Querying the hive mind here.. I've got a 91 Chevy S10 Blazer w/ the 4.3 V6. I use it as a plow/logging/yard truck.. When I'm plowing with it, after I use it for about a half hour or so, it'll start surging & stalling, unless I lean on it, in which case it'll straighten itself out.
What would be the first thing you guys check?
Idle Air Control.
I will bow to those who know that engine better, but that's kind of how my old Neon behaved with a dead O2 sensor.
Sky_Render - Can you clean those on this engine, or is a "it's done broked, replace it" kinda thing?
I got this thing for free when I bought my house last year, it's rusting apart, but good for plowing :) I'd like to keep my investment to a minimum..
Duke - Wow, they must have made the ECU really slaved to that O2... You should have just got crappy gas mileage on the highway for an ECU of that vintage.
I'd check it with my brick (Snap On MT2500) scanner...that vintage truck is so simple that most sensor failures are extremely obvious. Do you have any check engine light? If so, check the codes first. If not, a failed coolant temp sensor, idle air control, and oxygen sensor can cause the symptoms you're experiencing. With a scanner, you can monitor the inputs for the temp and oxygen sensor (or a voltmeter, in a pinch). The IAC should only cause problems at idle, not while driving, but it can cause some goofy stuff (and is less likely to fail on that throttle body setup in my experience).
Bryce
Good idea, Bryce, but I don't have a brick, nor the title & registration of the truck to take it somewhere that does :) It doesn't show any codes when this is happening, though.
I was thinking possibly coolant temp, do they use a different temp for the ECU than the dash?
How are you going to check that truck with an OBDII scanner? 
WonkoTheSane wrote: I was thinking possibly coolant temp, do they use a different temp for the ECU than the dash?
Yes, the ECU has a different sensor than the dash. Check for obvious corrossion or harness problems for stuff first, of course. You can check the resistance (at a few temperatures...cold, not cold, hot) of the temp sensors if you don't have a scanner:
Swank Force One wrote: How are you going to check that truck with an OBDII scanner?
This truck is pre-OBD2. The brick is an ancient scanner that predates such novelties and is really handy for the older stuff; I got mine at a yard sale for $100, worth every penny when working on old stuff that it's compatible with.
Bryce
Is a 91 tbi or cpi?
TPS is another possibility
Nash - thanks for the chart! I'll check it out when it isn't -15 outside :)
Carbon - it's a tbi
Ice racer - I would think that a tps would fail the whole time or intermittently.. That's how it was on my rx7 when it went out, not only once it warmed up, consistently..
coolant temp sensor, was the problem on the one I fixed this week. <$20 too
unplug the temp sensor and see if that makes it run any better or worse..
wiggle the wires going to the temp sender- i've seen a few 80's and 90's GM cars and trucks where the insulation on the wires going to the sensor had shrunk back and exposed the wires so they can make contact.. sometimes, it runs good when cold then wants to die when warmed up- like the choke on a carb that is stuck closed... sometimes, it runs like crap when cold then good when it warms up, like a choke that is stuck wide open. sometimes it runs perfect until the engine vibrates just right and it dies or runs like crap when the wires make contact..
if nothing else, GM temp senders are pretty universal and you can get a handful of them for free to a few bucks at a junkyard and keep trying them until you get a good one. or just spend $20 at NAPA and get a new one...
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