I have a follow up question to a thread I started a few days ago. I have a 2000 GMC Sonoma V6 which is showing a hanging and hunting idle on stock tuning. Basically the spark timing keeps ramping up and down literally flying from 0 degrees all the way up to 34 degrees and carrying the idle all around with it. See this screen capture video and raw data.

Does anyone know what could be driving such instability?

A few things I noticed:

  • The MAP varies down to 25 kpa. Wondering if that's a threshold in the spark table? It says 25 kpa under "MAP/VAC Bndry"
  • The IAT is reading high (car was parked before this drive)
  • The IAC seems to be wandering as well but stays in the 80-120 units zone

Here are the spark timing bits:

After reading through the stock tuning, I see that the Throttle Follower is configured based on RPM (i.e. additional air added per RPM) which could cause the idle to hang if the air added was sufficient to keep the idle in the range:

I plugged up the PCV and the idle dropped about 150 RPM to about 650-700 RPM at idle with the IAC at 0 counts. It seems like an obvious vacuum leak but plugging PCV and the vacuum hose to break booster/climate doesn't get me down to 650 RPM with reasonable IAC. I am going to redo the vacuum leak smoke test (I think some of this is due to a worn throttle body shaft).