Been having this issue on this truck ever since I installed the engine and turbo kit in it. The owner pulled the old engine and did some organizing to the wiring harness, which was just adding loom and taping wires. I installed the engine and turbo setup and it runs great except for some idle surge. At first I noticed the throttle position readout on the HPT scanner wasn't reading 0%, but rather 0.39%. I see this stuff all the time on f-bodys, and its usually fixed by backing off the set screw on the throttle body just a tiny bit and re-tuning for idle airflow. But not this truck... In its present condition, at idle it will just reference the idle spark advance table and not change at all in response to idle rpm error. IAC is also unresponsive, however it does not really need IAC correction because the idle surge duration is very short and only 100-200 rpms. The specs of the build are mostly irrelevant to the problem, but since people will ask, here they are anyway:

2001 GMC Sierra 2wd ECSB
SBE 6.0; triple 12, springs, etc..
KB Racing kit, 76mm, etc..
4L80e
Walbro 450
90mm throttle body

So, a couple things I've noticed:

-Sometimes it randomly enters idle adapt spark control and idles like a dream. This happens without warning and without any trigger or encouragement

-Throttle position with key on engine off and no throttle applied does not match engine running throttle position with no throttle applied. Foot off the pedal and it'll read 0% (most of the time) with the engine off. Fire the bitch up and it starts moving between 0.39% and 0.78% usually. Sometimes it gets frisky and goes higher to 1.96-2.35%. Mind you, this is all without touching the pedal...

-It is unresponsive to the throttle position relearn procedure. Went ahead and replaced the TPS sensor and performed the relearn procedure probably a dozen times. Also tried modifying the procedure to see if excessive throttle position while performing the relearn would be beneficial in that I could unscrew the set screw after the procedure to ensure it is 0%. Not sure if the PCM interprets the signal from the TPS sensor as absolute throttle position or if it needs to be positive in order to register as opening the throttle....not that it matters, because that idea didn't work anyway.


Here it is randomly entering the idle adaptive spark control routine. It idled fine until I touched the throttle, then refused to control idle again.




This is after shutting the truck off, key on, engine off...




This is immediately after starting it




I did not touch the throttle while starting the truck in any of those scenarios, yet throttle position reports changes. I am out of ideas to fix this thing and am probably going to have to give the truck back to the guy for the time being. It occurred to me that there might be some stray current in the wiring allowing the TPS to pick up a voltage when the engine is fired up, but I have seen it sit at a constant 0.00% TPS and still not enter idle adapt spark control. So I'm at a loss...

 

I know you know much more about this stuff than me, but have you checked the 5v reference to the TPS? It seems weird you are getting a changing TPS value when the throttle isn't moving. Maybe the reference is not a steady number. I wonder if the customer hurt the connectivity of the wire somehow. Good luck with it, it sounds like a hard one. Maybe someone with more input will chime in.

 

Last time I dealt with a TPS voltage that was phantom i had a wire that had bare wire and came in contact with ground

Started acting as yours....then got worst...got to point all sudden it raised up to 4500 rpms with no throttle cable