This fault is becoming a serious problem on BMW cars with the N54 engine, as they become older they develop a serious rattling noise from the turbo area, and power problems and low boost codes can ensue as well.
Each small turbo is paired to 3 cylinders on the engine, they are designed to spin up quickly thus removing the necessity for variable vane technology as found on the larger modern single turbo design, and are fitted with a wastegate.
N54 turbo setup (note the total inaccessibility of the wastegate arm on bank 1 cylinders)
A wastegate is a valve that diverts exhaust gases away from the turbine wheel in a turbocharged engine system. Diversion of exhaust gases regulates the turbine speed, which in turn regulates the rotating speed of the compressor. The primary function of the wastegate is to regulate the maximum boost pressure in turbocharger systems, to protect the engine and the turbocharger
You can hear rattling from what sounds like the wastegate actuator arms and sometimes a tinkling as you come off the throttle similar to the sound of turbine blades catching...
The wastegate rod as it comes of the actuator has some play at the joint where it meets the wastegate arm that goes through the body of the turbo connected to and activating the wastegates.
Basically what you're all experiencing is turbo's where the exit wastegates mechanism has worn from poor design and manufacture, The combined slack from the actuator arm linkage through the wastegate arm bush in the turbo body to the play in the wastegate valve means that the actuator arm moves around 5mm before it can take up all that slack, so they cannot operate correctly.
BMW will try an ECU update first, but this merely masks the problem of the wear by readjusting the control actuator arms taking up the slack, eventually though because of the initial wear progressing your turbos are still on borrowed time.
The solution is expensive due to the difficult positioning and nature of the turbos, BMW will tell you you need 2 new turbos and removal is half a days work due to the inaccessibility of the said turbos.
The cheaper solution is to get the turbo and wastegate actuator mechanisms re engineered with more robust parts, this company will do it for a lot less than 2 new turbos cost, as mentioned it's half a days labour at a garage just to remove them though.
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