Hello,
I never asked the question about this situation here, but I see many other that have with not a lot of success it seems.

I thought I'd share my experience in hope it will help others.

Basically, I bought this '91 w the 3.0 and it was running, but poorly. It would run okay for 2-3 mins, but then begin to bog down upon acceleration. It would accelerate to 1k, then was just gutless til 3500-4k rpm. At that point, it would take off like I was being rear ended by someone doing 80mph. I knew this was a rich situation because of a small black puff at speed, and it just made sense that once the valves were opening at that rate enough to deal with the excess gas.

Previous owner seems to have taken the approach others had taken here and started shotgun replacing parts. Thanks to him, I got a new cat, distributor, cables, o2 sensor, egr, pair, VMAF among other things. Unfortunately for him, none of these guesses fixed the issue.

Being a repair tech myself (in a different field), I know this isn't how you do things, and...Read The F'n Manual! Went through the troubleshooting chart in the FSM and after testing however many systems before the solution- Coolant Temperature Sensor! It was intermittent when cold, and not working at all at temperature. Very hard to test in-situ because it gave false OK readings .

I missed it the first go around, but I took my ohmmeter with me and got it good and hot, pulled over and tested it. It was measuring something like 20M ohms which is essentially open as far as the ECU is concerned. I see why/how this could be overlooked. But, if it doesn't work, the truck stays in rich/cold mode and it doesn't even read some of the sensor input to adjust mixture

It appears the pins had come loose of the protruding brass end. A small, but not easily seen crack running between the base and the body had let corrosion in. I broke it open and saw The small internal wire on one leg was just barely touching when cool

So, there you go. If you have this issue, follow the troubleshooting chart, and don't overlook the coolant sensor. This truck, although slow, runs like a champ now!

Except that CV boot that just tore....ugh...it never ends!

Hope that makes sense and helps someone in their frustrating search.
Thanks!

 

Your perseverance and thorough troubleshooting is what makes a good tech. you found the $20 broken part and proved definitively it was the culprit instead of throwing $1000 at it. Now you have an extra $980 to spend on goodies! Thanks for sharing your experience!