Hey guys had a few issues with my BA XR6T overboosting, took it to a tuner and had the diagnostic pc hooked up, displayed an overboost fault code, the issue was the car shuddering at high boost when seccond gear was engaged, watched on my boost guage and turbo would boost from 7psi all the way up to around 12 then the pc would retard timing to prevent a big bang. I am a mechanic by trade and even I had some trouble diagnosing this one, to confirm a faulty wastegate acuator remove the vacume/boost line to the acuator and blow into the chamber you can do this while it is still mounted to the car, I just layed underneath the engine and did this with ease, if your lungs can force air past the diaphram then it is split, on a new one you will be blue in the face and will not be able to force ait into the chamber