Hi all, I've tried to find answers on many forums but so far I'm still
in the dark.
1991 GMC S-15 Jimmy, 4x4 4.3l TBI "Z" engine, 150 000+ miles
Was stopped for a year before purchase 1 month ago.
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 ;-)
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
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
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.
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 :-)
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