TDI Boost Problems: Troubleshooting Guide

Hi everyone.

Unfortunately, I've been dealing with a bug for months that hasn't been fixed.
"On my 1999 ALE 1.9 TDI 90 hp D3 engine, the maximum boost pressure achieved under full load is only 1570 mbar, which is just slightly above atmospheric pressure (approximately 0.6 bar). The boost pressure control valve is being constantly controlled at approximately 30%, which I assume is the lowest possible duty cycle, meaning the valve is fully open, which would correspond to the boost pressure."

It's problematic to drive with partial load. The pressure repeatedly exceeds 2000 mbar, resulting in an error message related to "boost pressure control difference" after a while, which leads to significantly reduced power.

I have already replaced the boost control valve (with a new part, not a questionable used one), as well as all the pressure hoses. I have tested the wastegate multiple times with compressed air, and it moves audibly to its limits. While doing that, I also oiled the wastegate shaft where it connects to the turbocharger (with Balistol), and I opened and closed the wastegate with compressed air to ensure it moves freely.

What other types of malfunctions are possible? Could the wastegate valve be defective in some way?
And why does the engine control unit immediately command the wastegate to its full opening (approximately 30% control signal and a resulting boost pressure of 1570 mbar) even when the error code has been cleared, the ignition has been turned off, and then the accelerator is immediately pressed to full throttle?

Thank you for the responses.

Best regards, Matze.

Are the LMM values plausible? At full load, it seems to ignore the value somewhat, if I remember correctly.
Best regards,

Hello!

Is the exhaust blocked? Is the catalytic converter clogged? That would explain why the exhaust flow isn't sufficient under full load.

Hi.
Test drive without the exhaust pipe, directly from the loading area, was conducted, and no differences were observed.
I need to take a closer look at the LMM values; when I quickly glanced at them, the target and actual values seemed to be close.
Greetings.

Has the turbocharger boost pressure been checked for plausibility? Could there be a cross-leak?

Hi.
The boost pressure sensor is located in the control unit, so a simple cross-check isn't possible due to the software. However, I'll compare it with a manual pressure gauge.
The values seem reasonable; the wastegate should open at an overpressure of 0.5 to 0.6 bar, and when fully activated, the diagnostic device shows approximately 1570 mbar absolute pressure.
Greetings.

AGR?
Is the intercooler duct sealed?
Was war das Problem bei beiden N75-Modellen?
What does the analog LD display indicate?
Is the atmospheric pressure plausible?

Hi.

No change after replacing the N75 valves; a new Pierburg valve was installed, and the hoses are connected correctly.

Atmospheric pressure is consistently around 1000 millibars absolute, which is also reasonable.
I need to install a manual boost gauge, but I suspect the values being displayed are correct, and everything seems reasonable.

What bothers me the most is the fact that even after clearing the error and restarting the engine, the valve opens fully under full load and boost pressure, resulting in a maximum boost pressure of 0.6 bar. It's as if the error is still stored in some kind of "hidden" memory, but it's not being displayed. Is something like that possible? The system installed is an MSA 15 control unit with a 68-pin connector (ALE engine series).

Greetings.