Question About Fuel Trim

Trying to learn here . . . bought an OBD2 scanner to ID an MIL (which later went away) and got drawn into learning about fuel trim. My numbers are out of whack, but my questions are:

  1. Is long-term fuel trim a running tally/average of the constantly changing short-term numbers?

  2. If so, why would LTFT numbers be consistently higher/lower than STFT?

  3. And, in my vehicle, specifically, I notice that LTFT numbers drop from -15 to 0 when the RPM increases from IDLE to 2,500 rpm . . . which undermines my understanding of LTFT being a running tally of the STFT?

Not too concerned about the numbers b/c there are no codes (for now), just trying to learn - thanks.

  1. Yes, that is the way I understand it, but it isn’t a very, very long running average so you will see it change.

  2. The STFT changes pretty rapidly and you don’t always catch all of that in a digital display. If you plot them out as a graph over time, the peaks and valleys become more noticeable and the LTFT makes more sense. I’ve been fiddling with this with my own cars for a while and this is what I have seen

  3. Running -15 LTFT (running rich, the computer is leaning it out to meet the A/F ratio) at idle and then rising to LTFT 0 at 2500 RPM tells me one or more of your injectors may be leaking. The ECU is removing fuel because the leaking injector(s) is adding un-controlled extra fuel at idle. At 2500 RPM, the fuel requirement goes up quite a bit so any leak or dribble becomes inconsequential to the overall mixture. Since the ECU can fix it, it does and no CEL is thrown.

I’d try a bottle of Techron to the next tank of fuel you run and see how it affects the LTFT.

I have the opposite on my truck, slightly positive LTFT at idle to +15-17 at 78 mph on the highway. Seems as if my fuel pump can’t keep up with the demand.

The numbers are not the actual A/F ratio. The PCM calculates the amount of fuel that should be injected to achieve the optimum 14.7:1 ratio based on inputs from the MAF and/or MAP sensors. The O2 sensor in front of the cat senses the amount of oxygen left in the exhaust and the PCM makes adjustments to fuel injectors to correct any deviation.

The PCM compares the actual fuel needed to the calculated amount and that is the fuel trim. If the fuel trim is consistently a certain percentage above or below a preset value, a code is set and the CEL comes on. Your engine is NOT running lean or rich, the O2 sensors make sure of that, but the code is telling you that something in the engine is causing it to use more or less fuel than predicted.

My understanding concurs w/the posts above. The long term is some version of a time average of the short term. The averaging algorithm could be designed in different ways, depending on what you wanted to accomplish, so comparing short term to long term might produce unexpected results for any particular experiment. It seems like the two should be more or less consistent for an engine (already warmed to operating temperature) idling in the driveway and nobody is messing w/the accelerator pedal.

Differences between long term and short term fuel trim would be especially noticeable during experiments where the throttle valve and engine load were rapidly changing.

As posted above a -15% fuel trim means the computer is finding it needs to inject 15% less gasoline for that particular operating circumstance to meet the o2 sensor requirement of a proper a/f ratio, compared to the amount it thinks it should inject ignoring the o2 sensor and based (mostly) on the maf and engine coolant temp sensors. A -15% fuel trim could be caused by the injectors injecting more gas than the computer thinks it is injecting (too high of fuel pressure, faulty or sticking injector) , or the actual air flow into the engine is lower than what the maf sensor is saying (I think that could happen if the maf’s wire was coated w/gunk), or the engine coolant temperature isn’t what that sensor is saying. A -15% fuel trim might be entirely normal, if it occurred in response to quick throttle angle change.

In other words the engine-control computer is constantly reading the o2 sensor(s), the maf, and the ECT and it feels those readings should be self-consistent with each other. When they aren’t the computer uses the o2 sensor by itself, which insures the correct a/f ratio, and shows its cognitive dissonance displeasure by outputting a non-zero fuel trim value.

That must be the very first time those 3 words have ever been used together on this site! I tip my hat to George!

Edit: It seems “cognitive dissonance” has been used several times before but not with displeasure!

Ethanol content also affects fuel trims, so it seems.
Once I had trims around 15%. I was concerned because CEL trigger level for that engine was 25%.
I thought trouble was brewing, checked for vacuum leaks etc. Couldn’t find anything wrong.
Next tank of gas trims fell back to single digits.
Gas in my area has “up to” 10% ethanol.

How old was your vehicle? Ethanol mixes have been around for decades now, and I remember my 94 Saturn Sedan’s owners manual said gasoline up to E10 would cause no damage to the system or require any adjustments…

I didn’t say anything about damage or adjustments.

my mistake, I misread it earlier :slight_smile:

A comment about E10. If the engine was tuned with E0 and the calibration tables built with E0 in mind and E10 is used, the A/F ratio needs to be lower (to reach Lamda = 1.0) since Ethanol has less energy per gallon. The ECU will add more and the LTFT should increase - richer - to compensate. It doesn’t need to move much, but you will see it.

Let’s see… pure gasoline stoichiometric A/F ratio is 14.7, pure ethanol is 9.
So ethanol is about 63% richer, E10 therefore should change LTFT about 4%.

1 Like