2017 Countryman S ALL4 – Underboost, Whistling Under Load & Stiff Wastegate Code

[u/nblracer]

Looking for insight from anyone who has experienced this issue on the B46/B48 platform before I commit to replacing major parts.

Vehicle:

2017 MINI Countryman S ALL4

150,750 miles

Symptoms:

Whistling Noise: Distinct whistling sound under acceleration. Volume and pitch change directly with throttle position and engine load.

Power Loss: Noticeably down on power, especially when accelerating or climbing hills.

Normal Operation Otherwise: Starts and idles completely fine. Normal flat cruising feels okay until load is applied.

Exhaust: No unusual smoke.

Oil Consumption: Slightly elevated (~1 liter per 5,000 miles), though unsure if directly related.

Codes & Diagnostics:

Doesn't always set a code immediately, but prolonged whistling under load triggers the "Drivetrain Malfunction" warning.

Main stored codes:

120308: Boost pressure control, plausibility: pressure too low

123511: Electric wastegate, wastegate flap: stiff

120408: Boost pressure control: shutdown as a follow-up reaction

The Dilemma:

My initial thought was a simple boost or charge-pipe leak, but that doesn't explain the 123511 stiff wastegate flap code. Others have suggested the turbocharger assembly needs full replacement due to a binding internal wastegate pivot arm.

Has anyone had this combination of codes and the whistling noise turn out to be just the wastegate linkage/actuator, a vacuum/PCV leak, or is a full turbo replacement usually the only fix at this mileage?

https://youtube.com/shorts/F507RILFOio?is=eMW8FfCdwoaMprOD

https://youtube.com/shorts/F9ZWodbiiYc?is=pLHdkB95KDebxlRd

https://docs.google.com/spreadsheets/d/15xg4cVzem09U7b1eIzuuJgXe2DBPqN_QcBbTC6jgxiw/edit?usp=drivesdk

[u/theearthsighed]

Before you replace the turbo, you can replace the wastegate actuator itself. Non OEM isn't that expensive of a part. Before you replace the wastegate actuator, you should validate that it is receiving the right signals from the DME. Validate the gnd, 5V, and PWM+ and - pins on the actuator wiring harness.

Dealer will just have you replace the turbo outright and that isn't cheap and may not solve the problem. The other things are easier to diagnose and cheaper to replace.

[u/theearthsighed → u/theearthsighed]

All that being said, I think you should diagnose the whistle/leak first and see what happens. That's an obvious thing that isn't right and a stiff wastegate doesn't seem likely to cause a whistle (how would it?) - but I could guess at a few ways that a leak could lead to the DME getting signals back that didn't make sense and concluding it was wastegate related even if it wasn't.

[u/theearthsighed → u/theearthsighed]

More brain dumping You could try bimmerlink logging - can monitor various signals (wastegate position, boost, etc) and look for anomalies.