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Boxster 986 O2 sensors Misfire and running rough.


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Hi,

 

I had my 2001 2.7L Boxster serviced in June and all was fine, it has done 93,000 miles.  A week or two later noticed an increased noise from the exhaust and on closer inspection there was a small hole in the CAT on Bank 1 (drivers side in the UK).

 

I got two CATs and had my local garage fit them (now July), no problems there and then filled up on the way home at a garage I don't normally go to as the tank was low.  The next few days I had random misfires, but only after deceleration and at low revs (less than 2,000 rpm) when feeding the power back in.  The misfire was random (sometimes cylinder 1, then 6, then 2, then 4).  I thought may be dirty fuel.

 

My OBD-II code reader showed ageing on all four O2 sensors, I guess they were just old and swapping them over into the new CATs finished them off.

 

After a long wait to get new sensors (now August), I decided to fit them myself, the old ones were difficult to get out of the new CATs, so I took both CATs off, fitted the new O2 sensors and all faults cleared with none coming back.  I took it for a test drive, no misfire and back to normal.

 

Roll on another week and the misfire was back, but not all of the time.  I tried a fuel cleaner and a couple of tanks of good fuel from my regular garage, but still got the odd misfire.  I then noticed that the battery was quite low and had been for some time, so recharged up to full on a recovery charge, the result no more misfire, although the engine is not running as clean as it was.

 

There is no problem at idle or when pulling, it's at small throttle openings or slow speed, it's not a misfire, but it isn't running very cleanly, so since O2 sensors was the last thing I have gone back to those.  I initially thought that the output looked normal, the two sensors pre-CAT move from rich to lean in nice smooth graphs and the post CAT sensors are fairly smooth, so I tried a re-learn and it was then that I noticed that Bank 1 Sensor 2 (Post CAT) is sitting at 0.2V soon after starting whilst Bank 2 Sensor 2 is 0.50V.  Once the engine warms up Bank 1 Sensor 2 does come up to around 0.5V when the car is driven, but soon drops to a fairly low voltage when idling, this shows the sensor is working but may be the heater isn't, is that what you would think?

 

You can see the big difference in the attached screenshot from Torque Pro.

 

I know Sensor 1 is used to set the mixture and thought that Sensor 2 is used to test the CAT, but is Sensor 2 also used to set the long term fuel trim and could a faulty heater in one sensor post CAT cause rough running?

 

Robin

IMG_4073.jpg

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Have you already tried cleaning the throttle valve ? At the same time clean the airflow meter with a spray developed for this purpose, CRC brand for example. Al this is inexpensive and takes little time, i would start with that.

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