8.3 ISC turbo flutter/surge

[akains]

Hey there, new to the forum and seeing lots of good info. I am working on an ISC8.3 2250 that has been de-mandated. Im getting a lot of turbo surge when you are under load at 1200-1500 rpm and quickly let off the throttle. I was thinking it was part of the file but I put stock file back on and can get it to do the same thing with harder load and throttle release. Can sticking vanes in the turbo cause this? Or beginning failure of the actuator? I have no codes and it runs really well going through all gears, exhaust brake, boost etc is fine. It is only under heavy throttle and quick to let off that it will flutter. any ideas? thanks!