High Negative LTFT Out of Nowhere

[ayau]

Per Hondata, the factory map is tuned with 0% ethanol. US gas typically has 10%, so that would already throw off the fuel trims by a little. That's why Hondata has a map that adjusts for the 10% ethanol.

Your car is most likely running in closed loop majority of the time (cruising, slow to medium accelerations, etc). Closed loop means that the car will automatically adjust the fuel trims so that you still hit 14.7 AFR.

In open loop (WOT), you may running slightly rich or lean, but I don't believe there will be any longterm negative effects. The ecu should throw a check engine light if something was really wrong.

I wouldn't be too concerned and just continue to monitor the fuel trims over the next couple fill ups.

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[bbeem]

I read on the prl cobra intake street maf they target the same -4 LTFT that the factory tune aims for. So apparently the OEM target is -4 and not 0.

From PRL website:
Technical Notes:
Our CNC machined billet aluminum MAF Housing was designed to provide *factory-spec fuel trims (-4% of factory STFT & LTFT) for optimal drivability and performance.

 

[ayau]

I've never done something like this so I'm scared...lol

All gas stations pretty much have some sort of cleaner already.

 

[ShadowFiend]

So what has your fuel trim been doing since then?

Mine also went down since my last post here. It is now mostly where many people report, about -7% typically, and it goes to -5% or -4% on a longer drive (longer for me recently is just 5-15 minutes without stopping). On idling it goes sometimes as low as -11%. So it changed since several weeks ago when it was higher (less negative) by about 4 % points.

So what is the reason for this change? Here is my theory: Due to the delayed switch to the summer gasoline blend this year in the US hnot sure about Canada), I believe the increased PCV flow of fuel into the intake could be ontributing to this.
With higher temperature, and the more volatile winter fuel in the tank, the fuel accumulated in the oil could be evaporating at an in reased rate, causing the ECU to apply a negative trim. In addition to any possible error in the MAF temperature correction..

My fuel trims have been okay... I actually re-flashed my ECU in the middle of the day thinking it might affect it...cause I flashed it during fall. But yeah, I got scared for nothing. Right now on idle it sometimes goes to -5% even during hot days and high humidity.

You could also be right about the fuel/gasoline blend! I haven't thought of that really...

 

[Gruber]

My fuel trims have been okay... I actually re-flashed my ECU in the middle of the day thinking it might affect it...cause I flashed it during fall. But yeah, I got scared for nothing. Right now on idle it sometimes goes to -5% even during hot days and high humidity.

You could also be right about the fuel/gasoline blend! I haven't thought of that really...

I did notice once that refilling with Shell 93 after having another brand 93 in my tank suddenly gave me high negative LTFT. I dismissed this initially, because also the weather got warmer at the same time. I then refilled with Shell 93 from another station and it was still even more negative.

So today my LTFT was down to -12% on town driving and wanted to go still more negative. I refilled with another brand 93 and went on a longer mountain trip. The temperature there was also about 10 °F lower and after some driving the LTFT reached -1%. I returned home and parked with -3%.

So I will now be able to see whether it's weather and the way of driving or the fuel, depending on whether it now comes back to high negative.

In any case, the car runs perfect and strong, and I only bother with these experiments for fun&knowledge because I see the trims displayed on my Ktuner all the time.

If the LTFT was always the same and remained at the specified -4% or close to zero all the time, it would mean a perfect agreement between the O2 and the MAF sensors. That in turn would mean that O2 sensors are not even needed because the MAF would do just as good job if it was used to control the fuel instead. Well, this obviously is not the case, as the control by the O2 sensors is superior, and it may deviate significantly from what the MAF sensor would order.

(Btw. the -4% LTFT, if Honda indeed specifies this, would be the specification for the calibration of the MAF sensor only.)

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