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Old 05-03-2015, 11:25 AM   #1 (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)
Thanks: 2,373
Thanked 2,157 Times in 1,463 Posts
Puzzle: vacuum leak, injector crud, or O2 sensor fail?

After a month of normal and flawless operation, I am back to the very same symptoms that caused me to replace the fuel pressure regulator. No CEL, no pending codes. But the ECU thinks the AFR is off and is therefore cutting fuel trim, sometimes at much as 35% and often 10-25%. when I hit the throttle, trim will be slightly increased, which seems normal for this Honda. Then trim steadily decreases fuel as I hold steady throttle, at either high or low load. It will even do this at idle. Though it has bucked a little once or twice, it has never stalled (yet).

My power is reduced. Fuel economy is suffering badly.

Last month I tested fuel pump pressure (normal) and fuel pressure regulator (normal). Replaced fuel filter (it was past time). Compression test too was good.

Yesterday I removed each spark plug, all black around base and gray around insulator.

No smell of gasoline in any cylinder.

Vaccum leak? MAP sensor? fuel injectors? O2 sensor?

I replaced the O2 with a good quality unit nearly 3 years ago. It has maybe 40,000 miles on it.

At this point I decided to drive it until it fails or I get a CEL. I have AAA+ and therefore free towing up to 100 miles.

I figure I'll patiently test items. Learn. Fix. I need my DD.

Any advice appreciated.


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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