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Old 09-13-2012, 06:51 AM
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ztom ztom is offline
I gotta have more cowbell
Location: Thousand Oaks CA
Join Date: Sep 2007
Posts: 892
Mein Auto: 1997 528i
Misfiring under load - KNOCK SENSOR!

I did a lot to track this down, discussion on this thread..

..but today it is solved - I disconnected the knock sensor and the stumble/misfire at freeway speeds under load (like going uphill) is gone. I've ordered a new knock sensor but driving around I don't detect any issue for now with the sensor disconnected.

I think for any e39 over 150K mi, I recommend at least to disconnect the sensor to see if some power is restored in the 3-4K rpm range. This problem started maybe 10-20K miles ago, at first at 3500 rpm and then most recently it happened under less load and at lower revs down to around 2800rpm.

Here's my story, may help others:

>Did easy misfire DIY first - swapped coils between cylinders and put in new plugs, and checked old plugs for signs of misfire.

>Changed the bank 2 pre-cat O2 sensor because I could see dynamically on the code reader it was lazy

>Swapped in a spare CPS and vanos solenoid, no change. Note in doing this it's easier if you take off the oil filter cap to get more room.

>Pulled the fuel rail and swapped out the FPR and injectors with another 528i, no change. Also cleaned the injectors in ultrasonic with 70% alcohol/water.

>Swapped in spare MAF

>Then I realized I could check the STFT trims in real time. I could see that I was not having a lean condition (as happens if MAF dirty or non-metered air getting in via any intake boot crack), rather STFT's were fine even under load. I did notice that when it stumbled the timing advance would drop from +30 degrees (typical at frwy speeds) in quick drops to say 27 degrees. I attributed this to knocking, but now I realize it was the knock sensor sending a false signal. Appears that under load the ECU is interpreting a bad knock sensor signal as a knock.

>Then disconnected a lot of things all at the same time - MAF, Vanos, both pre-cat O2's. Problem persisted.

>Finally, with everything connected again, I disconnected only the knock sensor. The plug is under the intake manifold next to cylinder 4. I did not need to remove the intake. Rather took off the bolts to the PS fluid reservoir, removed the oil filter cap, removed the +12V bolt on the intake and pushed the 12V socket down below the intake. In this way I could reach under the intake and pull off the knock sensor connector. Driving around, I'm still expecting to see a CEL for missing knock sensor, but I drove once so far for about 20min under various loads/speeds, no CEL yet.

One of the replies earlier suggested I replace the knock sensor. Whoever that was, thank you.

Last edited by ztom; 09-13-2012 at 06:54 AM.
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