LMM Values: Meaning and Impact

Good day.

Regarding LMM values and their impact on various engines:

For some time now, I've been trying to find a fault with my 1.9 TDI, manufactured in 1998, with the AFN engine.
Since 2000, a tuning box has been installed and configured by a trusted workshop. For about a year, the exhaust gas recirculation valve has been malfunctioning. Approximately 6 months ago, a VNT17 turbocharger was installed, and the boost pressure was increased by 0.3 bar using two 0.1-volt diodes connected behind the pressure sensor in the engine control unit.
- lowered, not a big deal, but stiff (important in the next paragraph)!

The actual incident:
On my way from the airport towards south of Vienna, I wanted to avoid a traffic jam and ended up driving through rough terrain (I would describe it as an asphalt field). Due to my time constraints, I didn't take any precautions this time and sped over bumps and potholes as if the devil were chasing me. At the end of this 'road,' there was this, well, unfortunate pothole, after which I thought it had blown a fuse in my car...
Suddenly, the car started running like it was in limp mode, which immediately caught my attention because of the lack of power. A truck that was turning nearby almost rear-ended me. A quick glance at the boost pressure gauge showed a maximum of 0.1 bar at full throttle, around 2500 rpm.
First, I opened the hood at home... Over the following days, I checked the VTG (Variable Turbine Geometry) - it was moving freely (using the spray method). I only checked the vacuum hoses between the solenoid valve and the turbocharger, because when I disconnected the vacuum hose coming from the vacuum system, I could hear a hissing sound, which indicated the presence of vacuum.
Furthermore, the box was removed independently, and the diodes were desoldered.
Unfortunately, these attempts yielded no reaction.

At the workshop (during the vehicle inspection), I explained the situation.
When I picked up the car, you explained that no error codes were found, but you were quite certain that the solenoid valve was defective because it couldn't be activated manually. - I thought that was a clear diagnosis.
I bought this part from a VW dealer, installed it right away, and now I realize I just flushed 90 euros down the toilet.

The last action I took was to measure the mass airflow rate while driving.
According to research, a Pierburg mass airflow sensor should read 1.5 volts at 900 rpm - which is okay in my car. At 3800 rpm, it should read approximately 4.8 volts, but in my car, it reads 3.3 volts.

I read several articles about Mass Air Flow (MAF) sensors on dieselschrauber.de. It seems there are many ways to disable a MAF sensor (partially), but the method involving strong vibrations, which can break the ceramic plates, seemed very suitable for my situation.
Since it doesn't really matter why the LMM is providing the low values, I'll get straight to my question:

How do LMM values affect different engines?
From what I've read, the performance can easily decrease, even to the point of entering a failsafe mode.
Could it be the case, in my situation, that the emergency procedure is activated even when the engine is first started?

I strongly anticipate that many people will point me to the search function, as this topic has been discussed numerous times before. However, I can't find any information regarding whether an AFN engine is capable of triggering a limp-home mode, as it compares engine speed with the mass airflow sensor signal and concludes that there is an absolute error.

Thank you very much to everyone who is taking the time and effort...!!

Best regards, Peter.

Hello Peter,

'Why shouldn't the engine control unit (ECU) have a limp-home mode? I'm not aware that a faulty mass airflow sensor (MAF) triggers the classic limp-home mode. With a completely dead MAF sensor, the ECU simply provides substitute values. If you don't have VAG-COM or a replacement MAF sensor for testing, I would suggest trying a diode as a temporary measure.'

@neubaupe:

Have you ever taken the time to carefully inspect the entire charge air cooler system, especially the area near the bottom (LLK), for leaks or cracks?

Perhaps one of the components came into contact with the ground and/or cracked due to the strong impact.

Always search diligently .

Regards,
Alex.

@Marco
Hi, thanks for the reply!

Actually, I'm trying to figure out when the engine control unit starts replacing the mass airflow sensor signals with its own, and whether the car is already running in a limp mode or not.
Since the boost pressure never exceeds 0.1 bar, and remains very consistently within the range of 0.0 to 0.1 bar, I thought it might be in a limp mode.

Another question would be how the mass airflow sensor (MAF) signal affects the turbocharger's performance. When I bought a new solenoid valve, the mechanic I spoke with mentioned something like this: 'The signals from the MAF sensor, the intake manifold pressure sensor, and the boost pressure sensor are added together and divided by 3... and then used, among other things, to calculate the fuel injection amount?' Of course, he didn't have time for a more in-depth discussion.

'What I want to avoid is another investment of almost 100 euros, and then having the whole thing not work again. If I take it to VW, it will definitely cost the full price, and unfortunately, the workshop where I usually go, which is usually reasonable with prices, doesn't have a diagnostic tool (I don't know what that device is called for this van).'

Sure, here's the translation:

'cu!'

This is a common greeting in Romanian, similar to 'bye' or 'see you'.

@donalexo
Yes, I checked the charge air duct from the turbocharger.
If there is a leak, it must be minimal, e.g., a hairline crack.

'Have you tried the diode trick? I was getting 4.5 volts on the MAF sensor signal going to the control unit. However, the control unit should compare the engine speed and MAF sensor signals, and if it detects an illogical difference, it should immediately switch to limp mode after starting.'
1.) Unfortunately, I lack the knowledge required for a more complex circuit.
2.) How can I actually tell if the car is in limp mode or not?

Sure, no problem!

msfg. peter

Hello again,

I finally found the error after searching for countless hours.
Human error!

The solder joint between the diodes and the signal output of the charge pressure sensor.
It has been resolved with this pothole (see the first post).
This, of course, wouldn't have happened if the solder joints had been made precisely.
It appears that the boost pressure sensor (in the AFN engine, located in the control unit).
a little bit, which was solved through crafting, and in corresponding cases.
Worked on the solder joints to correct any issues.

Conclusion:
Hasty action, pressure transducer, and mass flow meter replaced - cost: 150 euros.

Well, I'm glad it's being reversed, and I'm sorry that so few people have responded.
have, even though this error...

Good evening, Peter.

PS:
My actual question in this thread was whether zero volts appear at the signal output of the...
Drucksensors einen Notlauf verursachen kann. That is correct.

Best regards,
Peter