1997 s10 2.2 hesitation/stumble off idle

I’m working on a vexing off-idle hesitation/stumble on a 1997 chevy S10 2.2L manual 2wd. First post here, I’m a big fan of scanner danner and the info on this forums. I am hoping someone on here with experience with Chevy MAP engines can point me in the right direction.

Here’s the background of the problem so far. I got the truck with a broken timing chain, tore top end of engine down for all mechanical repairs. Reassembled with new TPS, new MAP, new AC Delco O2 sensor, new IAC, new plugs and wires. Upon reassembly, runs but has a code for idle higher than expected as well as this hesitation off idle. The hesitation I thought at that point was maybe just a bit of moisture in the fuel—truck had been sitting for a while with a half full tank before the mechanical repairs. Smoke tested the intake for vacuum leaks, found some smoke at the throttle shaft. Adjusted throttle stop screw some to compensate for extra air (tamper-proof cap had already been removed leading me to think previous adjustments had already been made). Eventually I figured out that the IAC screws were stripped allowing the body of the IAC to move some when the pintle moved. Repaired this problem with longer screws and the idle is now rock solid at a touch over 900 rpm.

However, the hesitation persists. The problem does not occur when the engine is cold. Great snappy throttle response. When cold I have -4.7 LTFT and similarly low numbers for STFT. When the engine coolant temp hits about 140*F, STFT jumps to something like -18 and LTFT soon compensates. LTFT at idle from this point on is locked at -12.5. Snapping the throttle when warm often results in hesitation. Sometimes I can even make the engine stall by opening the throttle. It acts like a dead spot in a TPS: if I hold the throttle just slightly open it will stumble and sometimes die. If it can move beyond this RPM range it will rev right up. TPS tests good, nice smooth voltage/resistance increase throughout the throttle opening range.

One theory is that the injectors are leaking such an amount that the computer can correct for it when the engine is cold but not when warm. It certainly smells rich when warm. Does anyone know if +/-12.5 is the maximum LTFT on this system? But stumble at throttle tip-in usually results from an over-lean mixture, not an over-rich one, so I would expect leaking injectors would not cause this problem.

I have also considered is that I have misadjusted the throttle stop screw. But I presume if the engine can idle at the right speed, it is getting the right amount of air—should it matter if this air is making its way around the throttle plate or through the IAC passage? Is there a maximum IAC position specified at idle for the purposes of adjusting the throttle stop screw? If the throttle stop screw is too far closed, and the IAC therefore too far open at idle, will the computer not be able to calibrate the values of the TPS and the expected drop in manifold vacuum? I am using a phone scanning app (OBD Fusion) and for some reason it cannot see IAC counts. If anyone knows of an affordable tool that will allow me to see IAC counts on this model, this would help a lot.

The only sensor that has not been replaced or tested is the Intake Air Temp sensor. Could a problem with this sensor result in an issue that crops up as the engine is warming, but before sufficient temperature to enter closed loop has been reached?

I’m really scratching my head on this one. Any help is much appreciated….

I'm running into virtually the exact same issue. Did you ever find out what the cause was/is? Also remember to check the fuel pressure regulator vacuum line, there may be gas in there... cheers.

Interesting. Perhaps you could post the history of your problem as well so we can compare notes. I ended up selling the truck, but offered to help the buyer (a neighbor) figure out the issue. It’s still ongoing.

After I posted this thread, the truck started fairly reliably throwing a throttle position sensor code. I tested the TPS a million times with a multimeter and found nothing amiss. Eventually got the scope out and repeated the test. I was able to create a spike in the signal by wiggling the wiring harness. Oddly, I could not isolate the source of this spike to a single one of the three wires from the ECM to the TPS. But after much wiggling, I somehow found a position of wires that was not producing a spike on the scope. Took it for a test drive and no check engine light. Still a bit of hesitation though. Maybe a separate issue. Someone had already removed the loom from the main engine harness. The truck is a bit of a basket case as it turns out.

So no, I haven’t found the issue. I may be in touch with the owner soon to check the status.