Vane sweep in the Ford 6.0L was developed to help with the turbo stiction.   Some signs of your vanes sticking is lack of power, delayed/erratic shifts, excessive popping through intake, black/white smoke. It acts like a turbo failure, which it kind of is, but in this case the only thing that fails is the vanes stick due to corrosion. Ford updated turbo design and as a last resort, applied the vane sweep strategy.

The PCM commands the variable geometry turbo actuator duty cycle from 15% to 85% in a step

pattern to help the turbo vanes achieve full movement. 

Vane sweep occurs when the following conditions are present: 

-The vehicle is stopped. 

-The EGR is commanded off. The engine speed is less than 1,160 RPM. 

-The engine oil temperature is greater than a calibrated value. 

-Vane sweep repeats approximately every hour if the previous conditions still exist. 

-Vane sweep may occur during scan tool commanded test functions.