Misdiagnosis

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Steve Smith
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Misdiagnosis

Post by Steve Smith »

When it comes to diagnosis possibly the bitterest pill to swallow is "misdiagnosis" and below is an outline of how I fell foul after testing and testing again before making the wrong decision. Even after rectification I am still bemused how this vehicle is now fixed and why it displayed a particular fault code. Even more embarrassing, this was my wife’s car!

Manufacturer: BMW
Model:X4
Year:2015
Model code:F26
Engine code:B47 D20A 2.0 Litre Diesel
Transmission: ZF 8 Speed Automatic
Mileage: 80754 miles

Customer complaint: Engine warning light illuminated; vehicle drives fine with no performance concerns
An initial scan revealed the following fault codes:
• 290900 Exhaust-gas recirculation: Duty cycle Air mass too low
• 28DE00 Exhaust gas pressure sensor upstream of turbocharger Signal implausible (dynamic)

The codes were erased and the vehicle road tested where the following codes appeared (without illuminating the engine warning light)

• 290900 Exhaust-gas recirculation: Duty cycle Air mass too low
• 28DE00 Exhaust gas pressure sensor upstream of turbocharger Signal implausible (dynamic)
• 2AB600 Exhaust gas pressure sensor upstream of turbocharger Pressure too high
• 2AB400 Boost pressure sensor Charge air pressure too high
• 290B00 Exhaust-gas recirculation: Duty cycle Air mass too high

EOBD returns
• P00BD ECM1 Fault pending Air-mass meter or airflow sensor A High flow rate
• P0471 ECM1 Fault pending Exhaust gas pressure sensor Outside specified range

I hope we can all agree the PCM is concerned about airflow (either too high or too low) and the exhaust gas pressure. With that said, a WOT road test capturing airflow figures at peak torque revealed a MAF of:

655 kg/h @ 3513 rpm with 2790 mbar of boost
(655 x 1000) / 60 = 10,916.666 gm/min
10,916.666 / 60 = 181.944 gm/sec

To put these airflow figures into perspective, the power output of the above vehicle is 190 hp
As a rule of thumb, if our engine was 100 % volumetrically efficient (100% VE) the peak MAF would be 190 g/sec. Typical VE figures for diesel engines are around 85 to 90% (90% is “ambitious”)
To calculate our VE
Our measured airflow = (181.944 gm/sec) / (190 gm/sec) x 100 / = 95 %
This value is most certainly ambitious!

At this point, I want to add how a brief check of “known faults” for the above airflow codes revealed 2 possible causes:

1. Air filter, air filter casing and modified PCM software
2. Air flow meter faulty

Oh, how I wish I had paid more attention to item 1 above!

An initial inspection revealed a number of faults which were rectified as follows:

Clean and seal DPF differential pressure sensor connector (No corrosion to sensor pins or connector)
Contaminated with screen washer fluid directed from the scuttle panel
Image 1
Image 1
Remove restriction from exhaust gas pressure sensor pipework (Scope confirmed low output at high rpm)
Blocked with carbon and soot deposits
Image 2
Image 2
Air filter soiled but not blocked or restricting airflow. (Vehicle performance is normal)
Note the fluid staining in the left-hand image below
Image 3
Image 3
Rather than replace the air filter above, I chose to refit the filter in an attempt to discover how and more importantly why the PCM detects “air mass to high” with an alleged contaminated air filter

The fundamental components responsible for the detection of air flow would have to be the MAF sensor in combination with the EGR valve.

A brief inspection of the MAF sensor revealed similar fluid staining to the air filter above (See below) Once again, the old MAF sensor was refitted in order to discover more about how our fault codes are generated
Image 4
Image 4
In addition to the above inspections, I can confirm no external leaks were present throughout the air intake system

To recap at this stage:
• Exhaust pressure sensor pipe unblocked
• Clean and seal DPF differential pressure sensor connector
• MAF fault codes indicating MAF too high and too low are stored (MIL not illuminated)
• Original air filter and fluid stained (Refitted)
• MAF sensor fluid stained (Refitted)
• Vehicle performs fine with great torque and power

Prior to further evaluation, an active test of the EGR valve confirmed the valve to be functioning as normal
Image 5
Image 5
Next, the vehicle was driven until hot, all fault codes erased and the engine allowed to idle.
Below we have captured the precise point where the EGR valve is switched off and fault code “290B00 Exhaust-gas recirculation: Duty cycle Air mass too high” is stored within the PCM
Image 6
Image 6
Please see the video below:
AIR MASS TO HIGH.mp4
Air mass too high
(16.53 MiB) Downloaded 88 times
As can be seen in the video and “TEST 15” capture above, fault code 290B00 is erased and the engine allowed to idle with EGR active, DTC 290B00 then returns indicating MAF is too high after a period of 30 seconds at idle speed!

Zooming in, we can see the effects on MAF looking at the “freq(F)” math channel. Note the pulsations associated with air flow when the EGR is active (i.e., exhaust gas pulsations “felt” inside the intake assembly)
Image 8
Image 8
The keen eyed would have noticed how the frequency of the MAF sensor increases with the EGR active! This is counter intuitive as we know air flow decreases with EGR active!

A quick study of the operation of this MAF sensor confirmed its frequency should increase with an increase in airflow! (The reverse of what we have above!)

Below, a WOT road-test was carried out to confirm the operational characteristics of the MAF sensor which proved to be the complete opposite of the research and literature previously studied (i.e., frequency of MAF sensor decreases with an increase in air flow)
Image 9
Image 9
A back-to-back test with a BMW 3 Series 320 d (2013 F31) confirmed the MAF sensor frequency to increase with engine speed and not decrease as above! With hindsight this was a mistake to use a “similar” but not identical donor vehicle!!

Given the fault code suggested MAF too high, I then chose to capture live parameter data between the BMW X4 and BMW 3 Series
Table
Table
Looking at the data above, it would appear the MAF for the BMW X4 is far higher than the BMW 3 series, which supports the fault code “290B00 Exhaust-gas recirculation: Duty cycle Air mass too high” Whilst these vehicles are not identical, they are both 2.0 litre diesel engines at idle speed with near identical EGR activation and therefore I would expect similar air flow values.

Summary:
• PCM returns fault code MAF too high when idling
• Serial data indicates MAF higher than a donor vehicle (28.30 kg/h)
• MAF sensor exhibits fluid staining
• MAF sensor operation is the reverse of the description provided in the technical literature (Frequency decreases with an increase in airflow rather than the described increase)

Based on all the above I felt there was enough evidence to replace the MAF sensor along with a new air filter. Of course, wanting to know if the MAF sensor was the sole offender I installed a new original MAF sensor (not a new air filter at this stage) only to find the fault was still present (Ouch!)

To clarify, DTC 290B00 “Air mass to high” remained after a period of idling, the MAF sensor frequency decreased with an increase in air flow and serial data confirmed near identical MAF values at idle. Embarrassing all round and where to now?

An extended road-test seemed like the next option in a bid to “settle” the engine management system (after periods of long idling) along with further data capture. Prior to the road test I felt it was “best practice” to install the new air filter.

Sure enough, erase fault codes prior to the road-test and drive for a number of miles where to my astonishment everything was fine! No fault codes appeared.

The vehicle was left to idle for long periods where DTC 290B00 did not return!

A word to the wise here, EGR will be switched off after prolonged periods at idle (e.g., 20 minutes) which is a characteristic operation of this engine management system. Cycling the ignition will then restore EGR operation and no DTCs are stored as a result of this normal EGR deactivation.

How on earth can replacing just the air filter remove the repeat detection of DTC 290B00 “Air Mass too high” at idle speed?

Further research into the detection condition of DTC 290B00 revealed the PCM is not only monitoring air flow via the MAF sensor but also pulsations attributed to the “push pull” effect of the 4-stroke cycle and EGR operation. Should these pulsations exceed a specified limit or demonstrate irregularities the PCM will set the relevant fault code (Either DTC 290B00 “air mass too high” or 290900 “air mass too low”)

Taking a step back, not only are intake pulsations attributed to the 4-stroke cycle and EGR but also the air filter itself! A combination of irregularities in the manufacturing of air filters and the varying levels of contamination all contribute to fluctuations in the air intake.

Armed with this information, the temptation to measure and compare intake pulsations via the MAF sensor between the new and old air filter (Image 8 above) was just too much to resist.
Below we have the EGR stable and active around 2.2 V (Image 8 above is 2.3) and our airflow pulsations at near identical min and max values.
Image 10
Image 10
Based on the above capture, I cannot see a difference other than a slightly different EGR position for the identical pulsations at the same frequency!

The new airflow meter was removed and the original unit refitted. The vehicle has since covered over 150 miles with no repeat of the above fault codes. I can therefore conclude that the air filter was solely responsible for DTC 290B00 “air mass too high”.

If I had followed “known faults” (Item 1) at the start of this post I would have been home and dry, but my curiosity got he better of me regarding “air mass too high” with a soiled air filter.

I am convinced there is more to this fault code than meets the eye and I would love any feedback or comments on the above as I am sure I have missed something or perhaps could have captured why this DTC 290B00 is flagged by the PCM

Looking back over the serial data and airflow in particular, with the new MAF and new air filter sensor installed, with EGR status at 39% the MAF was 25.20 kg/h, which is consistent with the table above given we have an increased EGR status (higher EGR status = lower airflow)

What has bugged me is the MAF readings/values from the BMW 3 series at 7 kg/h with EGR @ 33%, how is this possible?

My gut feeling is the units are incorrect for the displayed serial data with the BMW 3 series, I believe that the displayed 7 kg/h should read 7 gm/sec. (Yet another variable/curve ball to deal with)

I hope this post is somehow helpful and prevents others making a similar incorrect judgment on a MAF sensor

Once again, any feedback or theories would be most welcome.

I am now the embarrassed owner of a brand new original MAF sensor

Take care…..Steve

tintoverano
OneWave
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Posts: 20
Joined: Mon Dec 14, 2020 3:38 pm

Re: Misdiagnosis

Post by tintoverano »

hi Steve,

thanks for the post!

may I ask where did you learn about the pulsations for triggering the fault code?

just quickly checked here: https://bmwfault.codes/XMLDiagView?doc= ... AzADAANAA=, but no luck

cheers

KimAndersen
TwoWaves
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Posts: 235
Joined: Fri Nov 30, 2012 2:53 pm
Location: Denmark

Re: Misdiagnosis

Post by KimAndersen »

Hi Steve

Have you seen this TSB (SI B01 14 18) from BMW where the same DTC appears in the Ecu - as you experienced in your BMW.

Here they are talking about replacing the EGR valve with the following spare part number (PN 11 71 7 810 871) on the warranty.

I know your BMW Model X4 is not covered in this TSB, but I think it's very close.

This is despite a BMW with the same DTC as your model. :wink:

BMW_TSB_SIB011418.pdf
BMW TSB SI B0 11 418
(1.66 MiB) Downloaded 99 times

Regards
Kim

Steve Smith
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Joined: Sun Aug 25, 2013 7:22 am

Re: Misdiagnosis

Post by Steve Smith »

Hello and thank you for the feedback Kim & Tintoverano

I had seen this bulletin but tried very hard not to be influenced by the content as it applies to F10 and F15.

With that said, look at the description of the fault code:

290B00 - Air system, air to EGR air mass flow, plausibility: measured air mass compared to calculated air
mass too low. This is the complete opposite to the description of the code from the scan tool I used in the video!

I may have feel over here because the fault code description is incorrect (I would certainly lean towards a BMW publication being accurate)

If we follow this publication further it lists fault code......

290900 - Air system, air to EGR exhaust mass flow, plausibility measured air mass compared to
calculated air mass too high (Again the opposite of the scan tool used)

The description of fault code 290B00 (air mass too low) from the BMW publication certainly fits the underlying fault with this car which proved to be the air filter. I.e. The PCM will calculate the theoretical air flow based on engine speed, engine capacity, cylinder count, MAP, EGR position/PWM & VE and compare it with the value obtained from the MAF sensor.

If these values do not tally , then the appropriate fault code will be stored (The appropriate being 290B00)

Over and above theoretical and actual MAF values, the PCM conducts some form of intake "integrity check" based on the air flow pulsations detected at idle speed thanks to a combination of the 4-stroke cycle and EGR opertaion.

This information came to light thanks to our VM links across the world, however whilst the pulsations monitoring information was official, it was not confirmed if a modified air filter housing and PCM software update was applicable as highlighted by the scan tool "known faults" reporting system.

This places the diagnosis somewhere between "the devil and the deep blue sea" and reinforces the need for product knowledge and perhaps "brand specialization"

Given a new air filter has resolved the concern I have no doubt the vehicle will be fine for may miles to come, however, fault code 290B00 may return sooner than a vehicle with a modified filter housing and re-flashed software.

It's all food for thought as ever

Take care.....Steve

victor2k
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Re: Misdiagnosis

Post by victor2k »

Hello,
I think you must report this wrong description of fault codes,in ESI Ticket/Diagnosis,maybe you can add TSB with fault description as an attachment.
Best regadrs

Steve Smith
Pico Staff Member
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Joined: Sun Aug 25, 2013 7:22 am

Re: Misdiagnosis

Post by Steve Smith »

Hello Victor, thank you for the feedback and prompt.

I have sent an ESI ticket with 2 concerns, one surrounding the fault code description and one surrounding the Actual values of the MAF sensor. (MAF frequency decreases with airflow and not increases as suggested)

Thank you again, take care.....Steve

Steve Smith
Pico Staff Member
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Posts: 1295
Joined: Sun Aug 25, 2013 7:22 am

Re: Misdiagnosis

Post by Steve Smith »

Thank you again Victor, please see the response below to my ESI Ticket

"Dear Hotline,
Thank you for your important information.

We have checked your indication and implemented necessary software changes.
The revised software will be available in one of the next ESI[tronic] versions.

We are pleased with your cooperation and thank you for that once again
"

I have to say I am very impressed with their response time

Take care......Steve

Dyls
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Re: Misdiagnosis

Post by Dyls »

Hi all
I have a BMW 420d B47 2015. I was driving from work early in the morning and all of a sudden the CEL came on. I chacked the code and it was P00BC and P00BD which relates to a MAF. Took it to the garage and showed 2 different codes. 290900 and 290B00. So face i have changed the o2 sensor after cat and i have installed a new maf but the cel keeps coming back. Any idea guys

wireboy67
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Re: Misdiagnosis

Post by wireboy67 »

Hi Steve,

Just had a similar concern on a 2018 F15 with the N57 engine yesterday. Multiple faults stored. 28DE00 exhaust backpressure dynamics, 290900 air mass too high, 290B00 air mass too low and 2aB600 exhaust backpressure sensor before turbine signal too low. The test plan did ask whether the vehicle had the new style air filter installed. The air filter was replaced within the last year and was fine. Inspection revealed that the exhaust gas pressure pipe was plugged. The pipe was cleaned out and the adaptations were cleared. Vehicle was road tested and the faults never returned.

Bill

Steve Smith
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Re: Misdiagnosis

Post by Steve Smith »

Thank you for the feedback, it is much appreciated and once again, interesting how these fault codes (290900 & 290B00) appear to be spilled out regardless of the actual fault condition. (The airflow/pulsations throughout this engine would appear to be closely monitored and correlated)

Also interesting how the Test Plan made the suggestion about the air filter and with hindsight, it will be first port of call in the future (OE Spec too)

The pipe you describe Bill would appear to be something of a "Go to" check for these engines, especially if running short journeys.

I have a similar vehicle (F31 N47 @ 145,000 miles) that uses this sensing pipe arrangement yet never had a concern in the 100,000 miles I have covered (unlike my wife's car which rarely sees a Motorway journey)

Certainly one for the customer questionnaire prior to diagnosis

Thank you again, take care......Steve

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