Understanding Fuel Trims
 

The only thread I found useful to help me understand Fuel Trims, the OP had 100 Octane.

I moved from Florida where they have 93 Octane, to Colorado where 91 is the highest. I'm JDL Turbo'd tuned. To make this clear, I have NOT been getting insane Fuel Trim reads like (-28 or +30). However, the trims seem to love being in the Negatives. It does not look normal to me.

Keep in mind that I noticed this issue couple days after I got a revision from my tuner (due to the move) I went for a test drive to see what my Fuel Trims looked like.

On Idle

ST: Lowest: -4.7 Highest: 0.8

LT: Lowest: -4.7 Highest: -1.6

On normal driving:

ST: Lowest: -11.7 Highest: 2.3

LT: Lowest: -3.6 Highest: 0.8

Again my point is that I am heavily on the negative side. I was on 91 octane for around a week before the revision and I didn't see readings like this, this is why I don't think it was just the switch to 91 form 93.

maybe the ecu is compensating for higher elevation?

That's pretty good by my account. I think the temperature correction factors are a bit off on the OEM tune for some reason, so my winter ltfts are negative and summer is positive. I don't think anyone actually messes with temp correction. Colorado is likely a bit colder and dryer than Florida being my point.

Stft will vary quite a bit. Every squirt of gas is a bit different so it takes into account very short term (like 2-4 revolutions) trends and adapts immediately. I don't think twice about anything less than +-15 on these.

Ltft stores longer term trends based on stft. I'm usually ok with +-5 on these. Some people are more picky. Your ltft looks like it might decrease to about -5 or -6 all things being equal, based on your stft range.

Don't worry about idle and low rpms unless they get real bad(10+%).

I'd wait for temperatures to get into the 60 or 70s then see how it looks. If they get below -10, then you should start looking for a solution IMO.

Making another post about octane since it's a different matter entirely. I think you might be referring to 100 RON fuel, which is a different standard than we have in the states. As I understand it, 100 RON is equivalent to 87 ish here.

Octane is, in basic terms, the knock resistance of the fuel. Gasoline still burns at the same ratio to air regardless of knock resistance. Therefore it should not affect fuel trims.

Good information. For the future, what would most likely be the cause of really low fuel trims (like -20s or less)? I have read that if they're really high it can be an exhaust leak.

Also, how should my STs and LTs fluctuate when I'm have a bigger percent throttle. I haven't looked at them while doing a pull.

Large trims in one direction is likely to be due to incorrect maf or other sensor scaling.
Explanation: Under low loads, the engine runs in Closed Loop. This means that the maf sensor sees an amount of air going by, and tells the ECU the amount of air. The ECU gets input from other sensors, and uses that and airflow to determine the amount of fuel to put into the engine. The fuel and air combust to create other gasses. Then, the ECU CLOSES the loop by checking it's work through reading the lambda sensor in the exhaust and comparing to what it expected. It uses the fuel trims to adjust until it gets it right.


Ltft is stored in RPM bands. If your engine is running at 3500rpm, the 3000-4500 ltft value will be applied. It will be applied in both closed loop and open loop situations.

Stft does not get stored, so there's no point in having categories for it. Stft is only applied in closed loop operation.

Meaning that when you do a pull, ltft at any rpm will be what the stored value for that RPM range is. Stft will be 0 after a short delay.
More explanation: If you increase load or rpm past a preset point, the engine will enter open loop operation. Open loop is the same as my closed loop explanation above, except the ECU DOES NOT CLOSE the loop. It simply doesn't care how accurate it is, and ignores the signals from the lambda sensor. It will still apply the ltft because it has it available, but it does not change it. The reason for this is that AFRs are generally more rich at WOT than traditional lambda sensors can accurately measure, so the ECU expects the feedback to be junk anyway.

Hope that helps.

100 ron is between 91 and 93oct

I m quoting everything you said in this thread, everything on point
just for info, one could easily disable LTFTs learnt during closed loop, being applied to open loop... simply put at 0 both "long term fuel trim max" and "long term fuel trim min" ...many tuner do this and I like it too

Another alternative is to always run in open loop. Though I tried that once and my engine was behaving funny.

Or put closed loop afr maps directly at 14.7 on most cells

this way ltft is disabled both in closed and open loop


but i think at least on closed loop, is better to keep it working, since IAT is constantly changing

I would say most 100 RON would be the same/better than 93 octane. Our 99 RON in the UK is roughly 92.5 octane depending on the blend.

I guess depends really on the blend/Country/gas station... i got sometimes really bad 100Ron fuel here. in US they usually get better fuel

LTFT are stored in MAF ranges, not RPM ranges:

https://s9.postimg.org/wnye7fqfj/Scr...t_18.14.25.png

There is no problem if seeing large STFT for a short periode of time. Especially coming in and out of overrun and getting on the pedal. Often it's in unusual driving conditions. As an example going downhill with a very low throttle input (like 1% throttle).

You got to look at a complete log and not extreme values on the OFT screen. If the STFT generally is low in stabile conditions everything is fine. STFT values of +/-15 or even 25 occasionally and for a very short while is normal.

100 RON is 95+ AKI (at least German 100 RON). 98 RON would be 93 and 95 RON is 91. 91 RON is 87 and I don't think sold in any civilized European country for the past 20 years. ;) There is a bit of variation in fuel quality between southern and northern Europe (just like in various areas of the US).

Doh! I haven't looked at them in a few months. That's what I get for posting from my phone and not my PC lol. :bonk: