I bought an Powertrain Control/Emissions Diagnosis manual (my FSM didn't cover the emissions), and think I've found my fix.
There is a pink/tan plastic line coming off the TAD soleniod that should run to the diverter valve. I saw the line on the solenoid, but nothing near the valve. I traced the line around behind the manifold (it runs with some others in corrugated plastic conduit) and couldn't find the other end.
So, I disconnected it at the solenoid and blew air through it with an extension line, and listened for air escaping. I found the end of the line inside the corrugated plastic conduit running along the RH valve cover. It was open to the atmosphere.
Had this ever been hooked up? If so, how long since it was? Did I find a manufacturing/assembly defect? Had someone at the plant or a dealership accidentally broken off the end of the line, and just tucked it inside the conduit to hide it instead of fixing it? I'll never know for sure.
Anyway, I bought a rubber connector nipple and an elbow, and hooked it up like the service manual described. We'll see if it makes any drivability improvement.
Any comments or feedback?
Great find. Hard to say what lead up to this situation.