FUEL TRIMS QUESTION

[aidan5678]

I’ve been concerned about my latest fuel trims, they are at -9 to -10 for my long term fuel trims, and my short term fluctuates from 0 at acceleration, and when I let my foot off it spikes suddenly to -20 for a split second and goes back down to about -13 at idle. I have a mishimoto air intake and often run 18.5 psi, only on 93 shell premium. AFR is always at 14.7 idle and 12.5 to 11.9 accelerating. Knock control at .49-.59. All the hose clamps and everything are tight on the intake. I have a civic 1.5 hatch non sport. Is this normal? Thank you.

 

[Gruber]

It's probably normal, at least qualitatively. I'm completely stock and I see see generally the same behavior. The LTFT does get less negative on longer drives, (-7% to -4%) but it takes only a little idling to get easily -10% or more. The STFT reacts with a negative spike every time the throttle is closed.
What seems odd and probably not normal is this:
Normally, when going downhill or cruising above some speed with the gas pedal released, the STFT goes quickly to 0%, as the fuel should be cut off. It seems like sometimes, it somehow gets stuck and the STFT desn't go to zero, even on a long descent. Looks like the fuel still flows, when it shouldn't.
So I'm thinking, this might be a sign of the fuel injectors not reacting perfectly, maybe leaking somewhat, sometimes when they should be closed.

 

[aidan5678]

It's probably normal, at least qualitatively. I'm completely stock and I see see generally the same behavior. The LTFT does get less negative on longer drives, (-7% to -4%) but it takes only a little idling to get easily -10% or more. The STFT reacts with a negative spike every time the throttle is closed.
What seems odd and probably not normal is this:
Normally, when going downhill or cruising above some speed with the gas pedal released, the STFT goes quickly to 0%, as the fuel should be cut off. It seems like sometimes, it somehow gets stuck and the STFT desn't go to zero, even on a long descent. Looks like the fuel still flows, when it shouldn't.
So I'm thinking, this might be a sign of the fuel injectors not reacting perfectly, maybe leaking somewhat, sometimes when they should be closed.

Hey, thanks for replying. I decided to take off my air filter to check how dirty it was and it is absolutely coated in dust and dirty of all sorts. Is this possibly the cause to the negative fuel trims (running rich)?

 

[Gruber]

No, if the air filter is clogged, the MAF, if it works as it should, would still measure the actual air flow. Only the power would be limited. My air filter is almost new.

 

[gtman]

I wouldn't worry about those STFT peaks and valleys because that's what short term trims do.

The -9 LTFT is definitely somewhat rich but within tolerances. I mean, if you want to try to improve those numbers you could try adjusting maf scaling.

And not so much for fuel trims but for efficiency, go ahead and clean that filthy air filter.

Sponsored