Fuel trim believed to be rich

MATTBMENIFEE

MEMBER
1998 HONDA CIVIC
0.6L
4 CYL
2WD
AUTOMATIC
249,140 MILES

When I drive the car with scanner/live data feedback being monitored my STFT is very high negative numbers (-15 to -30) when there is no pressure applied to the accelerator/coasting. Once throttle is applied the STFT goes high positive numbers(10 to 25). I know the numbers should fluctuate but I feel that the numbers are to big of a fluctuation. Could these numbers be okay or is there a definite rich condition (stuck open injector or what)? Any ideas should be much appreciated.
Thanks,
Matt

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,896 POSTS

Hi Matt and thanks for using 2CarPros. Com.

Under perfect conditions, the fuel trims should be near 0. However, what you see is not really out of the ordinary. At idle, when they should really be checked, (STFT), I would like to see it closer to 0, but it isn't that bad. Since it is negative, that indicates to me it is a little rich, nothing bad, and the computer is trying to lean the system. The positive numbers on acceleration tell me the computer is trying to lean the system, which is normal. If your vehicle is running okay and there are no trouble codes stored, I would leave it alone. If it stayed rich (in the negatives) all the time, I would suspect a leaking injector, fuel pressure too high. If it was always in the positives, it is trying to add fuel, so suspect a vacuum leak, low fuel pressure. However, it is not. If you take the average of your readings, there are really close to 0.

I hope this helps. Let me know if you have other questions.

Take care,
Joe

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Friday, November 16th, 2018 AT 8:36 PM

MATTBMENIFEE

MEMBER
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It was consistently high negative numbers (-10 to -40's) never dropping lower than -10. The numbers just recently this past week went into the positives so I will keep an eye on it and see what happens next. My only real problem was MPG taking a pretty big drop. The car runs good other than fuel trim numbers and some intermittent idle/rpm issue. The idle is high on start up (normal cause of closed loop) and only troops down if I turn ignition off and back on again. Also it surges at 2,000 RPM up to 2,600 RPM's only in second gear every once in a while so I got something else not functioning properly. IACV would be my guess. It has been replaced about two months ago but $35.00 eBay valve no $200.00 Honda replacement. Also if I am trying to adjust the base idle the idle surges if I remove electrical connector from IAC valve which make setting it not very accurate but I have gotten it to a steady 950 with no load.
I really appreciate the help.
Matt B.

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Friday, November 16th, 2018 AT 9:52 PM

MATTBMENIFEE

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12 POSTS

I forgot my exhaust/catalytic converter combo has a crack in it right above the upstream o2 sensor. That is definitely not helping the diagnosis process out. That might be the whole problem right there.

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Friday, November 16th, 2018 AT 9:58 PM

JACOBANDNICKOLAS