2000 C3500 5.7 Vortec RPMs fluctuating

[Mannyarp]

I have an issue with my truck. My RPM fluctuate and eventually the truck shuts off at idle. If driving it runs horribly and sometimes shuts off. Seems like it doesn’t get fuel. The problem was intermittent now it’s permanent. At first I thought it was a fuel issue because I’m getting 60 psi Key on engine off and it starts dropping almost immediately. So I replaced fuel pump and connector, fuel filter, injection spider, distributor, distributor cap and rotor, spark plugs and wires, battery (read on another thread this fixed a similar issue). Now checking alternator but i believe it’s fine? I’m getting around 14.0 volts at alternator output and at the battery when accelerating about 1500 rpms. When I let go of gas I get 13.7 to 13.9 constantly fluctuating rpms and voltage. (Engine running horribly) something I did notice volts fluctuating 12 to 15 at ECM fuse only there no other place did I see that reading. Has anybody had this issue before? Maybe a bad ground or the alternator regulator? Or the ignition control module? Please help!!

 

[Schurkey]

Voltage seems low. Charge the battery overnight, retest alternator output.

As for the engine running badly,
The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.