Your experiences sound all too familiar.

Hotsauce observed:
Another thing wrong I noticed as I was taking the supercharger off. There were 2 loose bolts that could be causing a leak. The bolt behind the lower manifold that holds the fuel line bracket to the manifold, and the bolt that holds the EGR block bracket to the manifold both were loose. These bolt holes are through holes into the casting of the manifold, so any looseness is a vacuum leak. A much better design would have been to have them be blind holes.

Newthings observed the same thing:
I have codes of P0171 and P0174, which are FUEL TRIM SYSTEM LEAN BANK ONE AND BANK TWO, respectively.

As to the solution to my idle problem--
The EGR valve sits on an aluminum block which is attached in two places: A metal vacuum tube coming from the back of the intake plenum and a flat metal bracket which bolts to the S/C intake manifold. A 1 1/8" spacer is between the bracket and the manifold. A bolt goes down the center of the spacer.
The bolthole in the intake manifold goes clear through into the air path.
So, if the bolt falls up (!) and out of the bracket you have a 1/8" air leak and everything looks fine. My searching with starter fluid spray produced inconsistent results since I was squirting in the wrong places. I pulled off the EGR and replaced the gasket. When putting the valve back on, the spacer fell over. A search for it's home revealed the hole.

Problem soon returns:
Another 2 hr. trip to the Isuzu shop finely got results. They felt it was the intake gasket in the front area. Hmm, the codes say lean in both banks. Both gasket leaking? So off came the S/C and manifold. New gaskets and the same problem returns. Then the search goes to the back on the S/C chamber in the back next to the firewall. Another drilled through threaded hole holds on a bracket for the fuel line. The bolt is loose and inaccessible. Off comes the S/C again and the bolt is fixed in place with LockTite. Problem solved, at last. Two different bolt leaks and two different fuel line leaks in a few months has been a nasty exercise.

There is nothing like the sense of relief when a chronic problem is fixed.
Roy