Boost Pressure Control: Power When Boost Drops

Hello fellow DIY enthusiasts,
I'm constantly looking for ways to improve my Audi A4 (B9). Something I've been struggling with for a while is that the engine feels sluggish at low RPMs. The problem manifests as the engine straining when starting from a standstill, but then suddenly pulling strongly once it reaches around 1800 RPM. I haven't found any specific error codes or logged any unusual data. Initially, I thought it might be due to insufficient boost pressure, so I finally installed a boost pressure gauge to monitor it while driving.
Okay, I'm not experiencing insufficient boost pressure. What I've noticed is that once the engine starts producing power from around 1800 RPM, the boost pressure actually starts to decrease again.
I strongly suspect that my VTG (Variable Turbine Geometry) is too tightly closed, and the engine is struggling due to excessive exhaust back pressure. Alternatively, it could be a normal occurrence caused by an overboost event.
What do you think?

Greetings.
Reuselkopp.

You don't have any fireworks .

But if you already have logs, please show them to us.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

@dieselmartin,

I can only upload the logs tonight. But you don't see anything in the logs .
The logs are being generated at full throttle right now. But you're giving me an idea – I could create a log under normal circumstances. That is, start driving normally and record the data from that scenario.

Hello,
Here's the log I promised. You can clearly see that when starting up, it initially produces an overshoot of 0.5 bar and then regulates down to 0.3 bar.

Regards,
Reuselkopp.

I think it's quite clear that when starting up, it initially produces an overpressure of 0.5 bar and then regulates it down to 0.3.

Hello, I'm sorry, but where do you see this behavior again?
Or am I looking at a different log file?

Okay, if you ask me about the engine's behavior, I would say that the turbo is working very hard. You don't experience any overboost, so you remain completely within the smoke limit until 1932 rpm.
The amount of air being drawn in matches the expected value (LD), so it's probably not a leak.
But the turbo's boost pressure curve is a bit unusual:
Starting at 3200 rpm, the variable geometry turbine (VTG) must be closed again to maintain a constant load demand (LD).

Therefore, I would like to take a closer look at the surroundings and the turbocharger (including the catalytic converter).

Best regards, Jochen.

Hello,
No, no, that's the correct log file. If you look at lines 18-22, columns M-P, you'll find what I mean.

Everything around the loader is normal and in good condition; there are no issues or leaks. The Oxicat container is empty; there's nothing left inside.

Greetings.
Reuselkopp.

'You really can't use this section for a proper diagnosis because the acceleration phase is far too short. Instead, focus on the area after 20 seconds, where you can really push it.'
However, this area is good for one thing:
You can see that the charger starts charging too late and then overshoots the target voltage when only a small amount of charge is requested.

The turbo's wastegate actuator or vacuum canister seems stiff, which means a significant change in vacuum pressure is required before your modification to the LD (likely referring to a load sensor or similar) takes effect.
Furthermore, the target value for the LD (likely referring to a specific parameter or setting) is not reached at high speeds.
Has the VTG been rotated around its axis?

Best regards, Jochen.

Hello,
Perhaps the delayed response is due to the fact that I installed a VNT15 turbocharger with the larger housing, rather than the one with the smaller housing that's actually supposed to be used in this car.
The VTG mechanism is very smooth and easy to operate; there's no resistance when you test it with a syringe. I haven't adjusted anything on the VTG either.
I also noticed that the boost pressure isn't reached at high RPMs, but I thought it was still within the acceptable range.

Would shortening the VTG rod help with my problem?

Greetings.
Reuselkopp.

Perhaps the late response is due to the fact that I installed a VNT15 turbocharger with the larger housing, and not the one with the smaller housing that should actually be in the car.

Basically, larger injectors result in a higher fuel injection volume, which simultaneously produces more exhaust gas. Therefore, with larger injectors and unchanged software, the overshoot behavior becomes more pronounced.
You have absolutely no enthusiasm .
I don't think the large pressure vessel changes the steering behavior of the turbo. However, Ulf should chime in here, as he apparently also carried out this modification on his AHF Golf.
What does your vacuum pressure reading say? If there isn't sufficient vacuum pressure, the control of the VTG (Valve Train Gear) won't work correctly either.

What does your AGR (Abgasrückführung) system look like? Could there have been a problem with it here?
Perhaps it's not closing properly anymore?

Best regards, Jochen.

Hello Jochen,
I don't understand anything anymore . How do you come to the conclusion that I have larger injectors? I have the standard injectors for the AFN (0.205 µm).

My vacuum system is working perfectly fine (the pump was replaced a year ago). I don't think the problem lies with the vacuum supply, as it has been checked and replaced multiple times. The N75 valve has also been replaced several times.

My EGR system probably hasn't been active for over 100,000 kilometers because a piece of metal has gotten stuck in it.

Regards,
Reuselkopp.

Oh, sorry.

I probably mixed up 'Düse' and 'Dose' in my signature...

My EGR valve has probably been inactive for over 100,000 kilometers because a piece of metal slipped in between.

Okay, then please check the metal sheet again.
We recently had a case where the metal was completely rusted through.

If that's alright, then we really need to take a closer look at the charger.
Perhaps the wave is carrying more information than previously thought.

Best regards,

Okay, I'll take a look at the sheet metal, although I can't imagine that a copper sheet could rust.

Quote:

Perhaps the wave is more complex than we thought

.

What do you mean by that? Are you saying that the charger was incorrectly calibrated from the factory?

Greetings.
Reuselkopp.