Symptoms: misfire, stumble, intake backfire at idle or under load, only during engine warmup. No CEL.
Replaced plugs, wires, cap, rotor, CPS, and TPS. No improvement. Did wiggle tests at all computer and electrical connectors. Swapped multiple parts with known good ones. Countless hours connected to various scanners. O2 sensors operating within specified parameters.
After fretting over this for a couple of years, kept digging. Finally, after reading stuff here and on other sites, decided, just for giggles, to unplug the upstream O2 sensor. No stumbles or misfires over the past 24 hours, will continue to monitor over the weekend, but suspect that the heater circuit was borderline, not bad enough to trip the CEL but enough to cause the symptoms until the exhaust gas heated it enough to compensate. New NTK brand sensor located and priced, hoping this is it. Just Empty Every Pocket.