Evidently, older style OEM O2 sensors make a difference. I've been fighting STF issues on cold start for months. On a cold start up (tune is closed loop), both STF drop to .60 lamda for 45 seconds before the PCM intervenes for which they recover at 1.40 before slowly recorrecting to 1.0. Basically, I couldn't drive the car for about 3 minutes on initial start up without bucking and sometimes stalling. Smoke testing, gasket replacements, and many hours with a very patient tuner (thanks BJ, excellent tuning service!) are just a fraction of steps done. Followed a suggestion by JDM and ditched the newer version of OEM O2 sensors (GU2Z9G444A) and replaced one bank with the older style (5C5Z-9F472-AA - green in color) for testing and....theory proven. Using the older style O2 sensor rid me of the issue (see attached for datalog comparison). Blue line is the older sensor displayed and operating as it should. From what I'm told The newer version 02 Sensors are a universal part now for all older Ford applications. However, they are actually a monitor not a sensor and therefore, don’t read properly.
Bottom line, after months of frustration, my issue is solved and thought I'd share in case anyone else has experienced this issue.
Many thanks to BJ for excellent tuning service and JDM for the helpful suggestion.
Bottom line, after months of frustration, my issue is solved and thought I'd share in case anyone else has experienced this issue.
Many thanks to BJ for excellent tuning service and JDM for the helpful suggestion.