Understanding: Fuel Trim: Long Term, and Short Term Fuel Trims (LTFT and STFT)

Created by Anthony Cardinale, Modified on Wed, 28 Nov 2018 at 11:41 PM by Anthony Cardinale

Fuel trims are supposed to move around a bit. This is normal, and is what they are designed (and intended) to do.


Trims that are within +-10% are typical, and should be expected.


There are some moments where fuel trims may swing substantially. These moments include:


  • Quick throttle stabs
  • Throttle lift off
  • Accelerating up hill
  • Coasting down hill
  • .... and more


Remember: Even trims that are showing +-12% to 15% can still be considered OK.


Fuel trims can never be looked at as a snapshot. This means that you should never focus on only ONE instance of a single fuel trim (either STFT or LTFT) at one RPM (moment).


Recognize this: Watch your STFT and you LTFT and add them together.


 LTFT + STFT = Total Trim


Example: LTFT is +20, while STFT is -20; when added together, then total trim at that instant total fuel trim is +-0 = perfect


Total trim understanding is whats actually important. When discussing fuel trims, look at fuel fuel trim trends (history/histograms) ... not instantaneous fuel trim moments.


Example: Histogram showing TOTAL FUEL TRIMS, vs RPM and LOAD -- This trim history of combined LTFT/STFT shows this highest average was ~+7%, and ~-6% = within 10%



Below, you will find an example from the SAME datalog that shows how reviewing only an instant of fuel trim data can make you believe there is a fuel trim issue -- a fuel trim issue that doesn't actually exist.





For additional education find the histogram for ONLY LTFT below, and then STFT, respectively as comparison.




Datalog attached to this post for additional review as you please.


~Anthony

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