Running way too rich, hard start, tough to idle

Going through fuel like a drunk with a bottle. 107000 miles, running rough about a week, replaced plugs/wires, added engine treatment to fuel. Drove 125 miles, muffler shop diagnoses plugged catalytic converter. Round trip 50 miles, hard start, stalls at low rpm, really rough, barely make it back to town to Minute Muffler.

After cat conv cut open, still runs rough, shop checks plugs, all plugs fouled, remove cat-replace with straight piece of pipe, replace plugs, 02 sensor good, mass air flow sensor slightly out of range, but within normal parameters, still runs badly. 40 mile round trip, remove/inspect fuel filter, OK, replace it, find plugs blackened, cleaned up, re-gapped, cap/rotor OK, still poor performance, can't sort it.

On to Nissan, tell them all that has transpired. 3-hr diagnostic, mass air flow sensor, intake systems, vacuum lines, fuel sensor, pressure in combustion chamber, fuel pressure to fuel rail, spark plugs, plug wires, cap and rotor, cylinder compression--All no faults, systems are in specs. NO faults OR solution discovered.

I'm told that they have to keep disassembling to find problem, next would be to remove and inspect fuel injectors, who knows where it will end. I call time-out, and am told to run engine treatment and premium fuel (runs hotter) to see if that will help. Wish they had told me to eliminate that possibility BEFORE 3-hr diagnostic.