out of ideas

Check the fuel pressure if its within specs if okay-check the EGR and PCV valves and the MAF sensor-

replace the rubber elbow at the top of the engine;the elbow connects the pcv system tube (black plastic about 18 inches long to the intake manifold). These collapse, block vacuum and soon suck outside air, leaning the mixture out beyond hope of calibration. Factory obsolete (they sold you the tube with the elbow for $80). Auto Zone or whoever has a replacement elbow in a line of parts called "HELP". Fit a GM small block I believe. You owe me dinner.