Porsche Air Oil Separator Failure: Smoke, Codes, and What to Do Next

The air oil separator (AOS) is part of the crankcase ventilation system on Porsche flat-six and V8 engines. It separates oil vapour from crankcase gases before they are recirculated into the intake. When the AOS fails, liquid oil enters the combustion chambers, causing blue/white exhaust smoke, fouled spark plugs, and a cascade of fault codes. On M96 and M97 engines, a failed AOS also accelerates bore scoring.

Short Answer

The AOS contains an internal diaphragm and a series of baffles that separate oil droplets from blow-by gases. The diaphragm cracks or tears with age and heat exposure, typically between 60,000 and 90,000 miles. Once compromised, the separator no longer functions — oil vapour passes straight through into the intake manifold and is burned in the combustion chambers. The result is visible exhaust smoke (especially on cold start and deceleration), increased oil consumption, and lean/rich fault codes from the disrupted air/fuel mixture. Replacement is the only fix. The updated Porsche part uses a more durable diaphragm material. On 911, Boxster, and Cayman models, the AOS is accessible without major engine disassembly.

What You'll Learn

• How the air oil separator works and why it matters
• The symptoms of AOS failure — what you will see, smell, and measure
• Which Porsche models and engines are affected
• The link between AOS failure and accelerated bore scoring
• Replacement cost and procedure
• Why AOS replacement should be a scheduled maintenance item, not a reactive repair

Real-World Scenarios

Case 1: 997.1 Carrera — Blue Smoke on Cold Start

A 2007 997.1 Carrera with 74,000 miles arrived with a complaint of blue smoke from the exhaust on cold starts, lasting approximately 30 seconds before clearing. The owner had been told by a forum that it was valve stem seals. We connected PIWIS 3 and found stored fault codes for bank 1 running rich at idle (P2096). A smoke test on the crankcase ventilation system confirmed a ruptured AOS diaphragm — pressurised smoke exited freely from the intake side of the separator. The AOS was replaced with the updated part (Porsche part number ending in -05, the latest revision). Smoke eliminated, fault codes cleared, and oil consumption returned to normal: less than 0.5 litres per 5,000 miles. Total cost: £680 fitted.

Case 2: 986 Boxster S — Rough Idle and Multiple Misfires

A 2003 986 Boxster S with 91,000 miles presented with a rough idle, intermittent misfires across all cylinders, and oil consumption of 1 litre per 600 miles. The previous garage had replaced all six ignition coils and spark plugs at a cost of £850 — no improvement. PIWIS showed multiple misfire codes (P0300 random misfire) and long-term fuel trim corrections of +18% on both banks, indicating a significant vacuum leak or metering error. The AOS was removed and found to have a completely disintegrated internal diaphragm. Oil was pooling in the intake manifold runners. The AOS was replaced, the intake manifold cleaned, new spark plugs fitted (the originals were oil-fouled), and the fuel trims reset. The engine ran cleanly immediately. The ignition coils replaced by the previous garage were almost certainly not faulty.

Case 3: Cayenne S — Oil in the Intake Manifold During Service

A 2008 Cayenne S with the M48.01 V8 came in for a routine service. During spark plug replacement, we noted oil contamination inside the intake manifold ports. The spark plugs were heavily carbon-fouled. Oil consumption had been gradually increasing — the owner was adding 1 litre every 3,000 miles and assumed it was normal for a V8 with 95,000 miles. The AOS was tested and confirmed failed. On the Cayenne V8, the AOS is integrated into the engine valley and requires intake manifold removal for access — a more involved job than on the flat-six models. We replaced the AOS, cleaned the intake manifold, fitted new spark plugs, and performed an oil change. Oil consumption dropped to less than 0.5 litres per 8,000 miles. We also noted this as a priority to monitor for coolant pipe condition given the intake manifold was already removed.

Why Inspection and Diagnostics Matter

AOS failure is commonly misdiagnosed as valve stem seal wear, piston ring wear, or turbocharger oil seal failure (on turbocharged models). The symptoms overlap significantly. A proper diagnosis requires checking crankcase ventilation system integrity (smoke test), reading live fuel trim data via PIWIS 3, and inspecting the intake manifold for oil accumulation.

At Nine Torque, we test the AOS as part of every major service on applicable Porsche models. The test takes 15 minutes and provides a definitive pass/fail result. Given that a failed AOS directly contributes to bore scoring on M96/M97 engines — by disrupting combustion temperatures and increasing cylinder wash — early replacement is a cost-effective investment in engine longevity.

If your Porsche is smoking on startup, consuming oil faster than expected, or showing lean/rich fault codes, contact us for a diagnostic assessment. The AOS is one of the first things we check.

Frequently Asked Questions

How much does AOS replacement cost?

On the flat-six models (911, Boxster, Cayman), the AOS is relatively accessible. Parts cost approximately £180 to £280 for the updated unit. Labour is 2 to 3 hours. Total fitted cost: £500 to £800. On the Cayenne V8, the job is more involved due to intake manifold removal — expect £900 to £1,400 fitted. Always use the latest revision part number to ensure the improved diaphragm material.

Can a failed AOS damage my engine?

Yes. On M96 and M97 engines, a failed AOS introduces excess oil into the combustion chambers. This disrupts the air/fuel ratio, causing uneven combustion temperatures across cylinders. The resulting thermal stress accelerates bore scoring, particularly on the already-vulnerable cylinders 1 and 6. A failed AOS does not guarantee bore scoring, but it significantly increases the risk and the rate of progression if scoring has already begun.

How often should the AOS be replaced?

We recommend replacing the AOS every 60,000 miles or 8 years as a preventative measure. On engines already showing signs of bore scoring or high oil consumption, replace it immediately regardless of mileage. The updated Porsche part is more durable than the original, but it is still a wear item with a finite service life.

What are the symptoms of AOS failure versus valve stem seal failure?

AOS failure produces smoke on cold start AND on deceleration (overrun). Valve stem seal failure typically produces smoke only on cold start after the car has been sitting — oil seeps past the seals while parked and burns off when the engine starts. AOS failure also causes measurable fuel trim deviations on PIWIS live data; valve stem seals do not. A smoke test on the crankcase ventilation system will confirm AOS failure definitively.

Is the AOS the same part as the PCV valve?

The AOS performs a similar function to the PCV (positive crankcase ventilation) valve found on other manufacturers' engines, but it is a far more complex component. It includes the oil separation diaphragm, pressure regulation, and oil return pathways in a single integrated unit. It is not interchangeable with a generic PCV valve and must be replaced with the Porsche-specific part for the engine code in question.