Isuzu Repair: isuzu trooper 1988 running rich, mass air flow sensor, mass air flow

Question
I have a 1988 isuzu trooper 4cyl 2.6 litre. I'm having trouble finding out why it's running rich. The mass air flow sensor clicks when I rev it from idle.it hesitates when accelerating. and feels like it has a very slight miss. the o2 sensor is new. and timming is correct.new plug wires and plugs cap and rotor. new head gasket &valve job. rebuilt injectors. I feel like it might be in the wiring or the ecm due to the inconsistency of the problem. some days it runs better than others.some days it makes a very light ping/ringing noise in the exhaust. some days it doesn't. I'm out of Ideas and very few parts left to replace. I've noticed that the timing doesn't have much of a advance curve if any at all. It stays exactly on 12deg at idle and jumps to about 20deg or so when the throttle is opened from idle.and stays there throughout the revs.then returns to 12deg when throttle is closed.I've tried three different ecm's three mass air sensors two distributors all were used. and still no fix to the problem. HELP! I can't sell the stupid thing because I have too much money in it. Any help would be appreciated

Answer
I would bet on either a bad ecm, bad engine ground or connection. Have you had the catylitic converter tested? Or an egr valve stuck open maybe. If your vehicle is running rich your computer is overcompensating, alot. I would definately have my catylitc converter tested as the others Iv'e mentioned. I will due some more research tho on your particular year tonight and find an answer ok?