What can cause an error code 16 (low voltage) in a 1992 c1500

Hard to start kinda like no spark or no gas. Engine turns fine no start, then
a stumbling start  until finally running. Idle rises and falls a little then settles
out. rev's fine. I drive off go a mile maybe starts to sputter and spit. When
looking at throttle body the fuel flow looks good. I can't really see if in fact it
goes lean.
Was told that when I went up the road sputtering it also blew black smoke
(unburned gas?). Then all of a sudden it runs fine again. Driving me crazy
trying to troubleshoot it. Checked and cleaned fuel pump relay. I hear the
pump run when turning on the key. But really can't hear anything when
truck is running.