Help with 2002 4.6is rough running

[romeokc10]

Originally Posted by David.X5

I've been told that post-cat O2 sensors are not part of the loop controlling engine performance, but instead are used to determine the health of the cat system. So if this theory is correct, it may throw a code, but the post-cat O2 shouldn't cause poor performance.

See for example - 12v Pages - The Engine - How it Works

You are correct.

[upallnight]

The voltage for the 02 sensors after the catalytic converters should be steady and not fluctuate like the pre-cat sensors. Can you post real time data of the 02 sensors after the cats?

[bcredliner]

The opinions on whether the post cat O2 sensors have anything to do with fuel trim is another one of those long debated issues.

The Bentley service manual for 2000 to 2006 X5s states that O2 sensors contribute to fuel trim. It does not designate between pre and post. It is a general statement. It is generally accepted the post cat sensors monitor emissions. Seems if the intention is too monitor emissions and conveys that reading to the ECM it is not just for grins.

That said, with OPs symptoms, it could be the post O2 sensor but there is a possibility if could be the cat. I suggest disconnecting the post O2 sensor and see what happens.

[murf]

Thanks for the replies!

I changed plugs and no improvement. I have been studying all the signals looking for answers because I know it is in there somewhere.

I have been concentrating on the rough running values and was wondering why I was only seeing values on bank 2 when bank 1 was all zeros.

In the software I can disable one fuel injector at a time. On bank 1 when I turn off an injector the idle drops way down and the engine almost dies. On bank 2 it hardly makes a difference.

Look at the values below. When I turn off an injector on bank 1 the value for that cylinder slowly goes from zero and starts to climb so I know it is reading some real values. It also will make change doing the same thing on bank 2 but not as much.

I have disconnected, one by one, the coil plugs on bank 2 and with each one there was no noticeable change in running. I disconnected all 4 coils on bank 2 so it is only running on 4 cylinders and that is how it has been running all along. I put a spark plug in one of the coils and it does have spark. So it appears that the fuel injectors are not firing on any of the bank 2 cylinders.

Where do I look next for a possible cause?

[bcredliner]

The rough idle could be caused by current going to the wrong injector. You can hear injector firing if you have an auto stethoscope. It will be a clicking sound. That won't mean anything if the pulse is not at the correct time based on piston stroke. IMO the cause could also be an O2 sensor. Suggest you disconnect O2 sensors one at a time on problem bank and take a new readings.

[murf]

I just checked the injectors with the stethoscope and they are definitely clicking away. I confirmed the sound by pulling off the electrical connectors and there was a audible change in the clicking. I don’t think there is a way to mess up the sequence of the injector connectors since they all come off together in the housing. That’s not to say someone before me didn’t switch them up.
I installed new o2 sensor before the cats. I bought a universal one for bank 2 after the cat that worked for 15 minutes and then the signal stopped. the computer recognizes that it’s there but there is no voltage signal coming back from it. I am picking up a new one today that is not a universal one.
I will try disconnecting the o2 sensors and see what happens. I will clear the adaptation at the same time so the readings are not influenced.

[J.Belknap]

Swap ingnition components (plugs/boots/coils) to opposite head and retest.

[bcredliner]

Disconnect the pre cat sensor-read, connect the pre cat sensor, disconnect the post sensor-read.

[bcredliner]

Originally Posted by J.Belknap

Swap ingnition components (plugs/boots/coils) to opposite head and retest.

Seems the best that swapping all ignition components side to side is move the issue to the other bank? Why not just disconnect current to one cylinder at a time and see if one or more cylinders change the rough idle the least?

[murf]

I disconnected one coil at a time on bank 2 (problem side) and you can hardly tell a difference in the rough idle.

I can swap all the coils from one side to the other and see if the problem follows, but I kind of doubt it. If it did though, at least it would narrow down the problem point some more. Both pre-cat sensors are brand new. I will try disconnecting them to see what happens.