Well, it wasn't the plugs or wires. Just finished up putting in new AC Delco OEM wires and platinum plugs. Had high hopes heading out from the house, things felt pretty peppy while I was still not fully warmed up and apparently was on open loop control. But, right on cue, when the engine warmed up, the boggy response came back, essentially the same as previous. I picked up a diagnostic scanner that reads and records live data and ran some data runs at normal idle, nominal 2000 RPM idle, and during an acceleration run holding the throttle constant along the slightly rising straightaway where I always have the problem. Nothing jumps out of the data as abnormal yet. Seems like the MAF is showing reasonable amounts of intake air flow, the TPS is giving steady and reasonable outputs, the upstream O2 sensor voltage is switching up and down like it supposed to, the downstream 02 senor voltage is hanging around 0.5 volts, generally staying between 0.4 and 0.6, so it doesn't appear to be running too lean or too rich. Going to digest what the data means, and try cleaning the MAF sensor. One thing that seems odd is the ignition timing advance, at idle is showing something like 40 degrees, seems way out of the box. Anyone know what normal idle timing advance should be at idle for the 3.1 V6? BTW, nothing abnormal looking on the pulled plugs, all looked pretty normal, just a light brown color that is usually considered normal in those pictures of what normal and abnormal plugs look like. No erosion/cracking/burnt look, all of their gaps were a jut a little bit wide (.070 vs .070).
Given the recent history of the (now corrected) coolant leakage/loss, for which I never could see a whole lot of externally, and never could smell in the exhaust, I'm starting to suspect an at least partially blocked cat converter. Not as bad as some of the horror stories I've read about online, but on its way. If I cant get anywhere with the MAF sensor cleaning, its going to the shop to have my mechanic check the cat inlet BD outlet temps, and do an exhaust backpressure check. Also, although the scanner indicates that the cat efficiency is OK (and it has never thrown a P0420 code), there are some failure indications for abnormally low 02 sensor switching rate, that apparently do not trip a DTC (yet).
