The relationship between LMM values and boost pressure is unclear

Hi.
Just to preface this: I tried using the search function, but it didn't really help me.

Unfortunately, I don't understand the connection between a faulty mass airflow sensor (MAF) and the resulting lack of boost pressure .

The reason for my search is the intermittent lack of or delayed build-up of boost pressure in my AXR engine. I had this problem before /viewtopic.php?t=7479. Then, after a 650 km drive at full load, it was quiet for a few months. Now, it occasionally runs like an SDI up to 1800 rpm. After that, everything is fine; the boost pressure settles properly at the target value and remains there.
The values logged by VAG-COM for the mass airflow sensor were significantly below the expected value during the period when there was no boost pressure.
My friendly mechanic checked it yesterday and immediately identified a faulty mass airflow sensor (MAF) – the car is still under warranty, but the part can only be replaced on Monday.

Perhaps someone can explain this connection to me sometime.

Hello Hati!

The situation is similar to the chicken-and-egg problem .

If the mass airflow sensor (MAF) provides too low values to the engine control unit (ECU), the soot management system activates, and the fuel injection quantity is reduced. With a reduced fuel injection quantity, the power output of the turbocharger is insufficient to build up full boost pressure. Consequently, a significantly weak MAF sensor almost always results in reduced boost pressure. It can even happen that the soot management system shifts the engine operating point so much that the ECU attempts to regulate a lower boost pressure (smaller target value), meaning that the maximum boost pressure is never reached, even under full throttle.

Regards,
Alex.

Thank you, Alex.

On the other hand, I had understood it correctly (in my case, the egg came before the chicken ).
Low boost pressure caused by a faulty VTG (or pressure control line, switching valve, etc.) results in a reduced injection quantity due to the soot reduction system.

Okay, then I'll see how it looks with a new mass airflow sensor. It's just strange that sometimes it works perfectly on one trip, and half an hour later it's completely messed up again. Basically, the main issue seems to be a lack of power in the first 10 kilometers (sometimes quite significant!), but then suddenly everything works like it did on the first day. Try to understand mass airflow sensors – they're complicated enough as it is!

Hati wrote:

Basically, I can say that there is a lack of performance primarily in the first 10 km (sometimes quite significant!), and then suddenly everything works perfectly, as if it were brand new. Try to understand the LMMs (it's already difficult enough for women)

.

Hmm, when I see your signature...

Volkswagen Golf IV TDI Special, 03/2002, 74kW, AXR, AGR valve defective.

I wonder if you're actually driving in a kind of "limp mode" for half the time because the EGR function is very strictly monitored in EU4 vehicles... maybe your problem lies there.

Hello Hati!

Perhaps the VTG mechanism on yours is a bit stiff.
These low temperatures, as we are currently experiencing, are...
Certainly, there's likely another factor that could cause the VTG to get stuck.
After driving for 10 kilometers, the TDI engine typically reaches a temperature of approximately 60-80 degrees Celsius under normal driving conditions.
Perhaps the material will then expand in such a way that the VTG becomes movable again.
It's just an assumption.

Regarding the relationship between lambda (air-fuel ratio) and boost pressure, I would appreciate professional answers.
would be very happy to.
As far as I understand from reading the forum, the mass airflow sensor (MAF) usually doesn't affect the boost pressure at all. The boost pressure is determined solely by...
Gas pedal request controlled (including intake manifold temperature and altitude sensor). I believe this even...
(Written by Ulf).

Only in extreme cases of 'LMM failure' (without a fallback mode) is the resulting...
The exhaust pressure is then no longer sufficient to operate the turbocharger itself.

Best regards, Peter.

neubaupe wrote:

As far as I understand from reading the forum, normally , the mass airflow sensor (LMM) does not affect the boost pressure at all. The boost pressure is solely controlled by the engine control unit's request for gas (+ intake manifold temperature, altitude sensor). (I believe that's even...) (Written by Ulf).

Ulf also wrote the following:

Quote:

Insufficient boost pressure, especially in the lower RPM range, can also be caused by problems with the mass airflow sensor (MAF).

Okay, I'm ready. Please provide the German text you want me to translate into English.

@Ulf

I'm ruling out a faulty EGR valve because everything worked perfectly for over two months with the EGR deactivated. If the EGR is being monitored, shouldn't there be an error code stored in the engine control unit (ECU)? Since the fault memory is empty, I'm also inclined to believe that it's not running in a limp-home mode.

@neubaupe
The valve timing gear (VTG) is operating smoothly. I tested it with a syringe. Also, the turbocharger already reaches a significantly higher temperature than 60° after just a few hundred meters. You probably meant the coolant, but that's likely not the relevant factor here.