This is killing me, 1991 B2600i idling rough plus..

Cold start it (fast) idles fine. As soon as it warms up the idle drops to just below 800 rpm and then starts to sound like it is missing. The exhaust really stinks. Popping on decel after I rev it above 3000rpm. Definitely rich. It seems to be using much more gas than normal (usually 12-13L= 100km, now it takes over 17L). I have done the following:
Replaced rotor, cap, and ignition wires.
Tried a different mass air flow sensor, idle control unit.
Sent out injectors and fuel rail to be cleaned and tested (totally fine).
I had a local electronics guy repair my ecu (he was not able to put it in a vehicle and test it). He was not as thorough as AVPRO. This has me wondering.
The computer presents code 26 when I pull codes. 26 is the emissions solenoid (also called a 3-way valve) I replaced it. No difference. I am not sure if it is using antifreeze.
Checked O2 sensor and it is fine.
Truck drives fine otherwise, no lack of power under load.
I'm getting pretty frustrated, any suggestions? Cheers, S.
I had the head off, pressure tested and valves reseated about 25,000 (km) ago.

There is some kind of air mixture screw on the throttle body near the intake manifold that might need adjustment...?

Unlike many of the "help I've got problems" threads I am going to follow up with the solution.
I tried another ecu, turned out Mateo repaired mine properly so that was not the problem. Put in another distributor, not the problem. Tried a third MAS, not the problem. Finally I bought some carb cleaner and hunted (again) for vacuum leaks. Turns out the intake manifold was missing a bolt so I pulled it and replaced the gasket. Put it back together and it is running like a champ again! Total cost was $500 for misc parts and having my ecu and fuel injectors serviced. I still think a shop would have charged me at least that much to diagnose and fix it. Well that is what I am telling myself;) BEER time!

Your being nice to the shop I bet they would have charged double and made some statement that it was such a big deal to fix. LOL

Yes, at $110/hr I'm sure it would have been more and a PITA. Funny, the guy at the wreckers wanted to trade me his Honda civic for my (poor running) truck. Another guy came up and commented on what a great truck I had. Made me really want to solve this and keep it going.