Zero codes! I drove around town today and aside from sweating my butt off due to leaving the A/C off, the engine was behaving perfectly. So This is absolutely connected to whatever the brain is doing to due to the AC being on..

 

When the AC works it draws power causing the engine to respond. How old is your battery ? Have you checked your alternator ?

 

I can't recall the battery life. The alternator is about 3 yrs + old - a rebuild.

 

When A/C is on, it loads the motor from the power it needs to run the A/C system. The higher the outside temp the more the A/C loads the motor.
At idle it "would" lower the Idle due to that load.
The A/C turned on sends a signal to the computer to >>compensate< for this extra load at Idle or you would have just the opposite problem, a drop in Idle or even stalling..
Obviously the 'signal path' is holding the Idle high for some reason when it should not with the Clutch in..
On automatic trans truck it's almost never a problem.
On Std trans trucks it is often a problem when the clutch is disengaged.
What is in the circuit that is not in an Auto truck?
A Clutch switch. The compensation signal come from a relay contacts in the A/C system.
The IAC has a lot of control over Idle for many reasons, one of which is A/C Idle control.
If one of those sourced has a fault it can override many of the others.
Go from there.
I don't have your answer but that's what s going on.

 

I had what sounds to be the same revving issue in my truck, 99' 4.2L. It took me several years to fix. I went to several different mechanics with several different fixes and included all of the ideas that I just read about in the replies. It ended up being the lower head gasket. I had never heard of a lower head gasket. I'm 99% sure that's what he called it. It was definitely a lower gasket of some type. Lol. It has been a few years since I got it fixed. It took the guy one day to figure it out. He is my only mechanic now. Good luck!

 

Probably the lower plenum/intake manifold gasket. That was a weak area in the 97 and early 98 4.2's. That can cause lean codes. Any unmetered air entering the engine after the MAF can cause it.