I have a 1995 Chevy Astro Van V6 4.3L vin W that is failing state inspection emissions testing in Texas. I proceeded to do all the normal things like plugs,
wires, cap, rotor, oil change but to no avail. Since the engine had 240K miles, and was loosing coolant, I decided to overhaul it. After re-building the engine,
the emissions test was still the same. I then tried EGR valve, O2 sensor, but
it still is failing for low and high speed HC'*. I noticed that the engine has a
"stutter" sometimes at idle, and it has always seemed to have this for as many years as I can remember. Upon some advice from a friend, he said to put the timing light on the other cylinder wires. I noticed that wire #1 and #4
have a very steady pulse at idle, #2 and #6 have extra ticks sometimes,
and #3 and #5 have repeated extra ticks. It is hard to see visually, but you
can hear it in the timing light itself by putting it up to your ear. I tried new distributor, coil, and computer, but this had no effect. I have replaced just about everything with no success, but the only thing left is the computer PROM. Could this be the culprit?

did you reuse the pickup in the dist? could be lean air fuel too, did you check fuel pressure, fuel filter, injectors etc?

Substituted new complete dist which included pickup coil and ignition module, tried external coil, the computer - but re-used old prom module because computer does not come with it. I replaced central injector unit, new fuel filter, new fuel pump. Pressure is still 60 psi.
I was just wondering if anybody out there had ever had a symptom like this that was caused by a faulty PROM.

Incorrect timing will increase HC'*. Have you put a timing light on it?

Running lean is another cause. Check for vacuum leaks.

Defective cat. Any power loss when driving, or odd smell coming from cat, or rattling coming from cat?

Low compression. Check compression on all cylinders.

I set the timing when I installed the distrbutor, and also when I subbed in a replacement
distributor. I unplugged the wire to disable computer control, and set the timing to the mark
on the front cover. Now - this cover is plastic, so it is supposed to be replaced after one
use. The engine re-builder gave me a new cover with the engine overhaul. I wonder could
the timing mark be off on the replacement cover? I will check it against the old one. It
appears to be running rich - O2 sensor is sootted up when I removed it. The compression
is lower since the engine was overhauled about 120 - 125 on all 6 cylinders, versus 140-150 beforer I took it in for the re-build. The engine re-builder said it needs some
miles on it to seat the rings. Cat is original, but the guy at the the muffler shop said
if the car is passing Nox, then the cat is probably OK. Again - I think the electronics
are somehow firing extra pulses - I can hear it in the timing light, and I think this mis-fire
is causing the emission HC failure, sooting up of the O2 sensor, and causing the stuttering idle. Question is - what is causing this. Can the PROM module in the computer
be the root cause? I think the only thinks I haven't replaced or substituted are the PROM, the wiring harness, IAT sensor, TPS sensor, MAP sensor, Temp sensor, and IAC valve. I tested the sensors according tho the procedures in the Haynes manual just before the engine overhaul in November, and they all tested good.