Fuel trims

[Larrylu]

I'm trying to understand what I'm seeing but it's not making sense yet. First I started watching LAMBDA and the readings made sense. Mostly displaying 1 with a slight drop to .75 or .8 at WOT and shooting up to 2.0 on deceleration. Then I added short term trim and found that made sense as it was dancing back and forth between a - 1 or -2 to a + 1 or +2. Then I added long term and I don't know what I expected but what I'm seeing is a positive number usually between 7 and 12 and that instinctively bothers me. Is what I'm seeing normal or am I seeing a chronically lean condition being corrected by the long term trim by adding fuel?

[ecoboostsho]

This is exactly how they should work.  If you were to "reset" your LTFT then your Short term trims would be off by that value.  The LTFT's are learned over time.  The STFT are used to just make small adjustments to the air fuel ratio in real time.  The tune calls for more fuel which tends to make them positive over time.  You aren't lean as long as your AFR is good...

[Larrylu]

Great. I'll just enjoy the display and stop worrying then. Now I have to go out and add some more gauges to look at!  Thanks for the consult.

[FoMoCoSHO]

On  A 30% E-85 I'm seeing my LTFT's peak 20-21ish.

Your numbers are what I saw before E-85.

[StrawHatShinobi]

Dang my LTFT w 16 gal of E85 are about as high as yours w/ E30. Mine peaks in the high teens with E30. I don't think I see as much timing as you do when I get on it though. 16ish maybe

[dalum]

Long term fuel trim is what the ecm has learned it needs to change to meet the commanded programming.  Short term fuel trim is what the ecm is calling for on top of the LTFT.  For example after a fresh flash they will both be at 0.  Start driving and stft can goto +5.  After its at +5 long enough the ltft will goto +5 and the stfu would go back to 0.  If ltft 5 is too rich then stft will go -1.  After a while stft at -1, ltft will eventually lose that 1 going to +4 and stft goes back to 0.

In reality stft flops around a lot and there is a large lookup table of ltft at different input values the ecm has learned.

[FoMoCoSHO]

Long term fuel trim is what the ecm has learned it needs to change to meet the commanded programming.  Short term fuel trim is what the ecm is calling for on top of the LTFT.  For example after a fresh flash they will both be at 0.  Start driving and stft can goto +5.  After its at +5 long enough the ltft will goto +5 and the stfu would go back to 0.  If ltft 5 is too rich then stft will go -1.  After a while stft at -1, ltft will eventually lose that 1 going to +4 and stft goes back to 0.

In reality stft flops around a lot and there is a large lookup table of ltft at different input values the ecm has learned.

Thank you for a simple easy to understand explanation of fuel trims!

[FoMoCoSHO]

Dang my LTFT w 16 gal of E85 are about as high as yours w/ E30. Mine peaks in the high teens with E30. I don't think I see as much timing as you do when I get on it though. 16ish maybe

I am on my second tank at E-30 and now my trims are 17-18 ish.

Fuel mileage is improving as the car continues to learn and trim.

[metroplex]

This topic came up while I was doing research. When tuning Ford and GM MAF systems, I normally look at the combination of LTFT and STFT (LT+ST) and tune based on that. Looking at my stock 2014 SHO data, the fuel trim data clearly shows the engine is running lean on average about 5-6%, probably within manufacturer's tolerances. But this is one area that I really wanted to be able to tune with the EcoBoost V6. For example, if I added an Airaid cold air intake (or any cold air intake), I know it will require retuning to gain the max benefit. On a MAF system, I'd adjust the MAF transfer function. On the EcoBoost speed density system, I'm lost. The Cobb Tuning guide talks about adjust the WOT Lambda table since the system uses a wideband O2 sensor in a closed loop environment (much like the GM LNF), but that table is for ECT vs RPM and there's no correlation for that table with what needs to be adjusted for non-WOT operation. Basically, what really needs to be done to correct for the AFR/fuel trim on an EcoBoost Speed Density system?

[AJP turbo]

You are talking about adjusting the airflow model.....i dont even think the pro tuners know.

You dont need a tune for an intake or even catless downpipes like you would for a maf tuned car

The desired lambda at wot is simple...set it to what you want...when you are at the pedal position that is considered wot then the ecu targets 1.0 lambda

There is no more lambda vs load vs rpm.....once you are wot its simply desired lambda vs rpm vs coolant

[AJP turbo]

Used to be that the tuning process was dialing in air fuel to match what is commanded....that easy with the sho...now the challenge is finding the right combination of switches to turn and limiters to remove and how to command boost where and when you want

[metroplex]

Reading Cobb's tuning guide over and over, it seems that the closed loop system would just try to achieve the scalar stoich (14.0x for my SHO) unless I'm at WOT which it then uses the commanded lambda - like you're saying.

But datalogging my fuel trims, it's pretty apparent my stock SHO is running fairly lean. On average, it's about 5% across the board, but plotting LT+ST across a table of RPM vs load, it goes as high as 11-12% on in the higher load/higher RPM cells. Do I just ignore this with the SHO? I remember back in the day that if the fuel trims are too far off, the system will tend to overcompensate and the swings weren't good.

On my MAF tuned S197 GT, the same datalogged table swings between +/- 5% in the high load cells but on average is within +/- 1-2%.

So I could run an open conical air filter on the SHO and the system will adjust accordingly, even if the fuel trims are really skewed? Or theoretically, no filter or airbox at all, and the system will still compensate?

[AJP turbo]

The only time i would say fuel tuning is required is when u drastically change volumetric efficiency such as cams, intake manifold, turbo size, compression ratio

[metroplex]

So when VE is drastically changed, that is when the speed density tables and the quadratic equation becomes involved?

[AJP turbo]

Probably

Im surprised ur trims are that high.

And actually you arent running lean...it gets corrected with the trims.

And even when you are wot its a form of closed loop....there isnt a true open loop with this