Isawicki, I tried your suggestions and here's what happened:
1. Started the truck and removed the air bypass hose from the throttle body and the engine died after a few seconds.
2. Restarted the truck and operated the throttle by hand to keep the engine running while removing the air bypass hose.
3. Unplugged the connector to the idle motor after five seconds and reconnected the air bypass hose.
4. Clamped the air bypass hose and used a hex wrench to adjust the idle up to 600 rpm. At this point the engine seemed to be be running very smoothly even kicking up and then releasing the throttle by hand.
5. Reconnected the idle motor, unclamped the hose and the idle returned to about 370 rpm.
6. There is now a distinct and loud harmonic whistle at idle (emanating from the opening of the air intake) which I can only imagine relates in some way to the change made to the idle adjustment.
I am becoming convinced that there is some kind of serious vacuum issue. I also wonder if the new idle motor I bought is defective or if I should try running without it connected. Any ideas?
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