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The flapper can be moved inboard/outboard )as opposed to rotating it open/closed) and give erratic boost levels as a result. I had this happen on the 14b that was on my car. Once removed I could see it was really obvious. That is what Shawn was eluding to.
Also, the seat area and not just the main collector inlet for exhaust gases, are the two main zones where the carbon pulls from the casting and you get deep fissures. You would have seen this before installing the turbo, but with the flapper in the proper place, many people have gotten decent boost and never realized there was an issue, while others get low if nothing on the gauge. All depends upon how much is not sealed in and blows by the flapper.
I visually inspected for any cracks inside the turbine passage & around the flapper port, had even connected the wastegate arm to the flapper while the o2 housing was removed to check for flush contact. Just for trying, I removed the arm from the flapper last night. I confirm it is being held flush naturally.
Keep thinking the spool is going right back into the air tract to indicate a problem with the BOV somehow seizing open....but again, a boost leak test (without vacuum testing the bov) should have ruled it out? Pretty hilarious, really. 14b and the tires won't break loose in 1rst gear. :^/