Did some more wrenching today. I lost count of how many hours I've drained on this damned post-tune misery.
First I went down the rabbit hole regarding wastegate calibration, particularly through
this good thread. Post #63 was of interest to me. Following the advice there, knowing that my CTS turbo actuator is the same style as the original IS38 actuators, I tightened the threaded rod and set
Charge air pressure actuator: acknowledgement to 3.54v, and
Charge air pressure actuator: adaptation for lower stop to 3.6v. I took a spin (with the heat shield removed) and was met with immediate disappointment: rattle and lag are both still present.
Tonight I finally had the time to dig deeper to run the end stop test and take a look at the flapper position. I removed the downpipe and took a close look. Here's the flapper with ignition ON (engine off of course) - closed tight as expected with the CTS turbo/IS38-style actuator:
I ran the end stop test through VCDS and the flapper opened as instructed:
I thought I was on to something when I noticed that the disc has a stop to prevent it from rotating freely, but it looked to me like it was broken off, potentially allowing a rattle:
It's worth asking here whether the flapper disc is suppose to move freely like that. I'm fairly certain though that this is normal. A quick eBay search of replacement flappers reveals there's two pins on either side of the arm and they don't appear to be connected:
I went ahead and reassembled everything. This time going a tiny bit tighter with the wastegate threaded rod and set
Charge air pressure actuator: acknowledgement to 3.46v, and
Charge air pressure actuator: adaptation for lower stop to 3.55v. Re-did first adaptation, reassembled everything and went for a drive - no change whatsoever. Lag machine, with that irritating rattle/crunch under load.
So wastegate voltage/preload makes no difference, and removing the various components involved in removing the downpipe as well as the turbo bracket, and reinstalling/retightening also made no difference with the rattle.
I truly wish I never tuned this damned car.