Turbocharger Problems: 1Z Engine Issues

Hello, esteemed community of DIY enthusiasts,
It's that time again when I need your help. I'm driving a 35i Variant with a 1Z engine and, as you know, a wastegate turbocharger. The mileage is approximately 220,000 km. About 30,000 km ago, I replaced both the mass airflow sensor (MAF) and the turbocharger pressure control valve. After a long period of problems, everything was fine until yesterday.

After starting (without any problems) and the usual slow warm-up, I noticed shortly after getting onto the highway that the power suddenly dropped noticeably. Repeatedly pressing the accelerator didn't bring any significant improvement. The car could only reach a maximum speed of about 145 km/h.

Fortunately, I had my laptop with VAGCOM software with me. I connected it and logged the data, which confirmed my impression: the boost pressure is only reaching about 1250 mbar (including atmospheric pressure). Disconnecting the connector from the boost pressure control valve didn't improve anything, so it couldn't be adjusted to 0.6 bar. When starting and at low RPM, the car still feels quite responsive, although even here the actual and target values are noticeably different. I would have liked to attach the log file, but something seems to be broken; the data can only be analyzed using VAGScope, and in Excel, all the data is jumbled together and not displayed as a table.

Yesterday, I checked the hoses from the boost pressure control valve. I pressurized the blue hose leading to the pressure sensor and noticed that the bypass valve opens (the pressure reading at idle also dropped from 1030 to 995 mbar). The control rod and lever seem to return to their starting positions correctly, as far as I can tell. However, I don't know how to test whether the bypass valve actually closes completely. The hose to the sensor was definitely not leaking. However, I noticed that after the tests, 2-3 drops of oil came out of the hose at the top end. The turbo itself is otherwise dry. The intake manifold (pressure side) is oil-free and doesn't whistle noticeably.

Here are the questions that, in my opinion, arise: Is the boost pressure being lost somewhere (perhaps through the exhaust gas recirculation system), or is the turbocharger simply no longer producing any pressure? Where did the oil in the blue hose come from? I've read that the pressure sensor might also be full of oil (I didn't want to test that yesterday in my nice clothes after the workshop closed). How can I empty the sensor if it's full? Could this be related? Based on my experience, the pressure drop manifested itself similarly to a faulty control valve, with multiple instances of power loss at full load, until it eventually remained that way.
How can I further refine this now? I'd like to consider a complete overhaul only as a last resort. I suspect it's more likely a failure of the controlling components.

I haven't been able to find a similar case by reading through the forum, despite my best efforts. If I've overlooked something, I would also appreciate any helpful tips. (I apologize in advance to any experts I may have bored with this question).
Thank you in advance to everyone who wants to help, or even better, can help.
Best regards, Sven.

Hi Sven.

Is the boost pressure system (including the EGR) definitely sealed? If the wastegate turbo isn't even delivering 0.6 bar of boost pressure, I would test that first.

Okay, I'd say it's probably the turbocharger or the wastegate valve.
Or, the wastegate valve inside the turbocharger might have burned out.

Hello Hati, hello FSEC,
First of all, thank you very much for your quick responses, and, to get straight to the point, Hati was also absolutely right!

Today, I took a day off and, with the kind help of a knowledgeable friend, I tried out pretty much everything that still needed testing. Among other things, we removed the beautiful cover over the engine. After a lot of fiddling with this and that, we were able to feel a crack, about 3 cm long, exactly on the underside of a connecting hose to the intercooler. I swear that neither during the drive, nor with the hood open, had anyone heard any whistling or hissing before. A small amount of leaked oil had run down behind the engine, well hidden. Yesterday, it was impossible to detect anything 'visible' from the outside. In this case, it's probably due to the size of the hole. In any case, everything is fine again after replacing the hose, the boost pressure is back to normal, and tomorrow I can go 'on the track' again.

I'm glad it was just a minor issue, and I'd like to thank everyone again for their input and help.
Greetings from Leverkusen.
Sven.