Grandpa,
I followed through this whole thread, and can relate to the idle problem. It was a problem for a couple years actually, and no mechanic I took it to could identify the problem. Of course, I was not a member of this forum at that time, and did not have the excellent troubleshooting listed above by moderator.
My idle problem would only occur after the engine was heated up, and I had stopped for a few minutes at a store, for example. When I would get back in, and stop at the first stop light, the engine would start to idle down, then back up, then idle down further, then back up. Eventually it would just stall if I did not rev it during that traffic stop. Then after driving it a few miles the problem would go away before the next traffic light or two. Bugged me for years.
You mention in one posting above that you are averse to replacing the fuel injection with a carb. Why not? You dont need to pass emissions. My truck is now over 25 years old in Texas and Neither do I now. I finally got fed up with all of it, and stripped out everything. Installing an Edelbrock RPM Air Gap intake manifold with a Holley 600 on top. Scrapped the computer and bought a transmission controller to keep the E40D tranny operating without the computer. AS I told the moderator yesterday, I will post pics and results this week as the conversion completes, and how it tests out. At this moment the manifold is installed, and carb is in place. And the engine compartment now has room in it. I never laughed so hard as I ripped out all that emissions junk... Revenge of 20+ years driving it with emission controls is mine now! And the best thing about going with a carb? EMP-Proof truck (with the small exception of an inexpensive tranny controller, that I can buy a backup of, and throw in the truck bag).
I cannot work on an EFI system, and it has cost me thousands in repairs over the years. But I do know a simple Carb. And as one member said: "Just let me know when John Force goes EFI". Well Said.
Best Regards,
Rod