Question about MAF and fuel trims

[jtrosky]

I've been monitoring my fuel trims and trying to get them closer to 0 by modifying the MAF table for the cells needed, but I'm finding that one day, the fuel trims will be perfect and then the next day, I'll get significantly higher LTFT's in the exact same cells. I'm mainly referring to idle cells. I just don't understand why the LTFT's for a particular cell will be perfect one day and up to +15 the next day (same fuel)?

Do ambient temps and/or humidity play a role? If so, how does one adjust the tune for those differences so that the fuel trims are generally the same all of the time for a partiular cell?

Thughts?

[Higgs Boson]

What's your process?

[jtrosky]

Not sure I understand the question. What I'm asking is why the fuels trims would be so different for a specific "cell" between logs (without any tune changes). For example, while idling, sometimes I'll see consistent +10 LTFT's for the 1950hz MAF cell. At other times (even the same day or the same trip), I'll see consistent +3 LTFT's for that same cell at idle.

I'm just trying to understnad why the fuel trims are so different without any changes to the tune. I would think that the LTFT's for that cell would be somewhat consistent without any tune changes - unless the ambient air temp or humidity or something like that affects it?

If you are asking about what process I use to adjust the MAF table, I'm basically using the average LTFT values from histograms to see where the LTFT's are consistently above high and then make small adjustments to those cells (actually to a range of cells) to try and bring the average values closer to 0. I have DFCO disabled and the logs don't include any COT or PE fuel ratio changes or anything. But really, I'm talking about idle and very low RPM cells here (becuase that is where I see issues in the LTFT histograms).

Thank you!

[TSloper]

The MAF is not the only part of the equation. You still have the VE airmass calculation that is going to change with MAP and IAT changes. At idle and low RPMs the airflow is so small it won't take much error to change the fuel trims. Also, are you refitting the MAF curve or just applying adjustments to specific cells? You don't want a choppy looking MAF curve especially at the low flow areas. I would refit the curve.

[jtrosky]

Regarding the MAF curve, yes, I'm keeping it nice and smooth (not just updating individual cells) - it's very smooth.

You're getting out of my experience and knowledge level when talking about VE and stuff like that, but that is exactly what I was trying to understand - what, if any, other facotrs would impact the fuel trims besides the MAF calibration (besides fuel). In this case, the IATs are very similar (55F vs. 66F, for example), but I don't log any VE-related data or even MAP-related data.

Also, even though I'm posting "generic" questions, my specific car is a V6, not a V8, so not sure what, if anything, that changes. It's a 3.6L LFX direct-injected engine in a 2012 Chevy Impala - bone stock mechanically. Increasing the lower values in the MAF table has helped a LOT, but like I said, while the trims are great sometimes at idle, sometimes I'm still seeing +10 LTFT's. Was just curious why.

I guess my answer is that the fuel trims are going to vary based on other factors at times and you just have to get it as good as you can... :-)

Thanks for the input!

[jtrosky]

For example, here are my low-MAF LTFT averages for two different trips (same tune, same day, within an hour):

LTFT1.jpg

LTFT2.jpg

Obviously, they are very different... Was really curious why...

[Higgs Boson]

with all due respect, there are a lot of tuning guides, threads, etc right on this forum, I suggest you start reading before you start adjusting....even if you start in the GenIII V8 section, then GenIV, then come back to GenV....you will learn the progression of the tune logic but you do need to know airflow based system tuning before you get into torque based, imo.

my suggestion is to read everything and come back with specific questions because right now you are asking how to tune the maf, start with the GenIII sticky with all the How To threads in it. ;-)

[jtrosky]

Actually, in this particular case, it was a suggeston from another forum member (@GHuggins) to increase the MAF tables based on some logs he reviewed. Was stock before that. Like I said, those change were definitely beneficial, but was just wondering why I was seeing different results at different times.

I can read "guides" until I'm blue in the face (and believe me, I have), but sometimes, other more experienced members can help answer something that I just can't get a directly from a guide. I mean isn't that the purpose of this forum - to communicate and learn from one another? It's not like I'm just asking folks to tune my car for me, I'm just trying to learn from the experiences of others, along with research, trial and error, etc.

Anyway, thanks for the input - was just curious why the averages change so much and if it was considered "normal" for them to change without making any tune changes - sounds like it is normal. This "issue" existed with the stock tune and was not created by any changes I made (actually, was helped by the changes I made based on recommendations from another member). Just trying to understand it better.

Thank you!

[Higgs Boson]

if the purpose of a forum was to have a new conversation every time, all of the threads from the previous day would be deleted. they still exist for a reason. not trying to be a d, but I think having some base foundation will help you when asking questions and getting answers, which there is no problem with.

to answer your question, how long are you logging your trims against your MAF table and have you disabled VE when you log? are you resetting your trims every time? how many counts per cell are you getting? there are 1000 questions I could ask which means there are also 1000 answers that could contribute to what you are experiencing.

[jtrosky]

I think you are mis-understanding the reason for my post. I'm really wasn't even asking for help tuning my MAF or fuel trims. I was simply asking whether or not it's "normal" for long-term fuel trims to change so much for a single idle cell from day to day - or even hour to hour. That's all that I'm asking and I think I got that answer a few posts back. :-)

I was originally under the impression that once a cell is "dialed in", that it would stay consistently dialed in. This question even applies to a completely stock tune on a stock car (say the same thing with stock tune).

So like I said, I think my question has been answered above by @TSloper - that something like idle fuel trims are easily affected by other items like IAT, MAP, etc. So in this case, I don't think that there is really anything to "tune" based on that answer - it's just going to fluctuate depending on other (non-tune) factors.

So I think I'm good now. :-) Do you know how long it would have taken me to find that specific answer by browing the existing threads or guides? Could take months! :-)

Thanks againf for the input.

Thanks.