Leitschaufeln des VTG-Laders festgefressen

The movable vanes in a variable-geometry turbocharger bound by soot and carbon so they no longer sweep freely. The vanes exist to change the turbine's effective size — closed at low engine speed to accelerate the gas and build boost early, opened at high speed to let it flow. Seized, the turbo behaves as a fixed one stuck at whichever position it jammed in.

Which position decides the symptoms. Stuck closed gives boost that overshoots and an overboost cut; stuck open gives no low-speed boost at all and a flat, gutless engine. Stuck part-way gives both, at different points in the rev range.

Deposits are the cause, and driving pattern is the cause behind that. Short cold journeys never get the turbine hot enough to keep itself clean, so soot accumulates on the vane pivots. Repeated short trips are why one car seizes at sixty thousand miles and another does not at two hundred thousand.

Cleaning genuinely works on many turbochargers and is worth asking about before a replacement, provided the shaft and bearings are sound. A seized mechanism is also what usually lies behind an actuator that cannot reach its learned positions — so replacing the actuator without freeing the vanes fixes nothing, and the actuator is the more expensive part.