I’m reaching out to the community for some guidance on a recurring misfire issue with my 2004 Porsche Boxster S (986 model). I purchased it a couple of years ago, and it has now clocked 88,000 miles. Recently, it has started misfiring again, and I’m trying to pinpoint the cause.
To give you some background, I had a minor service performed by a Porsche specialist back in June before a planned road trip. The service included an oil change using 5w50 Mobil A40 oil and a manual gearbox fluid replacement. Shortly after this service, I noticed the engine misfiring and some white smoke, which was concerning.
My initial investigation revealed a slightly loose coil pack and a significant amount of oil in the plenum, originating from the air oil separator (AOS). To address this, I ordered a new AOS from Design 911, along with new iridium spark plugs and Beru ignition coils as preventative maintenance, given the unknown service history of these components. This seemed to resolve the problem temporarily.
For those interested, the Design 911 part numbers were:
- Air Oil Separator: 99610702602
- Spark Plugs: 9991700IK20
- Ignition Coil: 99760210404/1
However, the misfiring issue has unfortunately returned. It’s now severe enough to trigger the check engine light. Initially, it threw a P0305 code (cylinder 5 misfire). After driving on the motorway, it escalated to P0300 (random/multiple misfire), P0305 (cylinder 5 again), P0304 (cylinder 4), and P0303 (cylinder 3). These are the same codes I encountered before replacing the AOS a few months prior.
My troubleshooting steps this time included disconnecting the hose from the AOS to the plenum. I observed a considerable oil buildup in the hose, even after the car had been sitting for 12 hours. There was also a small amount of oil on the throttle body side. More alarmingly, I discovered a crack in the plastic hose, which unfortunately snapped completely while I was trying to remove it. I haven’t yet inspected the coil packs directly, as I need to get the car safely elevated for proper access.
Referencing a video from La Rose Porsche regarding 986 Boxster AOS failure, they demonstrated a test involving the oil filler cap. With a failing AOS, removing the oil filler cap while the engine is running is supposedly impossible due to vacuum pressure. After replacing the AOS, the cap should come off easily. Testing this on my Boxster yesterday, I found it impossible to remove the oil filler cap while the engine was running, suggesting a potential AOS issue again.
Has anyone experienced an AOS failing again so soon after replacement, in just 4 months? Could a split hose be sufficient to cause these symptoms, mimicking a failed AOS? Additionally, I’m curious if anyone has experience using a lighter oil with the AOS in a Boxster. Any insights or advice would be greatly appreciated, as I’m starting to worry about a more serious underlying problem. I’m really hoping it’s something relatively straightforward to fix.
Thank you in advance for any help you can offer.
Kieran.
