Boost pressure builds up too slowly (Cupra TDI), VAG Com log

My Cupra TDI (engine code BUK) feels sluggish when driving uphill. I've logged the lambda (Ld) and mass airflow sensor (Lmm) values (in 4th gear, going uphill). The boost pressure doesn't reach the target value until 1800 rpm, and it's about 300 mbar lower than the target at lower speeds.

rpm, load should be, load is, difference.
1344 1692 1344 348
1407 1764 1440 324
1491 1884 1572 312
1554 1992 1680 312
1638 2112 1836 276
1743 2268 2148 120
1869 2376 2484 -108
2016 2448 2736 -288

The LMM signal behaves similarly, reaching the usual 850 mg/H only at 1600 rpm, and then increasing to 1250 mg/H at 2000 rpm.

What do you think about this? Is this normal for a BPX, BUK, or ARL device, or does it indicate a problem/defect?

I can't say for sure whether this is normal, but my car's turbocharger behaved the same way! As far as I know, both engines have the same 1749VB turbocharger. While the lag isn't ideal, it's still within an acceptable range at 350 mbar, and the subsequent overboost is almost in the same range. Do you use off-the-shelf software?

It would be interesting to see how things develop further. Why don't you create a log with a level of 4? When driving with the accelerator fully pressed, between 1,400 and 4,000 RPM, you can better assess the overshoot.

Hello,

The usual suspects in this case would be the mass airflow sensor and a lack of vacuum.

Best regards, Rainer.

@ Rainer: Can a limiter also be triggered from below, or do I only know it from being triggered from above, so that, for example, it prevents reaching the maximum speed, etc.? So, the power output according to the manufacturer's specifications is approximately 175 hp / 360 Nm (original software).

@ RTDI-Tom: So, the software is original. The boost pressure goes up to a maximum of 1.8 bar, and then it regulates itself back to the target value starting around 2500 rpm.

SeatLeon wrote:

My Cupra TDI (engine code BUK) feels sluggish when driving uphill. I have logged the Ld and Lmm (in 4th gear, going uphill). The boost pressure doesn't reach the target value until 1800 rpm, and it's about 300 mbar lower than the target at lower speeds. What do you think about this? Is this normal for a BPX, BUK, or ARL, or does this indicate a problem/defect?

I don't have any comparisons to other BUKs, but don't forget:
You're driving a 1.9-liter engine with 160 horsepower!
In order to create the EU4, certain compromises in performance are unavoidable, which may be less disruptive in the BPX / ARL (EU3).

@ulf: That sounds plausible, but I wonder why they set the target curve so high, knowing that it won't be achievable with EU4. Or do you think these minor cosmetic details were overlooked? I also get the impression that it revs up or reacts more spontaneously when it's cold or lukewarm. Can you come up with a rhyme for that in this context, or is it just nonsense?

Hello,

MKB: ARL, OBD, AGR malfunction.

I'm also currently working with setpoint/actual pressure profiles and can share similar experiences.

Measured in the 3rd. However, I still have significantly larger calibration errors (> 1000 mbar), even though I have no complaints about the driving behavior.

Perhaps someone could comment on my logs.

Sheet 1, MWB 11 and subsequent. Graphic.
Sheet 2, MWB 3-8-11.

BTW: Is the duty cycle an absolute value with respect to the VTG control, or does it depend on other parameters (speed, boost pressure, etc.)?

Can we, in principle, determine the exact degree to which the VTG (Variable Turbine Geometry) is closed based on the keying ratio, and thus how strongly the turbocharger is being controlled (assuming the hardware is functioning correctly)?

Best regards,

Bo.

SeatLeon wrote:

However, I wonder why the target curve is set so high, knowing that it cannot be reached with EU4. Or do you think these cosmetic details were overlooked?

Roughly speaking. Additionally, the LDR remains at a maximum even with an "unattainable" target value. The pressure builds up and reliably delivers the maximum possible output.

Quote:

I also have the impression that it revs up or reacts more spontaneously when it's in a cold/lukewarm state. Can you come up with a rhyme for that in this context, or is it just "nonsense"?

No explanation yet - except for the full load boost, which is only active when the engine is partially warm.

Quote:

Is the duty cycle an absolute value with respect to the VTG control, or is it dependent on other parameters (speed, boost pressure, etc.)? You can essentially read off the exact degree to which the VTG is closed from the keying ratio, and thus how strongly the loader is being controlled (assuming intact hardware).

In principle, yes, as long as the pneumatic pressure conditions are constant. However, in high mountain regions, the pressure ratio between the internal and external pressure is lower.
However, since the same membrane force is always required for a specific VTG (Valve Travel Gauge) position, the relationship between TV (Travel Value) and the VTG position also shifts accordingly with the external pressure.

If I were to set the EGR valve to 'closed' and adjust the mass airflow sensor (MAF) to achieve full cylinder filling, the torque limit should theoretically always be below the smoke limit. This would, in turn, increase the power and, consequently, the turbocharger speed, keeping it below 1800 rpm. Or am I mistaken? Because the smoke limit only reaches the torque limit level starting around 1750 rpm...

SeatLeon wrote:

If I were to make the EGR "CLOSED" and adjust the MAF sensor with a resistor to achieve full cylinder filling, shouldn't the torque limit always be below the smoke limit, which would therefore increase the power and, consequently, the turbocharger speed below 1800 rpm? Or am I mistaken? Because only above approximately 1750 rpm does the smoke reduction reach the level of the exhaust emission limit...

Almost correct, but a resistor against +5V on the MAF sensor signal would likely generate an error (short circuit to +). A silicon diode would be more appropriate, as it has a voltage drop of approximately 0.7V, resulting in ~4.3V, which is roughly equivalent to the maximum air mass.

Best regards, Rainer.