1991 chevrolet 350 code says running lean what sets the check engine light

SOURCE: Check Engine Light - O2 Sensor - bank 1 running lean

you should have three oxygen sensors on your car, one for each bank of three cylinders; should be towards the Y pipe on each exhaust manifold. Additionally you should have one on the exhaust pipe before the cadilitic converter.

SOURCE: losing gas mileage, left bank running lean

There are 3 oxygen sensors on your van. Left bank would be the front left. Try replacing it and see if that helps but remember to reset the computer. Where I work the sensor costs a little over $100.00.

SOURCE: CHECK ENGINE LIGHT ON , WITH CODE SYSTEM TOO LEAN

Possible failed bank 1 sensor 1 oxygen sensor..

SOURCE: 1990 CHEVROLET K1500 350 MISSES AFTER WARM

My first thought for a solution of your problem is plug wires. You have replaced some common problem areas for this situation but the plug wires are the most susceptible for causing a miss after engine is warmed up. Try replacing the spark plug wires and let me know what happens.

Lefty64

SOURCE: running lean

check fuel pressure and vacume leak arond intake /tbi