We changed a turbocharger today which had failed in a new and interesting way, to me at least. The car is a 1990 740 with the mitsubishi turbo, and only 170,000 km on it. The rod from the wastgate actuator had seized solid on the pivot where it connect to the arm on the wastegate, which prevented the valve from closing completely onto the seat in the housing. The result is a hole burnt into the casting you can put a pen through and absolutly no boost. The hole looked exactly like an exhaust valve that has burnt due to carbon sticking on the seat. I would think the first sign of a seizing pivot would be excessive boost on first hitting the throttle hard, with the boost leveling off as the actuator forced the valve open, possibly followed by low boost on the quick reapplication of power, as the valve would not yet be closed. A touch of antiseize on the pivot once a year should eliminate any trouble. This is one of those failures that could be avoided easily if only a person knows.