Diode as a replacement for LMM - Solution and answer needed

Hello.

I've done another experiment and replaced the Lambda sensor (oxygen sensor) in my Passat AJM with a diode because I have the feeling that the Lambda sensor is starting to show the typical signs of failure again.
A standard, off-the-shelf diode was used - the AGR (Abgasrückführung) system is still defective. The following posts related to this topic have already been found and read using the search function.

I already identified a weak mass airflow sensor (MAF) through diagnostics. Currently, I'm experiencing a loss of power that is related to problems with the turbocharger boost pressure regulation. The faulty MAF sensor is also contributing to the issue. Initially, the boost pressure is above the target value, then it is reduced. Afterwards, it is below the target value. Due to the weak MAF sensor, the soot limiter is now active, and it can reduce the fuel injection by 20-30%, notably at full load. I believe that this significantly accentuates the acceleration hole compared to a functioning mass flow meter.
I still haven't quite grasped the issues related to boost pressure regulation, despite having read a technical article. When acceleration begins, the actual pressure increases from 1300 to 2300 mbar within a single data sample! (It doesn't seem to be stuck at a certain level). To reduce the pressure again, it requires 3-4 samples (with 3 logged groups, one sample is approximately 1.2 seconds long).

The following is the impact of this measure on driving: Below 2200 RPM, no change. Between 2200 RPM and approximately 3050 RPM, there was a terrible loss of power, as if someone were holding the car back. Above 3050 RPM, there was a massive surge of power, a willingness to rev up, and finally, like before, a maximum speed exceeding 200 km/h was possible again. Previously, the speed would usually be limited to around 175-180 km/h.
Driving with the power dip between 2200 and 3050 1/min is more than just annoying, and it's also nothing like typical TDI behavior. Therefore, I have now reconnected the MAF sensor connector. The MAF sensor is likely the culprit for the lack of power, as revealed by the performance results with the diode. But how does this power dip explain the diode? Did I do something wrong? Perhaps the connector needs to be reconnected so that +12V and ground are still present at the MAF sensor? Should the diode perhaps be installed in parallel with the MAF sensor? It is supposedly possible to use the diode permanently as a replacement for the LMM...but certainly not with this power hole?

I'm also struggling with this power dip – between 1900 and 3000 RPM, it feels like someone has tied a rubber band to the back of the car. Then comes a sudden surge of power, and the top speed is also well achieved.

However, what is absolutely dangerous is the excessively high boost pressure in this area of reduced power. On my analog LDA, the charging pressure is maintained at 1.5 bar for a duration of 5 seconds. The standard charging pressure is 1.1 bar, with occasional overshoots of up to 1.3 bar.

Can anyone provide more specific information about this, or reproduce the same behavior? I'm starting to get worried about my loader if it's subjected to this kind of stress.

Regards,

hb2000

hb2000 wrote:

I'm also struggling with this performance issue - between 1900 and 3000 RPM, it feels like someone has tied a rubber band to the back of the car. Then comes a sudden surge of power, and the maximum speed is also well reached.

. . . and this lack of power is not noticeable with the mass airflow sensor, even though the engine control unit should be receiving a higher air mass reading from the sensor due to the diode?
Are there any logs available that show air mass data and any limitations related to this peculiar behavior (either from you or other drivers)? I don't have time for a thorough search right now.

Quote:

What is absolutely dangerous, however, is the excessively high boost pressure in this area of reduced performance. On my analog LDA, charging pressures of 1.5 bar are maintained for a duration of 5 seconds

. 1.5 bar is already quite a lot for the AJM engine - but in my opinion, it's not yet dangerously high, because it only occurs in the middle RPM range (of the power dip).
Logs from the MWB 11 would also be interesting here, as they might help pinpoint the cause.

Hello.

I've now installed a DC mass airflow sensor (MAF), and the strange behavior during boost build-up has completely disappeared. The maximum boost pressure is 1.1 bar, and the pressure spikes reach 1.3 bar. Unfortunately, I can't create any logs right now because my laptop can't run on battery power for more than 3 minutes, and I also don't have the time. It's possible that the diode isn't suitable for the test, but that's very unlikely. There's no chip tuning installed, the error memory is clear, the VTG adjustment is smooth, the EGR is permanently defective, the charge air system is leak-free, the intercooler is working perfectly, and everything is fine except for the MAF sensor, which is slowly starting to fail. When performing the diode trick, it's important to note that first, the EGR valve must be closed, and second, the boost pressure should be monitored using an analog LDA (Lambda Digital Analyzer). Of course, use the diode only for testing and checking.

Regards,

hb2000

Hi,


I was also able to observe this behavior with the performance dip in an ALH engine in an Octavia, manufactured in 2000. Different types of diodes did not provide a solution, and even using an LED, which already has a significant voltage drop, did not work. However, a DC-LMM (Direct Current Mass Air Flow sensor) is also not a solution, because it reports too little air mass in the idle to 1500 rpm range, which makes driving in the low RPM range unpleasant. For that reason, the perceived feeling of a powerful boost is all the more pronounced .

However, the DC-LMM has one positive aspect: when the boost pressure is modified, the overshoots are lower compared to a new VAG-LMM.

Best regards, Georg.

Hello Georg,

I have to disagree with you three times right away:

I've had completely different experiences with the DC.



The DC-LMM is also not a solution because it reports too little air mass in the range from idle to 1500 rpm, which makes driving in the low RPM range unpleasant.

Driving with the engine running at very low RPM is no different from normal engine operation.

Therefore, the perceived rush of energy is all the more pronounced.

The turbo boost is not as pronounced as with the original mass airflow sensor, but rather more consistent.

With modified boost pressure, the overshoots are lower compared to a new VAG mass airflow sensor.

The overshoots are almost even more pronounced than with the original mass airflow sensor.



Is it not actually possible to connect the existing LMM in parallel with a diode, in order to obtain only the positive aspects of both methods – just a silly idea...

The original LMM has this peculiar internal wiring connection, which seems to be intended for something – possibly a diode or a resistor for testing purposes.

Regards,

hb2000

Hello hb2000.


Perhaps the different regulatory characteristics of the AJM engine compared to my ASV engine explain this completely different behavior of the DC-LMM in your car and in mine.
"In my case, the DC mass airflow sensor reports a lower air mass than the VAG mass airflow sensor, up to approximately 1600 rpm. I logged this data before; if I dig deep enough into my hard drive, I'm sure I can find it again." Due to the lower reported air mass, the fuel injection amount is likely also reduced.
However, this only applies until the full boost pressure is reached. After that, the torque limiter kicks in, and the mass airflow sensor (MAF) value no longer determines the fuel injection amount.
When the topic of the DC-LMM came up, there was a link somewhere around here to a diagram that compared the characteristics of several LMMs. It clearly showed that the DC-LMM reported significantly lower values than the VAG-LMMs, especially at low engine speeds.

Okay, here it is, I just pulled it out.

/viewtopic.php?t=3297&highlight=dclmm

Best regards, Georg.

Hello Georg_G.

I suppose you're right that there are certain differences between the AJM and the ASV models when it comes to using the DC LMM. It's guaranteed to work perfectly in the AJM and ATJ models without any compromises or drawbacks. The main reason for using the DC LMM is its durability, not necessarily its superior performance. If Volkswagen offered reasonable prices for their spare parts, I would consider using them again.

Best regards, hb2000.

There is a dedicated section for VAG group vehicles in the automotive parts store. He should accordingly also be adjusted.

For AJM:

The Bosch/Pierburg part number should be either 7.22684.07.1 or 7.22684.08.0.

Comparison numbers that:

Ford 95 VW12B529 BB
Skoda 028 906461 and 06A 906461.
VAG 028 906 461 and 06A 906 461.

There is a DC (direct current charger) available for the VAG group in the accessory trade. He should accordingly also be adjusted.

For AJM:

The Bosch/Pierburg part number should be either 7.22684.07.1 or 7.22684.08.0.

Comparison numbers that:

Ford 95 VW12B529 BB
Skoda 028 906461 and 06A 906461.
VAG 028 906 461 and 06A 906 461.



Everything I posted was actually done about a year ago.

At that time, I had already pointed out the difference between DC LMM sensors from Pierburg / Pierburg LMM sensors for VAG as a replacement.

At that time, there was also a lot of confusion/mix-up between the two Pierburg mass airflow sensors.

Since I also drive an AJM, I can only say that the Pierburg MAF sensor for DC has a different characteristic curve, and I experienced very large differences.

@hb2000, just try it out with the correct LMM from Pieburg. I included the test report back then.

I won't buy another Bosch product, but that's a personal choice.

Okay, I'm signing out now.

Regards,
Thorsten

Hello,

Why do different Large Language Models (LLMs) exhibit different levels of 'overfitting'? Actually, the LD has nothing to do with the LMM value.
I can only imagine that certain LMM (likely referring to Lambda Measuring Module) behavior during a sudden gas surge could cause the injection quantity to increase rapidly, resulting in a large exhaust pressure gradient. The VTG regulations don't handle this situation very well.

Best regards, Matthias.