Originally Posted by
DmaxMaverick
It takes very little WG movement to dump a lot of boost. The reason for the high PSI shop air test is there's 1/2 of the force missing. Exhaust back pressure, which is nearly 1/2 of what is necessary to overcome the WG actuator spring tension. Once exhaust back pressure is bypassed (via the WG), boost can be dumped, or regulated, depending on how much the WG is allowed to open. Technically, with this system, we are regulating the exhaust pressure, which in turn regulates the boost pressure.
Black smoke could indicate an air filter due for a change. The overboost code at the same time is curious, though. I'd start by inspecting/replacing the WG actuator air line. Could be blocked internally, which could delay WG actuation.