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Thread: MAF and O2 causing M73N Secondary Air System failure?

  1. #1
    Join Date
    Dec 2016
    Location
    Springfield, VA
    Posts
    1,006
    My Cars
    2001 750iL DD74441

    MAF and O2 causing M73N Secondary Air System failure?

    Greetings. I have posted this in the E38 forum but taking a guess there may be more M73/N expertise here. Any assistance appreciated; good natured ridicule welcomed.

    Model: 2001 750iL
    Odometer: 130k miles

    TLDR: still having secondary air system (SAS) failure after doing nearly everything in that system. What are the chances of a failing MAF and lazy O2 sensors causing a false SAS failure? If plausible, which O2 sensor should be done: pre, post, or both?

    Full entry
    Symptoms:
    • -Check Engine light on
    • Codes present on both DMEs.
    • -P0410 Secondary Air System as read by DISv44 through ADS (20 pin connection).
    • -Occasional (1 in 5 instances) other codes may be simultaneous or solo, again read via ADS.
      • -P0103 - MAF High Voltage.
      • -P0170 - Fuel Trim Out of Range.
    • -Occasional “hiccup” of roughness (correlated with P0103) with cruise control (if engaged) ceasing control

    Actions taken:
    • -R&R Secondary Air Pump.
      • -Validated Pump runs during activation.
    • -R&R Non-Return Valves.
      • -Validated vacuum activation and clear of “junk”.
    • -R&R Vacuum Lines.
    • -R&R Vacuum Canister.
    • -R&R Vacuum Solenoid


    Discussion: Having those symptoms and taking those actions I still have SAS failure (CEL). Knowing a failing MAF causes fun issues and O2 sensors become lazy with age, I think my SAS failure is actually that and not an issue with the actual SAS.

    Theory: MAF is “dirty” and requiring more voltage thus sending a higher volume of air to the DME than actual. The O2 is lazy and won’t respond sufficiently fast to register when the SAS kicks in. Additionally, with the MAF sending incorrect (high) levels of air, the DME may consider the O2 reaction as responding to the “base” airflow and not the SAS.

    Potential Remedy: R&R MAFs and precat O2 sensors to remove false baseline and ensure responsive O2s

    Questions:
    • Is this plausible?
    • Should I also look at R&R postcat O2s?
    • Is there anything else in the SAS that I missed?




    2001 750iL DD74441
    Stock

    RIP: 2003 540iA Sport GS56111
    H&R front springs, Ultimate Cup Holder, Euro Dash & Armrest, Grom, BavSound Stage1

  2. #2
    Join Date
    Jan 2019
    Location
    Canada
    Posts
    25
    My Cars
    f10 535ix
    Here is how to rebuild auxiliary pump and check the relay, might help.

    https://www.youtube.com/watch?v=tQ7Q09f5qAs&frags=pl%2Cwn

  3. #3
    Join Date
    Jun 2014
    Location
    Blue Springs MO
    Posts
    708
    My Cars
    M70 6sp. 540iT, 535is
    I've seen post O2 stuck in lean, will throw off mixture on that same bank (have seen it on Japanese cars, never seen it German, yet)
    Check for exhaust leaks, happened to me with a Range Rover. https://youtu.be/VX0w4ggvZEQ
    Last but not least, check you fuel pressure, and fuel pressure vacuum regulator(s).

    -I assume that you did checked vacuum leaks at both of the intakes, right?

    Note:When you removed MAFs have you seen any signs of water/condensation? On one of my 8s, secondary air valve wasnt closing fully, causing condensation at the air pump and at the air filter box, corroding my MAF sensor..
    Last edited by bmw528e11; 01-23-2019 at 01:27 AM.

  4. #4
    Join Date
    Dec 2016
    Location
    Springfield, VA
    Posts
    1,006
    My Cars
    2001 750iL DD74441
    Bmw528e11 - I did have condensation in the pump with the original codes (start of journey). So replaced the valves, pump, and vacuum solenoid. Still had the issue.
    Also, did have an aged O2 driving me lean (as you pointed out) so replaced all 4 (as long as I was there).
    After all that still had the failure/code. Went back to F107 and looks like I have a partially blown fuse. Allows current for the DME 1 & 2 functional tests but the total command on pops the circuit. Replacing the fuse and we will see what happens. More to follow...


    2001 750iL DD74441
    Stock

    RIP: 2003 540iA Sport GS56111
    H&R front springs, Ultimate Cup Holder, Euro Dash & Armrest, Grom, BavSound Stage1

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