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California98Civic · 02-20-2015, 07:41 PM

"Black & Green" (a 1998 Civic DX) has developed a lurching stumble associated with extremely lean AFR readings on the UG. At about 30 or 40% load at low speed this evening with the car fully warmed, I saw short-term AFRs bounding around -34% ... and then when it hit -45% I cut the then stumbling engine and coasted to a light. This problem has emerged gradually in the last week or two. It became really obvious a few days ago. And it is progressive. Yesterday longterm AFR was -9.4%, so I took off the WAI to see if it was the culprit. Stumbling continued. And today AFR has gotten steadily leaner. Longterm AFR stood at -10.9% when I pulled into the driveway.

Cold or warm, the car will do this stumbling thing that seems to be associated with the lean AFRs.

Injector failing? Vacuum leak? Fuel pump?

No CEL has lit up, and there are no pending CELs either, but I'm taking it off the road for the weekend to figure this out before I get stuck on the side of the road.

Thoughts?

EDIT: At idle, with no throttle, the AFR gets steadily leaner, 0%, -5% -12% -28% ... I observed that sequence within a few seconds at a stop sign.

EDIT 2: pulled each plug they all seem fuel fouled, and cylinder 1 and 3 smelled of gasoline in the cylinder still (especially #1)

02-20-2015, 07:41 PM	#1 (permalink)
California98Civic Cyborg ECU Join Date: Mar 2011 Location: Coastal Southern California Posts: 6,299 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 2,174 Times in 1,470 Posts	Abnormally lean AFR "Black & Green" (a 1998 Civic DX) has developed a lurching stumble associated with extremely lean AFR readings on the UG. At about 30 or 40% load at low speed this evening with the car fully warmed, I saw short-term AFRs bounding around -34% ... and then when it hit -45% I cut the then stumbling engine and coasted to a light. This problem has emerged gradually in the last week or two. It became really obvious a few days ago. And it is progressive. Yesterday longterm AFR was -9.4%, so I took off the WAI to see if it was the culprit. Stumbling continued. And today AFR has gotten steadily leaner. Longterm AFR stood at -10.9% when I pulled into the driveway. Cold or warm, the car will do this stumbling thing that seems to be associated with the lean AFRs. Injector failing? Vacuum leak? Fuel pump? No CEL has lit up, and there are no pending CELs either, but I'm taking it off the road for the weekend to figure this out before I get stuck on the side of the road. Thoughts? EDIT: At idle, with no throttle, the AFR gets steadily leaner, 0%, -5% -12% -28% ... I observed that sequence within a few seconds at a stop sign. EDIT 2: pulled each plug they all seem fuel fouled, and cylinder 1 and 3 smelled of gasoline in the cylinder still (especially #1) __________________ 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. Last edited by California98Civic; 02-20-2015 at 08:22 PM..