I just had the same problem with my turbo. Turned out the overboost protection module was plugged. In the bottom of the module it was completly plugged. I am not sure how to vacuum test the module but my understanding is that it should let air flow through it freely when the electrical connector is unhooked. A new one is around 40 bucks from the MB dealership.

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Owen

87 300TD 603 I-6
94 Jeep XJ I-6 4.0 (see the pattern)
89 750il
seventy-two hundred feet above sea level (kinda cold up here)