Update
I decided none of the other items in the rough idle symptom table were likely causes, so I put the UIM back on with a new gasket. Before installing, I carefully cleaned the LIM surface with a razor blade to ensure a good seal. I followed the bolt tightening order prescribed by the FSM. I don't have a torque wrench that goes down to 8 ft.-lbs., so I went by feel...
...Put the ECM connectors back in, glove box back in, plugged in IPDM and alternator connectors, IAT connector, PCV hoses, throttle bodies, and bolted hard vacuum line brackets back onto the UIM. Checked everything a couple more times and plugged in the OBD scanner before firing it up.
Everything was fine until it tried to drop idle speed below 800 rpm...threw a P2101 (bank1 throttle control actuator). I was monitoring OBD II live data up until then, and all parameters were normal (IAT, timing, MAP, etc. all good).
I remembered seeing something in the FSM about running throttle position and idle learning after disconnecting TB connectors, and forgot that unplugging the ECU probably counts as doing that. ECT was above 158F, so I turned off the car and performed the throttle/APP/idle relearn sequence. However, it still had the P2101 code after doing that. I'll have to look into it some more.
Question: should the Z throttle bodies make a rattling/springy sound when gently tapped? One of mine has done that since I got the car, but no codes ever appeared until today. The one that rattles is coincidentally on bank 1(
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