<<My dad has a 1992 GMC Vandura 1/2 ton with a 4.3 liter work van.
Recently he just replaced the engine. Before he replaced the engine,
when the engine was cold or when it was sitting for awhile, when you
start it, the rpms would pulsate, from like 500 -1000 rpms, up and down
for like 5 mins unless you stomped on it or drove it. Since replacing
the engine, it still does this. We took the throttle body apart, cleaned
it, put a new TPS on it, Idle Air Control Valve on it, and it still has
the same problem. The van also has a new oxygen sensor on it, but it has
no catalytic converter. I have some thoughts of what the problem might
be, but would like any input on what you guys think it might be.
Thanks>>

Your problem appears to be heat related. You say that your van has no
catalytic converter. Did it come from the factory like this? Does the
engine have a smog pump? If the engine was produced with a catalytic
converter and it now doesn't have one, this could be the problem.

Typically, the next phase would be to start replacing everything, such
as the distributor, cap, rotor, wires, plugs, MAP sensor, fuel pump,
oxygen sensor, fuel filter, throttle position sensor and EGR. However,
if the computer thinks that there is a catalytic converter present when
there is none, this may cause your open loop performance to be erratic.

I made the mistake of posting my bad experiences of the 4.3L Vortec V-6
on this newsgroup and found that this engine is a sacred cow. In spite
of the large number of problems that I had with 3 of these motors, many
others on this group testified that this engine is the greatest motor
that GM has ever produced. I believe that the 4.3L V-6 made its debut in
1988, but it wasn't until 1993 that the 4.3L V6 engine received an
internal balance shaft to reduce vibration, as well as some other
improvements.

In any event, my post appearing under a "4.3 Liter" subject heading will
draw a large crowd and assure that you receive the help that you need.