My impression - early in the morning and without searching out or re-reading all your threads, is: If this was in my driveway, I'd try to find out where in the control range the idle valve is.

Let me try to explain, from a theory perspective -- not experience here; I haven't needed to do this, and don't really expect I ever will, unless by some chance someone with your symptoms shows up at my door:

To oversimplify it, the idle speed regulation occurs in a feedback loop. The RPM signal from the crank sensor is passed by the ICU to the ECU which uses it to control the "on time" or duty cycle to the spring-loaded idle valve.

To be effective, the idle valve must be able to pass some air around the throttle plate in all temperature and engine conditions. How much air, is determined by that duty cycle. If it was instructed to pass no air -- fully close itself -- the feedback loop would be at its limit, and useless. Likewise, if it got enough on-signal, it would fully open, and again, break the loop.

My approach to isolating the nature of the problem would be to find out where in that operating range -- between fully closed and fully open -- the idle valve was working. Simply pinching the hose is the quick diagnosis for high idle. If it starts to stall, you know the idle valve is what is keeping it running.

A dwell meter can tell you where in its range it is. The idea is to find out if the air (to create fast idle) is being ordered by the brain or involuntarily provided by another leak. Or is the brain sending the right signal, but the muscle isn't moving...

I would assume your shop has already determined the computer is telling the idle valve to open too far, which means you could stall your car if you pinched the idle bypass hose, and maybe bring the idle down to normal by disconnecting the idle valve's electrical connector. With no signal, the valve is open a "limp home" amount by design.

If the problem were related to a faulty signal from the crank sensor, as would result from any mechanical wear (I would not think loose bearings - but might consider an incorrectly installed flex plate or loose tone ring damaged during a transmission service) that faulty signal would be easily identifiable using a scope on the ICU signal to the ECU (ICU pin 17).

I have to think how many times I've learned things only to remind myself: None of us, including pros, were born knowing this stuff. It is an honest person who can admit that and deduct that learning from the food-on-the-table income stream. You owe them a debt of gratitude for that honesty, nothing more. Repay with your own honesty.


--
Art Benstein near Baltimore

A Stranger is just a friend you haven't met yet.