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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.
Thanks for your reply. MAP voltage is about 1.4 at idle. My MAP readout on my scanner interprets this as 10-12 in-hg. I will get an analog vacuum gauge on it as soon as I get a chance.
Here is a link to a forum where several members with this vehicle report 900 rpm idle speed. Perhaps these are relevant data points.
I have tried to adjust the throttle stop screw. I adjusted it out approximately 2 full turns (closing the throttle plate) and that seemed to make the problem worse. From that point I adjusted it in 3 full turns (net 1 full turn in) and that seems to have helped some. From there I adjusted it in one more full turn and this caused a high idle. (Note I am performing IAC reset procedure after each adjustment.) so I backed it off to the best position of the three.
Vacuum gauge shows 19 in-hg at idle. I think the scanner is oppositely interpreting the map voltage as it shows 30 in-hg when the engine is not running!
