A4 B6 AYM: Boost pressure dropping

Hello everyone,

I'm currently having a problem with an A4 that noticeably loses turbo boost at around 2,200 rpm and then slowly builds it back up.

As a potential source of error, I suggest replacing the LMM (likely referring to a linear magnetic valve), as its age is unknown.

What I don't understand right now is: normally, a faulty mass airflow sensor (MAF) manifests as wavy movements because the engine control unit (ECU) is frantically trying to compensate, particularly with the smoke limiter, and this effect amplifies itself.

During my diagnostic test, it's clear that the boost pressure drops and then slowly recovers (although it never reaches the level it should), essentially creating a "wave" pattern.


Leaks are neither visible nor audible; there is some oil visible on the LLK (likely referring to a specific component), but nothing else.
VTG has not been checked by me, but based on my understanding as a computer scientist, it looks quite good: The boost pressure comes when it should and goes away when it should .

The car has an aftermarket DPF (Diesel Particulate Filter).


Am I right in my assessment? What other problems could be the underlying cause?

Regarding the logs: I don't fully trust the LMM, so I've included another, quite extensive, log entry. In case of doubt, do not pay attention.



Sure, here's the translation:

"Thank you in advance."

Marco.

anywhere wrote:

I haven't checked the VTG, but from my understanding as a computer scientist, it looks quite good: The charging pressure comes when it should and goes away when it's supposed to icon_wink.gif

Hi Marco,

I see it completely differently. The boost pressure should be regulated to approximately 2000-2100 mbar. Ideally, the boost pressure should form a nice, slightly increasing straight line from about 8 seconds to 48 seconds. However, your boost pressure shoots up to 2570 mbar at 22.5 seconds before sharply dropping. Your boost pressure regulation attempts to reduce the boost pressure from about 16 seconds to 22.5 seconds (throttle valve position drops from 85% to 23%). Only then does the boost pressure control react, and the boost pressure drops sharply.

This is a perfect example of a jerky LD (linear drive) control system.

Regards,
Guste.

Hello everyone!

It's great to hear from you again!

Thank you for your reply. I was a bit surprised to see that it looks completely different on your end, and I've also figured out the reason why I had so many question marks floating above my head.
VCScope doesn't seem to use the same scales; instead, it adjusts the scaling based on each individual waveform and its maximum value.

That makes your answer very understandable, and I'll see how I can get a handle on the VTG .


Thank you very much and best regards,

Marco.