92 4.3L s-10 stumbles when warm?
 

I just picked up this s-10. Its a 92 with the 4.3 and has about 185k on it. I've put about 4000 miles on it in the last month and never had an issue with how it ran, but recently I have acquired a "stumble" or misfire under load once it warms up. if you get the rpms up to above 1500 or so it smooths out and doesn't seem to have a loss of power or anything.

I had a similar issue with my 92 2.8L a year or so ago and it was the injectors, but they were ticking and one had a visibly poor spray pattern which made diagnosing simple. As far as I can tell the injectors in the 4.3L look to be working ok.

Anyone have ideas where to start or what else to check for? I'd rather not drop the cash for injectors if they really aren't the issue. Thanks.

Time for an update. Saturday I went down to the you-pull-it and pulled a set of injectors. put them in along with a TBI rebuild kit and a new pressure regulator. Found myself with pretty much the same problem, only this time it was a bit worse. Ran great when cold, didn't miss a beat. Let it warm up to take a test drive and as soon as I got to the other side of town it started to hesitate and backfire again in the low RPMs. I limped it home and pulled the air cleaner off and the right hand injector was ticking and the fuel spray was more like a "fuel rain." This was the only set of 4.3 injectors left out on the lot, so unless anyone has something else to try I think I'm gonna bite the bullet and buy 2 new injectors this week when I get paid.

Still puzzled...
 

I put in a set of reman rochester injectors. It definately perked up a bit ran a lot smoother than ever when cold. I backed it out of the shop to let it warm up a bit. Once warm it started to have a little trouble idling. The RPMs would drop down then pick back up.

I went over rev'd it a couple times and it would miss and back fire right way. If I slowly brought the RPMs up it would rev fine in the higher range. Eventually it got bad enough to die when put in gear and now wouldn't re-start. It would crank, fire a couple times, back fire then die out. I let it sit about 10 min then it started up again, it did start hard though lots of cranking and rough idle.

With it running now I started checking vacuum lines for cracks (no carb cleaner or anything on hand to spray at them) I depressed the EGR diaphragm slightly to see if there was any change, with just a little movement the idle got rougher more movement killed it. I pulled it off to clean, there was a lot of build up on the top port and I found the diaphragm had a pinhole in it so I will be replacing that next.

Could this be it? I am still at a loss for why it runs great when it is cold. or is it just a factor of the higher idle speed?

Also I jumped pins A & B on the OBD1 connector and no stored codes.

similar problem
 

I am having a similar problem with a 1991 s15 jimmy. did you ever find a solution.

What have you tried so far? How does your distributor look? The magnetic pickup in mine was corroded real bad. After everything else only temporarily helped, I put in a new distributor and re-set the timing and its been running strong since my last post.

I'll bet the problem is in the plug wires, stumble under load.

When rebuilding the TBI. Did you completely disassemble, clean and re-assemble including the idle air control valve?
Have you checked the function of the IAC valve as per instructions (if you have the manual).
Make sure, all the vacuum hoses are intact (especially look at the plastic connectors and the rubber knees)
Electrical connectors on the injectors? Wiring harness to the TBI?
Gasket under the TBI for air leaks?
Just to make sure nothing has been overlooked on the injection side.