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Intermittent rough running

Old 02-12-2018, 08:48 AM
  #1  
bigbenUK
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Default Intermittent rough running

Hello,

First post here having only just got my 2003 986 Boxster S.

The car has a problem where it will run fine for about 20 miles then start to run rough eventually resulting in a CEL. Resetting the codes on the engine clears the problem for another 20 miles or so.

The codes I have read are:

P0140 - O2 sensor circuit - no activity detected
P1126 - Oxygen sensing adaptation below or above limit
P0171 - Oxygen sensing adaptation idle range Bank 1 - above limit

However, the O2 sensors are OK on start up / post code clearing which leads me to think that the O2 problems are a symptom of the problem rather than the cause.

The DME clearly thinks it is running too lean so is adding more and more fuel resulting in very rich running (the exhaust smells very rich at the point of rough running). This is saturating the O2 sensors leading to the error codes.

So far the previous owner has tried swapping coils and plugs side to side, swapped cam position sensors and I have removed and cleaned the MAF (also running with the MAF unplugged resulted in a significant change for the worse, suggesting the MAF is good)

My thoughts are there is a vacuum leak somewhere downstream of the MAF.

Anyone have any suggestions?

Ben
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Old 02-12-2018, 01:11 PM
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Anker
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I believe it is the O2 sensors, A vacuum leak usually leads to rough running and wouldn't wait for 20 miles. When I had O2 sensor issues it would also take a while after resetting the codes before the CEL lit up again. My guess is that the O2 sensor heat circuit is broken.
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Old 02-12-2018, 01:16 PM
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bigbenUK
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Originally Posted by Anker View Post
I believe it is the O2 sensors, A vacuum leak usually leads to rough running and wouldn't wait for 20 miles. When I had O2 sensor issues it would also take a while after resetting the codes before the CEL lit up again. My guess is that the O2 sensor heat circuit is broken.
That is a good point. I guess I can try swapping the O2 sensors side to side and see if that fixes it.

Is a broken heater circuit usually from the sensor itself or the associated wiring harness?

Ben
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Old 02-23-2018, 02:25 PM
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bigbenUK
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Tried 2 different (brand new) Lambda sensors today and the problem persists.

The pre-cat Lambda on bank 1 is outputting a fairly constant Voltage at around 0.1V the other three seem sensible enough. NB the 0.1V was the same for all 3 units I tried today, that is to say the original and the two replacements.

Ben
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Old 03-11-2018, 11:00 PM
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butzigear
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Have you looked into potential Air Oil Separator issues? I've seen some similar symptoms with failing AOS systems on a few cars.
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Old 03-12-2018, 07:44 AM
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bigbenUK
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Originally Posted by butzigear View Post
Have you looked into potential Air Oil Separator issues? I've seen some similar symptoms with failing AOS systems on a few cars.
The AOS is not that old on the car but is probably worth investigating further. Various forum posts suggest that a sign of a failing AOS is it becomes very difficult to remove the oil filler cap when the engine is running. I can remove the cap no problem. What is a sensible next step?
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Old 03-13-2018, 08:52 AM
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bigbenUK
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Originally Posted by butzigear View Post
Have you looked into potential Air Oil Separator issues? I've seen some similar symptoms with failing AOS systems on a few cars.
I removed the AOS from the system yesterday evening, that is to say I removed the pipe from it to the inlet manifold and blocked it off in both directions. This did not make any difference to the engine's fuel trim / Lambda readings

I was fairly surprised to note the AOS ran at positive pressure, I thought it was reliant on engine vacuum to pull the air through.

Ben
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Old 03-13-2018, 12:33 PM
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Yes, but when you block the vacuum line the piston blow by pressurizes the AOS. When the vacuum line is attached it runs under negative pressure.
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Old 03-13-2018, 01:03 PM
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Brian in Tucson
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Originally Posted by Anker View Post
Yes, but when you block the vacuum line the piston blow by pressurizes the AOS. When the vacuum line is attached it runs under negative pressure.
Get some MAF/MAP sensor cleaner and clean your MAF sensor. Takes more time to take the engine cover off than it will to clean the sensor. BTW, don't use any other cleaning product. and treat the sensor very gently.
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