Thanks for the great replies. Haven't dug that MAP off yet, cold up here and busy. But I have more reliably replicated the symptom.
The boost gauge confirmed sticks. So something is wrong with the signal, either at the MAP or I've heard ECU issues? Going to read up on that, unless you have experience?
Replicating the symptom: On initial hard acceleration after startup, car has typical power levels for about 2sec. Boost on Torque app spikes to 22.9psi on hard accel, then after 2sec or so power begins to drop off about 50% (not limp). After about 5sec, limp kicks in and you can feel all boost lost.
However, the boost guage still shows 22.9psi the whole time!
Even if I idle the car down and put it in park, still 22.9psi. Even if I disconnect the app from ECU, then reconnect it, the reading shows 22.9psi even when sitting idling (which would be physically impossible boost levels).
The odd time I can unstuck the boost by driving it. Then instead of the gauge dropping to it's normal 0.4-1in/hg of vacuum under non-boost conditions when working properly, it goes crazy up to 10-25in/hg with erratic fluctuations. Note this is intake pressure, NOT the vacuum system.
So I think what's happening is on hard accel ECU is requesting boost. It gets the boost requested as shown by spiking 22.9psi sensor reading. Then because sensed boost is too high, ECU tells actuator to reduce boost with vanes. Because sensor still shows 22.9psi, ECU tells vanes to reduce even more, reducing actual power levels but not reducing the stuck sensor reading. Finally because sensed boost (not actual) is too high for too long, ECU triggers limp mode to protect against perceived damage.
This is the only plausible diagnosis I can come up with. Would be nice to put aftermarket boost gauge on to verify this, how do people usually do this? Drill IC pipe upstream and tap one in?
Considering how much oil is coming up the intake, it makes sense the sensor was dirty. Will update and document when I get the chance to pull it.
Mr Fiat: I haven't subtracted atmospheric pressure. When the car was working right I would get consistent boost and vacuum levels under varying atmospheric levels. Can you elaborate on what I may be missing?
Wonneber When I shut the car off it eliminates limp mode until hard accel reproduces limp again. Unfortunately I don't have VCDS. Not having it makes diagnosis a huge pain with lots of creative methods. Should get it, but am also leary with commitment lol
lovemybug Some context, I've had this entire turbo system apart multiple times in past 1.5yrs for other symptoms which have been fixed. The actuator was good, rebuilt turbo twice (cleaned vanes) etc, full vacuum line tracing replacing and confirming. I haven't re-checked vane movement with mighty-vac recently, so will add that to my list.
Thanks for all the help everyone, will update when I actually do something with it. Hope this documentation helps someone else in future though.