Very high engine vacuum, rough idle/hunting, stalling, negative fuel trims

[vtraudt]

What could cause the above symptoms?
Smoked the car for leaks (twice), all tight.

Deleted EGR/Kombi valve (block off plate).
Deleted evap solenoid, tank. Capped lines.
Deleted N249.
Tuned out.

Deletes have been done same way as identical sister car.

No codes (after a while, 'rich' code).

Swapped another TB in.
Did TB alignment/calibration with VAGcom.
Oil filler cap feels normal (dancing when loose, just like identical sister car/engine)

First few seconds car runs/idles fine, but then vacuum (on boost gauge; T'd into fuel pressure regulator line) climbs to -30+ and idle starts hunting (within maybe 10 seconds from initial start).

[PZ.]

How is the PCV? Does the oil cap continue to dance after the vacuum climbs? No collapsed hoses, air filter or snow screen?

[vtraudt]

Not sure how to test the PCV. Are you referring to the check valve sitting inside the large diameter rubber T down at the block next to the crank vent hose?
Or the hockey puck up on top?

I have NOT removed the crank vent hose and connected lines yet, since a) hard to get to and b) tend to break when trying to remove. Is a bad check valve in the a known cause for very high vacuum?

Oil cap dance is the same behavior as sister car (no issues). No change in behavior (but vac climb is so quick after startup that oil cap behavior is not conclusive).

[PZ.]

Yes, the check valve and the hockey puck are part of the PCV. Since it is plumbed into the intake, it lowers the vacuum slightly and is un-metered air. If it is blocked, it can raise the vacuum but not by an excessive amount. The only other thing that can cause a very high vacuum is an internal restriction in the intake. It would never show a leak on a smoke test as there is no leak, but it could be a collapsed hose, crushed fmic pipe or even a bad throttle body (which should be ruled out from when you swapped them).

Perhaps the issue is with the vacuum feed itself? Maybe a vortex due to the port shape is raising the vacuum at the port? It sounds wacky, but that would explain the rich code. Try another vacuum gauge on another port and see if it reads the same.

[vtraudt]

If it were just the reading on the gauge, I wouldn't be concerned. But car STALLS, dies, doesn't idle, hunts.
I will unplug the MAF again, and remove the intake/MAF itself; but little hope since the issue is AT IDLE. When running, boosting car runs fine. No codes.

I plan to get intake manifold gasket and crank vent piping (under intake) on hand then remove block vent and valve cover vent (just let breath to atmosphere) and see if anything changes. Suspecting clogged lines down there, or stuck check valve (in that rubber T). Want to have on hand since that stuff likely falls apart when trying to remove.

Still looking for other idea/suggestions/thoughts.