1997 E350 6.8 V10
I cleaned the IAC for the first time after 200,000+ miles. It was running fine before I did this. During the cleaning I tested the solenoid with 12v and it clicked but the plunger did not move .

Started the engine and it won't idle. Seems OK with some throttle. I double checked for any vacuum leaks but removing the valve was very easy and did not require disassembly of anything. Do I need to go for a drive? Did I damage the valve with my 12v test?

Update:

The obvious solution was to replace the IAC. I did this and it works fine.

I made some checks and measurments as I waited to the new IAC. The IAC works buy getting a ground signal from the PCM. This signal is PWM and normal ranges are 20% duty cycle for no extra airflow to the manifold (normal driving) and 40% duty cycle for extra airflow needed to properly idle.

The IAC has a plunger that is sucked into the "closed" position by the manifold vacuum. Even at idle there is enough vacuum to pull the plunger closed. The IAC is supposed to pull the plunger slightly away from the closed position. The movements involved are less than 1/8".

If you apply voltage to the IAC on the bench, nothing moves becasue the valve is already fully open. I tried to manually push the valve close and then apply 12v but the valve was not strong enough to create any noticible force. Bottom line: you can't bench test the valve without sophisticated equipment and/or test jig.

To make sure the PCM was working, I back probed the IAC connector and measured the PWM signal. I think the duty cycle was around 20%. Note the frequyency is 1.5 kHz.

Finally, I used Forscan to view live data and it showed the PCM trying to add air at idle using the IAC (duty cycle 45%) and the short term fuel trims were pulling fuel from the injectors (-20%) because there was not enough air. All of this confirmed the IAC must be defective.

Moral of the story? "If it ain't broke dont fix it"!!!