Mercedes-Benz W211 CDI: boost pressure too low and limp mode
Why does a W211 CDI go into limp mode with boost pressure too low?
Limp mode occurs when the boost pressure requested with the accelerator pedal and the actual boost pressure differ too much. Code 2348 means too low and 2349 too high a boost pressure. The cause may be a charge air leak, the boost pressure sensor, the turbo's control actuator or sticking VNT vanes (the turbo's variable guide vanes).
What should you check first for a boost pressure fault?
Start with the easily visible items: the charge air hoses and the O-rings of the pipes for leaks, the air filters, the boost pressure sensor with its wiring and the wiring of the turbo actuator. A small leak does not necessarily show in steady driving, but limp mode comes on under acceleration. A more precise test is done with the Star diagnostic tool.
How do you check the turbo actuator and the movement of the vanes?
Start the car and use a torch to see whether the turbo's lever moves from one end to the other. If the lever does not move or the vanes are stuck, the fault is in the actuator or the vane assembly. The actuator has its own control logic, so the part number of a replacement part must match the original. Replacing the actuator alone is not always enough.