I have a 1996 4.3 engine installed in a1989 jeep wrangler. Engine, tranny, wiring harness came out of a wrecked gmc truck. Engine misses when under load except at full throttle. I can disconnect neg. battery cable for a while reconnect it then start it up it will miss for a mile or two then suddenly smooth out (sometimes it backfires). It will run fine until I kill it then it will start missing again. Parts changed:fuel spyder, computer, TPS, temp control sen, thermostat, both upstream o2 sen, map, iac, Crank pos sen, egr, coil, icm, dist, cam pos sen,

I know I may be opening a can of worms, but what are you using for a fuel pump? What kind of pressure readings are you seeing? The Vortec V6 needs at least 55-60psi to just run.
Loren
SW WA

I forgot I changed fuel pump and filter. It runs between 58 & 66 psi by a guage.

Guess I'll just use the worms for fishing.
Loren
SW WA