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Thread: MAF and O2 causing M73TU Secondary Air System failure?

  1. #1
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    MAF and O2 causing M73TU Secondary Air System failure?

    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?




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  2. #2
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    Hey Peter -

    Since you have DIS running, can you run INPA instead and see what the the BMW-proprietary diagnostic text is from that?
    The fabulous thing about INPA and proper BMW code reading is it gives you additional data about the code in question... specifically RPM/temp etc. when last occurred, and, in the case of electrical failures, specific text about short to ground, or open, etc.

    That MAF code looks to me like an electrical code. Either a bad wiring situation or a real hard-failing MAF there. That's obviously probably sending the trims whacko and causing misfires yadda-ya.

    I do hate to post this, because I know a half dozen friggin chuckleheads are gonna read this post in 3 years and gonna swap new MAFs and O2s into their cars in a vain parts-shotgun attempt to fix SAI errors and when it doesn't work they're gonna be all "hey you said this would fix it how come it didn't!?!?!"... but...

    I wouldn't normally point SAI failures/codes at MAF/O2 and absent any other issues it would never be my first avenue for diagnosis ( <<--- part chuckleheads will miss/ignore )...
    but it is fer sure possible.

    Diagnostic on the SAI system is going to be based on a couple things:
    - Electrical diagnosis of solenoids and pump DME output pins (no shorts / opens)
    - Seeing a certain expected response / plausible range in Lambda (O2 reading) during activation (does O2 go normal-to-lean as soon as the pump clicks on)

    So IMO yeah its very possible scenario in your case that bad MAF is sending your car trims are way off, and then because of that the DME's aren't seeing the right range from the O2's.
    Also certainly possible that O2's dying causes bad general trim and not reading the SAI-responses properly.

    I wouldn't chase O2's yet given you have a MAF error. Are the MAFs old by the way? Reasonable to expect they are past their sell-by date? If they are original I'd maybe think about go parts-shotgun on 'em and see if that does the job, figuring they are near end of life anyway. If you tell me they only have 30k on them or something then maybe not.

    I guess the other question is (and possibly here's where my ignorance of M73TU comes in...) are the MAF errors completely symmetrical on the DME's? If the errors are symmetrical for MAF that'd almost be a little suspicious that both would be failing exactly same time / same way. If its happening on both I might want to rule out the possibility of some common wiring problem to the 2 MAFs.

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  3. #3
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    GG - thanks and appreciate your venture into this other territory.

    Quote Originally Posted by geargrinder View Post
    Since you have DIS running, can you run INPA instead and see what the the BMW-proprietary diagnostic text is from that?
    I do have INPA but don’t have the translations down yet. As the M73N is “easily bricked” I use DIS as a safety. But will head into INPA for additional details. DIS gives some of the information though. Here is what I have (summarized):
    MAF codes are generally at full operational temp (102-106°C) and seem to appear in the 1500+ rpm zone. Frequency seems to be a single occurrence between scans and symmetrical across DMEs (more below).
    Trim codes are generally at initial loop operational temp (85-100°C) and are generally idle +300 rpm. Again single occurrence and iirc about 50/50 if on both DMEs.
    SAS codes are at cold start (30-45°C) and at idle to idle + 300 rpm. There are several (3-6) instances each read on both DMEs. That makes sense as it takes repeated codes to set the CEL.

    Quote Originally Posted by geargrinder View Post
    That MAF code looks to me like an electrical code. Either a bad wiring situation or a real hard-failing MAF there. That's obviously probably sending the trims whacko and causing misfires yadda-ya.
    My thoughts are the MAF is aged, likely original but will check my records to validate. With the age comes crusting of the sensor requiring more voltage for the same airflow. The wiring appears good: no cracks, kinks, wear, or other signs of issues. And both DMEs are dry and happy.

    Quote Originally Posted by geargrinder View Post
    I do hate to post this, because I know a half dozen friggin chuckleheads are gonna read this post in 3 years and gonna swap new MAFs and O2s into their cars in a vain parts-shotgun attempt to fix SAI errors and when it doesn't work they're gonna be all "hey you said this would fix it how come it didn't!?!?!"... but...
    Agreed and why I tried to be as thorough as possible in laying out my issue. That’s all we need. Some moron gets an SAS code, finds this, and changes out the MAFs and O2s without even testing the pump. Hopefully with a detailed first post we can avoid that.

    Quote Originally Posted by geargrinder View Post
    I guess the other question is (and possibly here's where my ignorance of M73TU comes in...) are the MAF errors completely symmetrical on the DME's? If the errors are symmetrical for MAF that'd almost be a little suspicious that both would be failing exactly same time / same way. If its happening on both I might want to rule out the possibility of some common wiring problem to the 2 MAFs.
    I am still learning the M73 and I am not sure about anything. It appears there are two DMEs that do checksums through the EML to maintain torque balance etc. After all, it is the “lazy” engine of just strapping two I6s to the same shaft... I haven’t been able to discern if an error on one registers as a failure on the other, primes as a “first code”, or completely separate.

    Likely plan will be replacing the MAFs (easiest) and providing results followed by replacing O2s with results. Hopefully I can work the translation issue for more detailed views (or exploring DIS for same effect).



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    Although the MAF's could be on their last legs, the inlet manifold gaskets are the Achilles heel of the M73 (and M70) - and they are the primary cause of adaptations at limits. This will also cause Secondary Air System faults as the monitoring of the system is made by measuring the Lambda response to all that fresh air being shoved up the exhaust manifold.

    If the Lambda reading is already showing a lean mixture at the limits of closed-loop control then it will not register the addition of fresh air and will fail the SAS test.

    To check if this is the case read the adaptation values as this will determine if you have manifold leaks - all explained here:



    ...and more about tracing these kind of problems here:

    https://www.meeknet.co.uk/E38/Diagnostics.htm
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    SEE PETER. I knew some M73 expert would come around and make anything I had to say look stupid!

    A known manifold gasket aging problem sounds 100% plausible.
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    Timm- good points and I should validate it isn’t something else through examining other potential variables. The main reason I am hesitant on the manifold leaks is that the records show a full manifold repair conducted 18 months and 10k miles ago. While possible, I am thinking the likelihood is lower.

    GG- possibly but going through the records I see NO evidence of the MAF or O2s ever being touched.

    Nothing ruled out at this point, testing and validation needed. Luckily, I was able to get OE MAFs and O2s for ridiculously cheap. So now I just need to find a good translator for INPA and some time... kids activities are fun but definitely get in the way of maintenance...


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    Alright, so this is going to go in stages and “live” analysis. Thanks to Timm’s video and Google Translate (awesome that it will translate pictures live) I went in a little deeper into INPA. Not very but enough to see some interesting things that I will need to digest.
    So first, here are the error codes. First one is the SAS and is a current fault. The second is MAF and is a sporadic fault (not currently present if I translate correctly).

    Then, going into Timm’s money shots. Keep in mind I have two DMEs so twice as much info







    The interesting thing is that the high adaptations were almost immediately followed by a negative adaptation

    Will do some analysis after baking cookies with kids...


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    Alright, first batch is done so some analysis.
    First blush I don’t see anything really “off”.
    At operating temp, the MAFs are more or less aligned. There is a 10% deviation but I suspect that is allowable but not ideal. It does potentially show a vacuum leak on the right side (second set). It has some high trims with a lower MAF reading. As I understand it, that should mean the DME is compensating for a leak post MAF.
    However, that side is immediately followed by large negative integrators reducing the trims. I wasn’t able to cycle fast enough to see any changes in the other readings (if there were any). So that implies there is instability in the system...

    So either
    1) I have a vacuum leak masking the SAS
    2) I have faulty MAF(s)
    3) I have faulty O2s
    4) 2 & 3
    5) something else

    The issue I have with 1 is the previous work done (and admittedly I don’t want to tackle that right now) and there seems to be large corrections going on. I am not sure vacuum leaks open and close... that implies it is a sensor issue especially when considered with the MAF code.

    As for 2, I didn’t see large changes in the MAF readings but there are codes (sporadic) for the MAF and a deviation between MAFs.

    3, I did see the integrator change with adaptations accordingly. AFAIK the O2 is directly responsible for the integrator value but is also only reacting to the environment. A large change in the sensor readings therefor either mean that 1) the environment changed (vacuum opened/closed) or 2) the sensor reacted (implying laziness). I don’t believe that vacuum leaks behave that way so that implies laziness.

    4 & 5, I can’t rule out based on my knowledge but welcome any insights.

    One thing I would like to do is see all the feeds on one page... can’t do it in INPA AFAIK but maybe in DIS I can? Will have to research.

    So in summary, at this point, with the codes, deviation between MAFs, and swinging integrator values I think I am justified in replacing the MAFs and observing the results.

    Thoughts?



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    The values don't look too bad - but they do show something is not right. You have positive adaptations on one bank and negative on the other - plus an implausible MAF reading error. The MAF error indicates that the measured air is not plausible with the throttle angle of 20% - and that could be the MAF, but just as likely it could be a manifold leak.

    As you have a MAF error on only one bank and have two MAF's - the next step is pretty obvious - swap the MAF between banks - if the error goes with MAF then the MAF is the problem - if the error stays on the same bank then a vacuum leak is the most likely....
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    MAF and O2 causing M73TU Secondary Air System failure?

    Timm thanks! Second batch got done and was going to swap in “new” when I saw your post.

    Was hoping for a quick fault but no.

    Here are the shots (right then left). My only “worry” for this analysis is that I got the previous shots backwards based on the trims...
    Right




    Left




    Will have to take it for a drive later to see if I can get the errors. No joy revving in P.

    The other worrisome note is the post kat O2s seemed to be mirroring the prekat O2s. But that is for another time.

    Edit: and this is why one probably shouldn’t do this between activities... the multiplicative is crazy negative... didn’t notice that at the reading but the pictures don’t lie... will have to check that...

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    Last edited by TheAngryBear; 12-23-2018 at 05:52 PM.

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    Well it took a bit but ended up with the SES/CEL back on with codes.
    Ones I didn’t expect.
    MAF on both banks.

    The SAS is working and performing (including activation and function test), now it is just the MAF codes. Both sides had them with both the same info. Odd, but sort of goes with my theory.

    So I am going to swap those out with my “spares” and see what happens. If I get the codes still then something else is off...

    And for the record, the trims and smooth/rough values are looking acceptable.




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    Those sort of values are usually seen with Chinese MAF's - what have you got in there?
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    They were the original MAFs: Bosch units


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    Let's hope they are completely dead then!
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    Well good news and and bad, always the case here right?

    So the MAF issues appear to be gone... good.
    However, the SAS has returned... bad. Well, sort of. There are new codes (that sadly I forgot to print off). But they lead to new issues.
    P0410-SAS
    P????-Maximum control limit reached, bank 2
    P????-Aging Oxygen Sensors

    I went back through the SAS system testing the electrical and found an issues with the relays. Purely by accident I knocked one of them while testing and it gave continuity while not being energized. They were allowing intermittent connections while being moved/vibrated thus (I figure) turning on (or maintaining on status of) the SAS. I guess that it being on when the system isn’t supposed to be drives the mixtures lean and the system attempts to compensate. So I replaced those relays. This is definitely a reminder that just because something is new doesn’t mean it is good: No Evidence of Working.

    The O2s aging also explains some of the issues I am facing. If they are slow in reaction then when the SAS turns on there may not be a fast enough reaction to acknowledge the system being on. Those are scheduled for replacement soon (I have them, just need to find the time).

    My logic thinking through this is the MAF codes take priority over the O2s and once sensed basically causes the system to ignore all but failure of the O2s. Hence I needed to resolve the MAF issues first. Now that the system happy with the input sensors, it allows the codes for the reactionary sensors to be set. Additionally, with high MAF readings I am not surprised the O2s may have been damaged/contaminated by the excess fuel being dumped.

    Not guaranteed to be out of the woods and may still have a manifold leak but making progress. Hopefully when I change out the O2s I can put this to bed.



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    Yes MAF failures cause the DME("s" for you!) to fail over to what are generally called "limp" or "failsafe" tables usually. And that can mean other systems that are dependent on MAF-inputs go into limp/failsafe too. Basically you go out of "closed loop". So yeah your hypothesis sounds reasonable. Bad O2 sensors and max control limit reached codes easily could be same problem with the O2s.

    Good luck w/ the SAS. Are you sure its the relay itself and not maybe the relay connection/socket?
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    MAF and O2 causing M73TU Secondary Air System failure?

    The sockets seemed stable and providing current/continuity as expected. The mounts did seem “loose” as they wiggled and moved as I was testing them (but still had current). Not sure how to shore them up. The actual relays though were found faulty on the bench, so my bet (for now) are the relays.
    And as I get to remove the DMEs and TCU to access those relays (and the wiper relay), until I get get a solid concept to shore them up or truly need to go in there...

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    Here is how to rebuild auxiliary pump and check the relay, might help.

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

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    Alright, it had been a while since updated this thread. Between the cold snap and life there have been some delays but there have been interesting developments. Don’t have pictures as the cold made me race through operations but I will attempt to describe the issues and the distractions...
    The SAS and MAF bank 1 codes were coming back and setting off the SES light. Additionally, I was getting a fuel smell at startup with an accompanying shake/sound on bank 1 that implied a vacuum leak (intake manifold gaskets?). So I was distracted a bit thinking it was the injector orings or air hoses allowing me to emulate the Challenger along with the IMGs. I was not happy about that...
    Pulling the codes I had the “P0173 can’t compensate” but no accompanying misfires. The smooth/rough numbers were all below 100 so I figured/hoped something else must be happening.
    Switching from DIS to INPA allowed reading shadow codes and let me see the “bank 1 precat O2 heater” code. Doing a visual on the O2 sensor showed the O2 lines starting to split and corrode. Near as I can figure it was enough to do some voltage drops but not enough to definitively set the codes. This made the DME think bank 1 was lean and with the massive adaptations was “flooding” the fuel system allowing me to smell (likely via exhaust) the gas. This seems to be confirmed with this thread
    https://www.bimmerforums.com/forum/s...nues-need-help
    So I swapped out all 4 sensors (why? Because the other lines were showing cracking plus I believe in doing sets for balance) and the fuel trim went back to normal(no more fuel smells) and operations were great although I still had the shake/sound on bank 1...
    Then the P0410 with accompanying MAF code went back on and set off the SES light. That is when I made the connection that the shake/sound was there for about 60-120 seconds... disconnected the SAS pump and no more shake!
    Here is the weird part (because this hasn’t been enough of a saga so far): after clearing the code and disconnecting the SAS pump... the SES light hasn’t come back on. I know the engine has been started cold multiple times (after sitting overnight in 7-12°F ambient, that almost has to be the definition of a cold start) but no light. Haven’t checked for codes (or shadows) yet but will need to.
    Doing further googling I found this:
    European EOBD only checks the electrical connection of the secondary-air system, but not its action.
    The electrical connection is monitored for short to earth, short to power supply and open circuit.

    The American on-board diagnosis version OBD II monitors the action of the secondary-air system:
    As a check, the secondary-air pump is switched on once per driving cycle with the engine warm.
    The Lambda sensor thus registers excess oxygen. The probe signal is compared to the set values in the control unit.
    Full link: https://www.ms-motorservice.com/en/t...-air-system-2/
    This explains why the SES light for the SAS comes on with low engine temp vs during operational temp. But that means there should be an exemption for this vehicle as the protocols are for the destination market vs the market of manufacture. Unless the O2s are at temp then and that fulfills the protocol... Regardless this caused me to re-examine the electrical connections.
    The harness to the SAS pump is nasty (pulled the cover back and the insulation is cracking and the wires are looking pitted etc) and when it gets warmer I will further examine the lines and probably repair that portion of the harness. I suspect (but need to test and validate) the harness is shorting and while connected to the pump is causing the shake/sound. If it is bad enough to “fool” the DME into thinking the circuit is complete while disconnected that would be the perfect confluence of wrongs making a right; in my mind not likely.
    Going back through my codes, it seems like there is an alignment of the MAF codes to the SAS pump being energized. Shorts in the system can cause other surges and drops so maybe related? Not sure but the MAF codes seem to be done since disconnecting the SAS pump...
    So at this point it looks like I had a myriad of issues that may have fed each other or their sums led me to believe an interdependence that may not be there. I have no SES light (good for inspection as a light = fail) and will need to validate I have readiness on all but 1 monitor so I can pass in April.
    More to follow as this continues...



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    E46M3Cic E39.540iT E84X1
    Interesting. Yes I think you probably should just have the SAI monitor not set. You should have at least a code now for a SAP pump electrical fault.

    I agree with your hypothesis - very well might be a cracked wire short so the SAP electricity is getting where its not supposed to be - the SAP pump comes on and takes out that MAF sensors signal temporarily or something? And by being disconnected the DME sees nothing is connected therefore doesn't try to run the pump?

    Sounds like time for some painstaking electrical troubleshooting checking for shorts and opens and whatnot... not always the most fun but often "what needs to be done"...
    2003 M3CicM6 TiAg
    2002 540iT Sport Vortech S/C 6MT LSD TiAg
    2008 Audi A3 2.0T DSG (the daily beater)
    2014 BMW X1 xDrive28i (wifemobile)

    Former:

    1985 MB Euro graymarket 300SL
    1995.5 Audi S6 Avant (utility/winter billetturbobattlewagen)


  21. #21
    Join Date
    Dec 2016
    Location
    Springfield, VA
    Posts
    1,005
    My Cars
    2001 750iL DD74441
    Yeah, for the inspection leaving well enough alone is the way to go. For the anal “gotta have it right” part of me that doesn’t suffice.
    Maybe I can get the kids into it so we are spending time together (single largest source of not working on the vehicle). Cause having 5 & 6 year olds with electricity and hot soldering irons sounds like a good plan... although it would be some powerful life lessons in doing things correctly.


    2001 750iL DD74441
    Stock

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

  22. #22
    Join Date
    Dec 2016
    Location
    Springfield, VA
    Posts
    1,005
    My Cars
    2001 750iL DD74441
    It has been a while since posting but I believe I have found the culprit... maybe.
    Checked the harnesses and continuity and no visible abrading. Still getting the random spikes etc.
    So I went all the way back to F107 and things looked good there: 12V with .3 Ohms so that was giving about 40A. I figured that it was a rounding error in the DVM...
    Not so much apparently.
    Near as I can tell, the fuse allowed enough current to complete the connection for each individual test (DME 1 command on and DME 2 command on). However, in actual operation both commands are sent whereupon the Amps were exceeded, fuse “went”, and weird electrical stuff happened. In testing that theory I would get continuity at first, pump kicks on and gives OL, pump turns off accordingly, and then a few minutes later back to continuity. And each switch of continuity to OL and back gave a “spike” that would stumble the engine. The idle would actually “stumble” as though everything just stopped for a split second.
    I assume the spike/stumble was “overloading” the MAF giving that error. The error timing followed and seemed to be during the functional test at temp.
    I would like to give a nice clean diagnosis on this: it was X and here is the proof. My situation seems to prevent that though with my partially blown fuse. Not sure how that works as I thought they were like pregnancies: yes or no, rather hard to switch back and forth... one switch to the other is done but going yes/no/yes/no/yes rather difficult...
    Should be an easy swap of new fuse and hopefully it will correct the situation but... all the local stores are carrying the large “box” fuses that don’t fit vice the flat fuse required. Apparently flat fuses are a special order around my area with a wait of a week...
    So at this point I have placed the online order with some other parts for future non-SAS repairs. Should be in this week, so (again) more to follow.
    Who knows, maybe I will get to invoke the lifetime warranty for a fuse...


    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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