I'm dealing with unintended acceleration (full IAC open while driving) on a 2000 GMC Sonoma 4.3 Manual stock which I purchased recently for the princely sum of $1900. Been chatting over on the S10 forums about the IAC control algorithm trying to figure out what has gone awry -- several other users report having the same problem and we have yet to find a solution. Quick summary of the problem:

  1. When starting cold, the car idles up briefly quite high to 2k rpm. Some have said this isn't correct but I can't tell.
  2. At stationary idle, the car doesn't attain its desired RPM of 650, rather stays at 780 RPM even with IAC at 0
  3. When driving cold, the car seems to behave as best I can discern "normally" for this series, which is to say the car idles up in between shifting gears between 1500-2k rpm.
  4. At some point, the car begins exhibiting the problem, which is that at some point (usually shifting from third to fourth gear) the car idles up to 3,100 RPM and stays there for a while. The only thing that resolves this completely is bringing the wheels to a full stop (VSS=0). The idle tends to hunt up and down while moving with clutch depressed. The engine load calculation goes up when the idle pegs at 3100.
  5. The problem is somewhat intermittent, in that some accelerations the idle will stay below 2k, sometimes it pegs at 3100 with full IAC open. I can feel the IAC going full open while accelerating sometimes.

Basically I believe the following points are correct:

  1. Vehicles typically have an IAC control algorithm designed to more gradually decrease engine RPM when you lift off the accelerator.
  2. Some have suggested that GM applies additional increases in engine RPM in the manual transmission S10/Sonoma/Blazer platform for some reason (some say emissions, some say transmission longevity)
  3. Something is causing this algorithm to go awry and result in full IAC open along with a hunting idle while driving.

Things that DO NOT have any substantial affect on the problem/things we have verified:

  1. Unplugging the MAF
  2. Unplugging the MAP
  3. Replacing the PCM
  4. Somewhat thorough search for vacuum leaks suggests that's not the issue (but there's a lot of vacuum on this vehicle so still working on that).
  5. TPS read out on Tech 2 scan tool does not correlate with the problem.

Things that do have an effect:

  1. Unplugging the TPS results in idle at 650 (according the mechanic that looked at the problem; suggests TPS being gone changes the algorithm since TPS readout is normal).
  2. Unplugging the VSS totally fixes the problem since it's clearly wheel speed dependent (according to another forum user at S10, see thread linked below)

I'm convinced that one of the inputs to the IAC control algorithm is fishy, and that resolving whatever is wrong should be the first step before I begin tuning to adjust the idle behavior to my liking. Though we're not sure exactly what is going on here. The best hint I've found is the fact that the problem didn't repro for the first 10 minutes of driving, which might be before the O2 sensors warmed up when the O2 sensor heater fuse was blown.

Some resources I've found:


Good news is unplugging the IAC makes the car drive like a dream BUT I am a nerd and think figuring this out would be fun A few things I'm going to try:

  • Have an OEM TPS and IAC I'm going to swap in since my current IAC is an $8 one.
  • Double checking PCM ground -- supposedly it's under the passenger foot carpet in this vehicle?

After that I got nothing :/