I found your thread in searching because I have a similar problem. Unfortunately, I cleaned everything mentioned and I still have the two main symptoms:
1) extremely bad gas mileage (9-10 mpg highway) and
2) running rough at low speeds and occasionally it'll fall on it's face with no power from a stoplight.
And recently when it's warmed up, sometimes it'll be hard to start
I took it to my mechanic and we did a full tuneup, replaced all filters, replaced the catalytic converter, checked fuel pressure, compression check and checked for codes. Nothing at all. The puzzling part is he mentioned something strange that he thought it might be. It's a fuel injected car obviously but he said there's some sort of "carb-like" part that he thinks is the problem. That and he can't seem to source the part new or find it at Pick-U-Part.
Does anyone know what that part could possibly be called or have any other things to try? I greatly appreciate it!
Mory
1988 Pathfinder SE V6
Los Angeles