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The car is a VW Passat B6 2.0 TDI from 2008.

There is sometimes a flutter-like sound when going from about 3000 RPM down to idle (that is, by pressing the clutch). At over 100 km/h, depending of when the engine is pushed a bit more, it goes into limp mode (less than 100 km/h if it's doing a DPF regen).

The diagnostic says that is a turbo overboost condition. My best guess is that it is caused by a faulty wastegate/turbo actuator, but before having a shot at replacing it, I'm looking for a bit of advice, other things that could go wrong or other tests I could make to check what could be the issue.

One recent change was that the O2 sensors were changed - the flutter sound was present sometimes before (especially during regen), but not as pronounced and not going into limp mode.

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  • I would venture to assume you're probably right about a wastegate. If the wastegate isn't functioning properly, it will allow for overboost, especially under the conditions you're talking about. Mind you, I'm not a Turbo Yoda, so I don't know if that's right (but it sure does sound right!). Any which way ... I wonder if @BobCross might know. He knows a lot more about them than I do. Aug 25, 2022 at 20:35

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It's pretty common for the VNT actuator on the turbo to get stuck due to buildup of carbon gunk. It usually gets stuck at a low boost position, but it can get stuck at high boost too.

If you have a hand-operated vacuum pump such as a Mityvac and you can access the vacuum port on the turbo from under the car, you can see if the actuator moves when you pump it down. If it's stuck, you will see little or no movement of the actuator rod. If it's not stuck, it will move about 1 cm.

The actuator can sometimes be freed up by repeatedly pumping it down and encouraging it to move with hand pressure on the actuator rod, then releasing vacuum and encouraging it with hand pressure to move the other way.

I did this on my 2000 TDI 3-4 times (different model, same problem) until several years ago when I started doing at least one extended full-power acceleration during each drive cycle. This has prevented sticking by keeping the entire travel path of the actuator cleared of carbon.

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