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Old 06-28-2018, 12:07 PM   #15 (permalink)
Cyborg ECU
California98Civic's Avatar
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,297

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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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.
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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