1997 Discovery I 4.0 150K miles. No Fault codes.
With the engine running, normal temperature, normal idle RPM.... if I unplug the IACV what should happen? Anything? I just unplugged mine, and nothing happened, the engine kept idling just right, is that normal?
I have read every post about idle problems. Cleaned everything, tune up including 8mm Magnecor wires, still having an INTERMITTENT problem of the engine RPM dropping when stopping at traffic lights, engine shut off. Starts right up but have to keep the RPM up with pedal. Next stop, it may be all normal again. Repeat, it is intermittent, most of the time the idle is just right with no issues. I just don't like to replace parts guessing for a solution.

 

If you unplug the IACV with the engine running it will just be "frozen" in its current position.
If you were to shut the engine off, unplug it, and then start it again you would have issues.
But dont do that, you will mess up its memory.

Have you checked the coolant temp with a live scanner?
I had a similar problem, it was a bad coolant temp sensor, it was giving bad readings and was dumping max fuel into the engine at idle so when coming off of the expressway and stopping at the light at the end of the exit ramp it was wanting to stall because it was being flooded.
My temp readings went from -40*F to +250*F, then to +100*F to -35*F and so on.
So when the ECU got a -*F reading it thought the engine was that cold and adjusted the fuel trim accordingly and flooded the engine.
There are 2 temp sensors on the 4.0 GEMS, one for the gauge, one for the ECU. The live scanner will read the ECU one and you can watch the actual readings the ECU is getting.

 

Thanks Spike. Maybe this helps, I confirmed today that when the RPM go down, there is a humming sound coming from the engine. If I push the gas pedal the RPM go up and the sound stops, let go of the pedal and the sound comes back. When there is no humming, the idle is perfect. It makes me believe that it may be a vacuum leak, but I cannot find it. What do you think? How do I look for vacuum leaks?

 

The humming sound you hear is prob the IACV.
To find a vac leak you can use brake cleaner and spray all the vac lines with it running, if the idle speed changes you found your leak because the leak will suck in the brake cleaner.
But it is not a vacuum leak, vac leaks do not hum.