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I have a 2001 Honda Prelude SH (Honda H22A4 engine). I was trying to determine how the reported short/long term fuel trims fit in to the fuel control loop (in particular I was wondering if the long term and short term values were added together to provide the actual applied value -- they aren't).

According to https://www.hondata.com/index.php?route=tech-closed-loop-oxygen-sensors:

The ECU uses the short term adjustment to alter the injector duration, and therefore the mixture, in order to make the oxygen sensor voltage swing around 0.6V.

...

Over time the ECU will look at the average short term oxygen sensor adjustment and determine if the engine is running rich or lean overall. The ECU will alter the long term oxygen sensor adjustment based on the average value of the short term oxygen sensor adjustment.

According to What exactly are fuel trims?:

  • Short-term fuel trims (STFT's) refer to the instantaneous fuel trim correction being applied by the engine management in response to rapid changes in throttle and load.

  • Long-term fuel trims (LTFT's) refer to corrections that are "memorized" by the engine management. This will factor in operational deviations like an leak in the intake tract or degraded MAF sensor.

And some random readings from my car over a period of about 12 minutes while driving:

enter image description here

The descriptions combined with the graph seem to confirm that the reported long term value is pretty much just a running windowed average of the short term value.

The short term value seems to be what's actually applied. The long term value is just the short term average over time.

So my question is this: Does the ECU actually do anything with the long term fuel trim value and, if so, what does it do? Or is it merely diagnostics info for humans?

If the long term value was being used, at least in closed-loop mode, then I would expect short term readings to be centered around 0, but it seems like all action is represented entirely by the short term values, and the long term values are just a report.

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    Oh cool I think I found the original patent on the O2 sensor based closed loop feedback system: docs.google.com/viewer?url=patentimages.storage.googleapis.com/… (also google.com/patents/US3815561 looks like an evolution of it) -- No time now but I think I'll be able to post my own answer here after I read it. – Jason C Oct 4 '16 at 15:12
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    Knee jerk to your question? If it didn't need it for something, why would it store it? From my limited overall knowledge of it, LTFT are the stored fuel memory which stays there when the vehicle is shut off. It is used to allow the engine to run well over a period of time. The STFT are in volatile memory which gets dumped when the vehicle is shut off. I could be wrong, though, so leaving this as a comment. – Pᴀᴜʟsᴛᴇʀ2 Oct 4 '16 at 15:47
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This isn't a 100% verified answer yet. I will search for more direct sources when I have time.

Pᴀᴜʟsᴛᴇʀ2's comment got me thinking about the obvious question:

If it didn't need it for something, why would it store it? From my limited overall knowledge of it, LTFT are the stored fuel memory which stays there when the vehicle is shut off.

In particular, there'd be no reason to persist the value across restarts if it was only for diagnostics.

So my hypothesis is the long term trim is used:

  • As the initial startup value, and
  • As a baseline in open loop mode.

I did some cursory research to see if a typical ECU actually cares about the fuel trim in open loop mode and found this interesting bit of info in a forum post (author blackbird_R/T):

Taken directly from the FSM:

"The values stored in long term adaptive memory [i.e. LTFT] are used for all operating conditions, including open loop and cold starting. However, the updating of the long term memory occurs after the engine has exceeded approximately 170°-190° F, with fuel control in closed loop and two minutes of engine run time."

So it will use the long term fuel trims in open loop such as after the WOT load point, but will only make corrections and adjustments to the LTFT when it's using O2 feedback in closed loop.

This supports that hypothesis and seems reasonable, but I don't have the direct source of that quote yet (I have not yet consulted the Flying Spaghetti Monster for confirmation) and also I have no info for my particular vehicle.

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    To better explain what I was thinking, it is my understanding the LTFT is the modified fuel map for the vehicle after it has been adjusted so the engine will run optimally. The STFT adjusts the LTFT to provide more precise fueling for the engine giving environmental factors and inputs from the various sensors. If you reset the ECU, the LTFT will go back to the base fuel map and will start the adjustment process again to compensate. – Pᴀᴜʟsᴛᴇʀ2 Oct 4 '16 at 16:19
  • @Pᴀᴜʟsᴛᴇʀ2 Yeah; I am starting to wonder if the LTFT is used exclusively in open-loop mode, and the STFT's are the primary action in closed-loop mode and also used of course to calculate the LTFT for open-loop / initial operation. That is, hypothesis: Without LTFT, closed-loop mode would be just fine (with the STFTs), but open-loop would be running purely from factory fuel maps = obviously not optimal. Just a guess. – Jason C Oct 4 '16 at 16:25
  • I think the LTFT is there so the system doesn't have to make the larger changes to adjust ... can make fine adjustments instead, allowing it to be a lot more precise. If you start closer to where you want to be, making the small adjustments through the STFT will happen a lot easier with less fuss/muss. Better performance all around. – Pᴀᴜʟsᴛᴇʀ2 Oct 4 '16 at 16:30
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    FSM has ALL the knowledge, put the strainer on your head and meditate. It will come to you. – DucatiKiller Oct 4 '16 at 17:41

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