Ladedruck im Leerlauf und verzögerter Druckaufbau | Posts 16+

Even at idle, with the variable geometry turbine (VTG) vanes fully open, the turbocharger rotates; after all, the engine still produces some exhaust energy even at idle. Let's look at two numerical values: The turbocharger speed at idle is approximately 10,000 RPM with the VTG vanes open and approximately 30,000 RPM with the VTG vanes closed.
'That's true that it still rotates even with the VTG open - however, mine also presents a flow resistance of at least 40 mbar when cold (i.e., a value that causes it to drop below ambient pressure) - but when the engine is warm, it's 0. As for the speed, it might run at around 1000 RPM with the VTG open, but not much more.'

Probably not - I already looked at it myself when I was working on the ARL engine.

Well, I can't quite imagine such a significant difference, even though ULF's explanation sounds logical.
How about an LDA (Leak Detection Analysis)? It should provide clarity regarding the pressure conditions and is definitely not a bad investment for the future.
I would rather consider the engine control unit (ECU) as the source of the problem, not the turbocharger.
Where are your measurements...?
How do you explain the different values of -40 mbar differential pressure with a cold engine versus 0 with a warm engine? Is this due to the viscosity of the oil, or the oil pressure (although it should be fully present after just a few seconds)? Or perhaps the variable turbo geometry (VTG) is controlled differently depending on the engine temperature?

@ZAK1976:

The difference can be explained by the different exhaust gas temperatures when the engine is warm and cold. The turbocharger is a machine that operates based on thermodynamics, utilizing the enthalpy difference, which means that the exhaust gases also come out at a lower temperature .
Therefore, it's not just the pressure difference across the turbine wheel that matters, but also the auxiliary gas turbine (AGT) plays a role.

Regards,
Alex.