The Magnetic Pick-up Coil is a sore spot on TBI engines, I've seen countless "New" Distributers cause identical symptoms as the old wore out OE Distributor did.
I was using new GM Performance Parts Distributors made for RamJet Crate Engines, But the supply has all but dried up.
A good Magnetic Pick-up/Pole Piece will produce at least .70 Volts AC at healthy cranking RPM.
These trucks have a decent Data Stream for a OBD 1 system.....One thing I've noticed with faulty Magnetic Pick-ups....
*The Accel Enrich PID will toggle from "Yes" to "No" at idle which it shouldn't do.
*The RPM PID will be erratic/unstable.
Reading GM OBD 1 fuel trims via scan data....
In the old GM OBD 1 world, There were the fuel control PIDs called Block Learn and Integrator. GM used a sliding scale between 0 and 256 with 128 being the center point. If the number was greater than 128, it reflected the amount of enrichment that the ECM commanded to compensate for a lean condition. If the Integrator read 139, the ECM had sensed the engine was 11 percent lean and made the appropriate correction. In addition, if the Integrator PID in the scan tool’s data stream read 120, it is reflecting the ECM sensing the exhaust is too rich and it made the appropriate correction to subtract 8 percent fuel to lean out or bring the Integrator back closer to 128 or the zero mark. The Integrator revealed the ECM's quick response where the Block Learn was the learned value that accumulated over time and learned the trends or cells for the Integrator corrections.
1. Check fuel trims in closed loop, should be 128, plus or minus 8.
2. If fuel trim numbers are high, Look at fuel pressure, Vacuum leaks. Add alternate fuel during the poor run condition to verify a lean condition.
3. If fuel trims numbers are low, Check the Exhaust Gas Recirculation (EGR) valve not seated.
People like to say a Scan Tool is worthless on TBI GM vehicles.....I wholeheartedly disagree! The GM Tech 1, Tech 2, And Snap-on MT2500 (Red Brick) are the best I've used for this system.
Also check the ECM block grounds at the Thermostat Housing, Engine to Body ground strap between the right cylinder head & the Body under the heater hose connections. Along with the Battery to Frame & Battery to Body Grounds.