Idles at 2000rpm
 

I've given up on this vehicle so I'm finally here. I'm working on a early 2000's suburban with the 5.3 v8. It has a mechanical linkage throttle body and a idle air control valve. This car originally came in because it would idle so low it wouldn't want to run. The "techs" that worked on it before me replaced the throttle body, idle air control valve, and throttle position sensor. The vehicle afterwards has had a out of control idle. My boss then believe there was a vacume leak on the intake manifold and put me on the job of replacing it. I replaced the intake manifold and no change. Putting a scan tool on the vehicle no codes besides a high idle code obviously. I tried commanding the idle valve to decrease idle with the scan tool and no change. I got a second valve just incase the first one was bad and no changes. I back probed the wires at the sensor and pcm and verified power and ground to the sensor. Pcm was sending things out occasionally but nothing is happening. Out of two or three weeks I've only seen the idle air valve move once even when demanded to close. I know there are no vacuum leaks because when I take the old sensor and force it out and install it I can get the idle down to 750 or lower. The pcm is just demanding the sensor to back out? We even tried replacing the pcm with no changes. So I'm total this is everything I've checked.
maf- changes with throttle also cleaned it.
map- reads the same as similar vehicles and changes with throttle
New throttle body
New throttle position sensor
New intake manifold
verified no vacuum leaks
Replaced pcm
New idle air control valve
Replaced fuel pressure regulator currently has 55psi and increased with throttle
replaced 1 ignition coil because of misfire.
I'm running out of ideas on what it can be and im utterly defeated. My boss also has no clue what it can be. This is a fire department unit for a small town and we are trying our best to get it fixed.