If the AOS has a catastrophic failure and causes a hydraulic or hydrostatic lock (hydrolock) condition, do not panic, remove all the spark plugs, turn engine over slowly by hand with wrench, get engine oil out of the cylinders slowly and gently. Perform engine leak down and compression tests on all cylinders to confirm engine condition and then replace spark plugs with new. Remove exhaust muffler(s) and drain engine oil from exhaust system. It is always recommended to remove central resonance tube from intake manifold to remove engine oil with a vacuum device (e. g. Mighty-Vac), complete assembly, replace faulty AOS and start engine. Run automobile at idle in a safe area until smoke subsides. Be forewarned, it will smoke a lot.

 

What is Hydrolock?

While hydrostatic lock is normally associated with damage from water or radiator fluid, liquid oil can be just as damaging to the combustion chamber. The M96 and M97 engines are vulnerable to hydrolock from oil even under normal driving conditions. Porsche engine experts who work one these cars on a daily basis will tell you that damage to engines due to symptoms related to hydrolock is as ubiquitous if not more so than the intermediate shaft bearing problem. They tell us that this issue may have been overshadowed by all the internet chatter, press and class-action lawsuit given to Intermediate Shaft Bearing. However, hydrolock should not be ignored as a serious risk for the 996, 986 (Boxsters and Caymans), and early 997s. One of the problems is that unlike the IMSB failure that can be traced to the source of the problem from metal fragments, hydrolock does not provide easily discernible evidence that allows the technician to trace the damage back to its origins, especially since oil is expected to be seen in its natural environment. Many times the correct diagnosis is missed.

The combustion chamber is intended to work in an environment of air and gas along with minuscule particles of oil from the PCV system blow-by. During the compression stroke at the point where intake and exhaust valves are closed that air/fuel mixture compresses in preparation for ignition. The next stage is called the power stroke where the spark plug ignites the mixture in the combustion chamber and literally explodes, and the effect is to push the piston down. That forces causes the crankshaft to rotate and all is good. However, like water, unintended large volumes of oil will not compress or ignite in the manner the chamber was designed to do. The oil then acts as a force against the piston and thanks to the laws of "ideal gas" and thermodynamics, the piston stops, rod bends, cylinder sleeve cracks, and the crankshaft is bent. The effect is almost instantaneous and the crankcase can be ruined in the process, the crankshaft bearings destroyed and the shock can even cause cracks in the engine block. The effect of hydrolock can be catastrophic.

Why is the M96.M97 motor any more vulnerable to hydrolock than any other vehicle? It is because PCV oil can accumulate over time at the top of the manifold in a reservoir-like cavity that allows it pool. The arrow in the picture below shows where the oil accumulates. Normally occurring PCV blow-by oil are tiny particles that build up over time in the cavity of the intake manifold, that oil will pool to to become a source of liquid oil that could then be absorbed by the engine as a gulp.

Should a small amount of oil make its way down to the cylinder head, damage is not likely but the evidence can be seen in the form of a random (or chronic in more serious cases) puff of white smoke from the exhaust. However, should a large enough quantity pour into a cylinder as a gulp, the results are not limited to environmental damage. A hydro lock event will cause major damage to the engine.

Unlike an IMSB event that can occur without warning, hydrolock fortunately provides a warning in the form of smoke coming out of the exhaust. If you see smoke exiting your exhaust pipes, one of the reasons can be due to small amounts of oil making their way into the combustion chamber. Of course smoking exhaust can also be caused by other problems, but if it is due to oil accumulation in the manifold reservoir the engine needs immediate attention.

 

Why should I upgrade to the Motorsports AOS Air-Oil Separator if I am tracking my Porsche Boxster, Cayman, or 911 sports car?

The M96 engine is wet sump, not dry sump like GT3, Turbo, and GT2 models and previous aircooled Mezger 911 engines. As such, the AOS is part of the complex system to control oil vapors in the crankcase and separate this oil from the oil laden air processed by the AOS. However, when you track any Porsche Boxster, Cayman, or 911 vehicle, the stock AOS simply cannot handle the extra load, allowing oil to be ingested by the engine through the intake which can lead to smoking and in the worst case scenario, cause catastrophic engine damage if sufficient oil makes its way into a cylinder including bent or broken rods, piston and ring damage, or cracked cylinders. The Motorsports AOS provides additional processing volume and has been proven to provide a solution to this problem on track when combined with our 2 quart deep sump that can help prevent oil starvation and also allows the oil level to be run at a lower level which also assists in reducing strain on the AOS. 5-chain M96 engines found in 1997-2002 Boxster and 1999-2001 911 (996) models can only be fitted with a Motorsports AOS with our adapter.

 

What is the Ultimate AOS?

The patented Ultimate AOS (USPTO Patent # 11,326,489), also known as the UAOS, will stop the worry of "catastrophic failure" of the engine due to hydrolock. It will also eliminate EVER having to replace the AOS AGAIN, which is a costly and time consuming job for most shops. Failure of M96 engines due to AOS malfunction is responsible for more engine damage than has been previously assumed. The Ultimate AOS, designed and manufactured by Charles "Skip" Brazeel, works by taking a systematic approach to the problems associated to to the Factory AOS and correcting/improving them. The Factory AOS design is well known for failing of the flow control diaphragm, and the result is increased oil consumption, or excess smoking, and in severe cases engine hydrolock causing bent rods and cylinder damage. When the flow control diaphragm fails in the Factory AOS, the present procedure is to replace the complete AOS. This procedure is very labor intensive due to the location of the Factory AOS, and involves hours of labor. In the Ultimate AOS design, the flow control diaphragm regulator is moved to easy access at a location near the throttle body and can be replaced in a matter of minutes. The "Vortex" part of the air-oil-separator is left as Factory designed and drains the separated oil back to the oil sump.

As an additional design improvement, the Ultimate AOS, incorporates an Accumulator in series between the Vortex and the Flow Control Diaphragm Regulator. The Accumulator acts as a secondary air/oil separator and collects any excess oil in the crankcase vent system. Also incorporated in the Accumulator is an infra-red sensor set to activate an in-dash warning if accumulation reaches a dangerous level of about 117ml , indicating a fault in the Crankcase Vent System and allows for operator intervention before any engine damage can occur. This feature will save engines from the threat of hydrolock. The Infra-red warning feature is Microprocessor Controlled and wired to the low coolant warning circuit, and will "slow flash" the red warning in the instrument cluster. This "built in" warning system is designed to end unexpected hydrolock failures in M96 engines.

Stop worrying over engine failure over hydrolock caused by failed AOS - The UAOS has a 2 year unlimited mileage warranty on all components except the pre-sealed AOS which carries a "Lifetime Warranty" and the flow regulator is easily replaced in just minutes!

Special Thanks to Tony Callas, Callas Rennsport for his assistance and providing the technical information this content is based on and also Charles "Skip" Brazeel, creator of the Ultimate AOS (UAOS).

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