1991 GMC S-15 Jimmy 4.3 [lack of power/horrendous gas mileage] (long)

Hi all, I've tried to find answers on many forums but so far I'm still in the dark.
Details: 1991 GMC S-15 Jimmy, 4x4 4.3l TBI "Z" engine, 150 000+ miles
Was stopped for a year before purchase 1 month ago.
My situation: I live in the Dominican Republic and have no access to esoteric tools and test equipment let alone good mechanics. Good thing I have some background and understanding and above all: a brain ;-) All I can rely on is a laptop with the WinALDL software. You will also have to excuse my limited vocabulary ;-)
Problem: The truck feels _almost_ right when cold but shows a big hesitation on acceleration when hot. If I take off from a complete stop an bury the pedal, il will accelerate slowly until around 2000RPM and will gain power gradually after that. If I try to take off slowly like a decent citizen, il will take off but almost instantly get "on his knees" and after a second or so accelerate normally...almost. It feels like a "go...no go...go" cycle. If I am cruising at 45/50 MPh with OD,I will have to bury the pedal or take it out of OD for it to accelerate, It feels like it has no power and I could almost swear it's slowing down! If it ever downshifts at an RPM greater than 2500 RPM it kicks ass but as soon as it goes under 2000, it's a snail. The truck starts like a breeze and the idle is as smooth and stable as it can be.
Things I've checked: When I hooked up the laptop I was able to monitor the following at IDLE. Truck reaches 195 after 10 minutes or so, then goes in closed loop. O2 sensor goes from .1 to .9 volts but seems to hover around .59V, when the truck is hot. If I disconnect the PCV vacuum the reading goes down to .12V as the idle increases but travels up slowly as the ECM tries to compensate. The "rich" flag goes on and off according if the reading is above or below +/- .6V THIS IS WHAT IS BOTHERING ME... SHOULD'NT THE ECM SUPPOSED TO STABILIZE THE O2 READING AT ABOUT .45V FOR THE MIXTURE TO BE OK??? The system sometimes goes to open loop but if I press the pedal it comes back to closed. I always have on: CLEAR FLOOD FLAG OLD HIGH GEAR FLAG Sometimes comes on the ASYNCHRONOUS FUEL FLAG. No error codes what so ever, the SES light never came on. Remember this was all taken at idle, can not play with the PC while driving...
Tests I've made: I checked the TCC lockup and it is working properly. I checked the EGR after removing it to check if it was blocked (it was'nt), if I move it manually or if I force vacuum in it, the truck will die instantly. Furthermore, if I disconnect it and block the output of the EVRV the ECM will throw me an error 32 after 5 minutes at 55MPH.(not while driving in the city, only highway). This suggests that the system works.
I have changed so far: Plugs (2 times, the first time I put R45TS and when I could finally find CR43TS I changed them again) The first plugs showed moderate carbon deposits that suggest a rich mixture??? after only 200Miles on them. Rotor Cap Fuel filter Air filter (of course) ;-) Rebuilt the TBI with a kit that includes Fuel pressure reg and injectors seals. (the PCV passage was clogged from top to bottom).
All vacuum lines seems OK and I have no leak that I know of, air deflectors, cruise control and 4X4 are working fine.
The ignition cables are looking good and I dont suspect them, I washed the engine a couple of times and it never failed or bucked.
There is no signs of oil in coolant or gas in oil or any kind of mixed fluids, there is no excessive smoking except a moderate "puff" of oil in the morning startup.
Gas mileage is 8MPG city and 13 MPG highway :-((( I know it can do a lot better, almost twice that.
Probably I forget a lot of details but I have been over this truck since a month and I can not find it.
Please understand that I'm not trying to be cheap or to sponge info without giving back, I live in this country for 3 years and it is impossible to find a mechanic that knows more than me (I showed them a few things actually)wich is pathetic because I'm NOT a mechanic. If I would be in the US or in Canada (were I'm from) I would have gone to the nearest GM shop a long time ago ;-)
Well that is it, If I remember any details, I will post them, I will also post any update or remark I found useful. I don't expect a quick answer and I don't expect a "fix all" solution either, but any advice will be taken into account.
Thanks for taking the time to read this rather long post, and have a nice day :-)
--
Olivier,

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wrote:

if the catalytic converter under your truck is REAL hot then it may be plugging up on you. That is one thing that can cause the exact problem you describe. Just replace ours yesterday and all is well. -- Regards Gordie
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The Nolalu Barn Owl wrote:

Thanks for your answer.
Will try to check this out. I heard that the temperature is suppose to be higher AFTER the cat and cooler BEFORE, by an amout of at least 100F. Is that correct?
--
Eaulive,


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If your CAT has failed, there is a good chance your muffler is also plugged. The exhaust will sound funny, shhhhhhhh instead of dumdumdumdum. I've had 2 cars do this, but they were earli 80s, when they used the CATs made of little balls. They broke up and plugged the mufflers. In both cases the CAT rattled, like a loose heat shield.
Another thing to look at, have you had to change the ignition timing? A lot? If so, your timing chain may have jumped a tooth, or more. Dan
wrote:

Colorado Springs, CO My advice may be worth what you paid for it.
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