I started seeing something like this with my GTP. For me, it only happens during cold startup and once the engine reaches a certain temperature, the idle surging stops. When I scanned it with HPT, I see the O2 is reading rich initially and slowly leaning out but not switching up and down yet. Then, the PCM starts switching back and forth between FTC0 and FTC2. The two FT cells have different LTFT adjustment values...so, this is what I think is causing the idle surging. Basically, one FT cell adds or pulls more fuel than the other FT cell.
So, what is causing the PCM to switch FT cells at idle? Using TinyTuner to peek deeper into the PCM, I noticed some settings that seem to imply if the O2 sensor is not switching fast enough, then the PCM will command this FT cell change.
...so, how to fix it? I am not sure. I already put in a new AC Delco O2 sensor. Is it the wires to the O2 sensor that are tired and not letting the O2 warmup fast enough? I have TOGs with an extension harness but I have had these for a couple years already before this surging started.
