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3.6L missfire

  • 1995
  • 911 Carrera
45 000

I have a 1995 3.6L motor in a dedicated track car.  The last time I had the car on the track it was running great until I was near the end of the session.  I was at about 6000 RPM, full throttle, when the motor reacted like I had hit the rev limiter.  I immediately lifted off the accelerator but the car felt like it was running on 4 cylinders.  There was no smoke coming out the back and my temperature was good but my oil pressure gauge was at the max.  I limped the car back to the paddock and as I came to a stop the engine which was still running very rough, stalled.  There was a Porsche mechanic who took a look at the car with me.  After sitting about 20 minutes I started to car and it ran idled fine.  I drove it around the paddock and everything seemed to be fine.  The mechanic said that it was possible that some dirt got on the Mass Airflow sensor causing the problem.  He suggested I take it out again and watch the temperature and oil pressure gauges.  When I went back on the track I was gradually building speed one lap at a time.  The car seemed to be running good so I kept adding speed.  All the while the oil pressure gauge was at the max, temp was fine.  After running about 3 laps at full pace the same problem occurred.  Again I limped the car back to the paddock and it stalled when I came to a stop.  I noticed as I was idling in the paddock the Druck Oil Pressure was at about 3.5 – 4.0.  Normally after a 20 minute track session the pressure with a hot engine will be about 2.0 or a little under.  After sitting about 20 minutes the mechanic was able to take a look and again when I started the car it ran fine.  He suggested 2 possibilities; 1) There may be a vacuum leak; 2) The MAF sensor is dirty.  At this point I put the car on the trailer and came home.

After getting home I did the following; 1) I took off the MAF sensor a cleaned it thoroughly with a MAF cleaner; 2) I checked for vacuum leaks on the intake manifold and all the vacuum lines.  I found one hose that was a little suspicious where it connected to the manifold so I replaced it but I really don’t think it was leaking; 3) I pulled all the spark plugs to see if anything looked unusual.  The plugs were only about a year old. All of the plugs look very dark.  They appeared to be covered in carbon (see attached photo).  I replaced all the plugs.

Because the car is a track car I have not been able to see if I have solved the problem.  This all happened during my last DE of the season.  I won’t be able to try it until spring.  But I am concerned that the spark plugs were all dark and the oil pressure seems to be running much higher than usual.  What would cause the oil pressure to suddenly run higher than normal and could this cause the engine problem that I described?  I was hoping you might have some input or suggestions as to what might be causing these problems.  Thanks!

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