here's a snapshot of my 6th gen ('99) accord 4cyl Short Term Fuel Trim(blue) and Primary O2 sensor(yellow) -



Everything 'seems' to look good except that one part where the O2 sensor is stuck in "rich" and the fuel trim is stuck at zero. This happens everytime under hard acceleration, or rather everytime the engine is revving higher than say 3000 rpm.
Long Term Fuel Trim is sitting at -7 to -9%
Does all this sound normal ? It doesn't to me.
Either way, why would it do that ? What could be causing this ?

While I am at it, another question I have is -
The fuel trim swings between negative and positive. I believe it should swing only in the positive i.e. above zero like between 0.5 to 1.7% or something.

so what do you think is going on ?

 

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Everything 'seems' to look good except that one part where the O2 sensor is stuck in "rich" and the fuel trim is stuck at zero. This happens everytime under hard acceleration, or rather everytime the engine is revving higher than say 3000 rpm.
Long Term Fuel Trim is sitting at -7 to -9%
Does all this sound normal ? It doesn't to me.
Either way, why would it do that ? What could be causing this ? </TD></TR></TABLE>

About the STFT jumping to zero... normal. The fuel system is jumping out of closed loop into open loop. When in open loop it doesn't look for the O2 sensor... it's the O2 sensor feedback that controls STFT.

Certain times the fuel system will be in OL, start up from cold, wide open throttle, possibly on you car high RPM.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AnAccordIsForever &raquo;</TD></TR><TR><TD CLASS="quote">
While I am at it, another question I have is -
The fuel trim swings between negative and positive. I believe it should swing only in the positive i.e. above zero like between 0.5 to 1.7% or something.

so what do you think is going on ?
</TD></TR></TABLE>

Yes FT will swing, especially STFT. What make you think it should only stay positive or add fuel??

LTFT will essentially stay in the same range, -7 to -9 for you. It's a bit low but well within spec. Usually plus/minus 15% will trigger the CEL for fuel system stuck lean or rich.

Everything looks normal to me. What's the time interval on each sampling point?

 

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About the STFT jumping to zero... normal. The fuel system is jumping out of closed loop into open loop. When in open loop it doesn't look for the O2 sensor... it's the O2 sensor feedback that controls STFT.
Certain times the fuel system will be in OL, start up from cold, wide open throttle, possibly on you car high RPM.
</TD></TR></TABLE>
thats what I thought too but since it was not WOT (it was ~65%) I was confused. I will verify that anyway next time I run it.

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Yes FT will swing, especially STFT. What make you think it should only stay positive or add fuel??
</TD></TR></TABLE>
I should haved looked up the Accord's manual, but for my other car '01 Acura MDX the manual says the STFT should be between 0.7 to 1.5% and LTFT should be between 0.5 to 1.2% (or its the other way around cant remember but basically its a small positive swing, thats why... but thats not a valid excuse )

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LTFT will essentially stay in the same range, -7 to -9 for you. It's a bit low but well within spec. Usually plus/minus 15% will trigger the CEL for fuel system stuck lean or rich.
</TD></TR></TABLE>
yeah it might be within spec of trigerring the 'CEL' but still a little uncomfortably off from zero. Me and 'Mr. Within Spec' are not usually friends.

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What's the time interval on each sampling point?
</TD></TR></TABLE>
when measuring just one PID I get a speed of ~7Hz. And for this particular run I was logging three parameters. I removed 'throttle position' from the chart. So essentially it would have been about 2-3hz per PID.