AUY: Limit Breached, Below Threshold

Hello everyone,

The gremlin of errors is at it again, and I can't seem to make any progress.
It's about a 2003 Seat Alhambra with a 1.9 PD engine, model code AUY, and 115 horsepower.

Throughout the entire speed range, it builds up insufficient boost pressure. Under high load, it then naturally switches to emergency mode.
Da die VTG sporadisch hing und das Wellenspiel schon recht hoch war, wurde der Turbo gegen einen Generalüberholten ausgetauscht. No change.
Ladedruckregelstrecke überprüft. Ein Riss am Hartplastikschlsuch der Unterdruckpumpe und ein Loch an dem Schlauch zur VTG gefunden. Ka change.
What else was done:
- Diesel filters
- Air filter
- N75 Pierburg
- Differential pressure sensor
- Original boost pressure sensor.
- AGR valve, Pierburg brand.
- AGR disconnected to rule out continuous activation.
- Intercooler pressure tested.
- Intercooler system checked, all connections are tight.
- VTG valves do not jam, and they can be completely adjusted even under slight negative pressure.

I have now measured the vacuum pressure. The tandem pump provides a vacuum pressure of 0.75-0.95 bar, which is dependent on the speed. A maximum of 0.68 bar reaches the VTG valve, and the N75 valve then regulates the pressure back down. This sounds quite plausible to me, considering the pressure loss due to changes in cross-section and surface roughness along the way.

Does anyone have target values for that?

You are correct that the mass airflow sensor is not involved in the boost pressure regulation, right?

Could there be something else, or have I overlooked something?

Best regards,
Leon.

Please provide a full system scan. Without the error logs, it's impossible to understand the problem.
hg
Herbert.