VTG Loader (AFN): Control Behavior

Hello,
'For some time now, my Passat (AFN engine, manufactured in 1998) has been experiencing frequent instances of going into limp mode. Each time, the error code in the diagnostic memory was the familiar 'boost pressure regulation difference.' During a test drive using VAG-COM, it was also clearly visible that the set boost pressure was momentarily too high.'

Time / Boost Pressure Target / Boost Pressure Actual / Control Value (%)
664.85 / 1877 / 1714 / 39
666.3 / 1163 / 1785 / 94.4
667.73 / 1040 / 1102 / 39

After that, I removed the pressure sensor, lubricated the lever mechanism with copper paste (even though it seemed smooth), and reassembled everything. So far, the error has not reappeared.

What makes me suspicious is the following:
If you disconnect and reconnect the vacuum hose from the control unit while the engine is running, you can feel the control rod being pulled. However, it takes about 1.5-2 seconds for the rod to be fully extended (full range of motion). Is this too slow for the control system? Considering that the pressure increases relatively quickly in the lower gears, I can imagine that the control system might become too slow in those situations.

Does anyone have any experience with this? Could the vacuum in the system be too low? Did I change something by adjusting it?

Regards,
Werner.

Hello,

Let me guess, your engine control unit (ECU) usually goes into limp mode during good acceleration, and then afterwards when you release the accelerator?
Regarding your second point: if you disconnect the vacuum hose, of course, everything inside that hose will be depressurized.
It would be interesting to see if pinching the vacuum hose on the adjustment canister tightly with pliers and then reattaching it would take just as long – which would be bad.
If the issue with the lubricated and now easily movable VTG actuator never occurs again, that's fine. Otherwise, you should check the magnetic valve for charge pressure regulation, as it can also get stuck or clogged, which can then cause incorrect vacuum values at the actuator.

Best regards, Rainer.

Hello Rainer,
You're absolutely right about the pressureless hose; I'll try a different approach tonight.
Your theory about removing gas is correct .
I had also suspected the solenoid valve before, and I replaced it with the AGR valve, but without success. Either both are faulty, or it wasn't that. However, the error hasn't occurred again yet...
It's just strange that the VTG linkage was actually movable before.

Regards,
Werner.

Hi,

Well, when the loader gets really hot, the adjustment mechanism can sometimes feel more difficult or even easier to operate than when it's cold.
Otherwise: if everything works, then it's all good . Keep going and observe...

Best regards, Rainer.