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Surprise!
Got a nice surprise.
I don't know if it will last, but suddenly the rich condition was gone on my return freeway run last night.
To recap, I tested and replaced failing ignition wires and spark plugs but the very high STFT readings (as high as -29) continued at idle freeway steady cruise across two drive cycles. I did my freeway cruising using P&G last night (third drive cycle) on the way out to appointment and the LTFT, as expected, improved from -9.4 to -5.4 as I expected. That's normal LTFT behavior for P&G. On the way home I wanted to see the negative fuel trims again and observe, so I steady cruised it, but all STFT readings were normal.
It seems possible that the ECU has "learned" it faces normal conditions now. But I am gonna wait a few more drive cycles and use the sensors for diagnostics. Maybe an intermittent failure has gone away temporarily.
The downstream O2 sensor reading is still fluctuating a lot, though, so I still think I need to clean or replace the CAT.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.
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