Mercedes GLS Turbo Issue Resolved

The symptoms suggest a turbo system issue. Whistling, hissing, and power loss indicate a boost leak or turbocharger malfunction. Before replacing the turbo actuator, check vacuum lines and intercooler piping for leaks or damage with a visual inspection and smoke test. What diagnostic trouble codes are being reported by the check engine light? DTCs will help narrow down the potential causes.

The DTC is P0299, confirming underboost. I did a quick visual check, but not a thorough smoke test. Where should I focus the smoke test? Are there common failure points on these Mercedes turbo systems to pay attention to?

P0299 points to an underboost condition, likely a leak or actuator malfunction. Concentrate the smoke test around the turbocharger, connections to the intake and exhaust manifolds, and the wastegate actuator arm. Pay close attention to the diaphragm within the turbo wastegate, as they crack. Given the vehicle's age and mileage, a qualified technician might save time and prevent misapplication of parts. The electrical bypass valve is also a likely culprit, best repaired professionally.

The mechanic found the electrical bypass valve on the turbo actuator was defective. They replaced it, cleared the codes, and the car is fixed. The total cost was 250€. Thanks for the insights, it helped!