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LTFT and STFT Negative at Idle?

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Old 10 February 2018, 05:22 PM   #1
LiamB
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Default LTFT and STFT Negative at Idle?

Does this look healthy or do I have an issue here? I took a look at STFT and LTFT today. The first half of the graph is at Idle. From a bit of googling a small negative STFT isn't out of the ordinary (correct me if I'm wrong). What about the long term though (-15)? Engine was warm after a short run (coolant temp ~75C)

After idling for a little while I cracked the throttle open and took the revs up to 2500rpm with no load on. That's where the short term shoots up to 20+ one the graph before taking a nose dive to -15 or so. Is this normal behaviour or do I have an issue. I took a look at the vacuum reading and it was a steady 19.3in/HG at Idle, going to 20in/HG with light throttle application.

Any input or advice very much appreciated.

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Old 11 February 2018, 10:16 PM   #2
bludgod
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if your car is a bug/blob/hawk then +/- 15 is the correction maxed out and would usually indicate there's a problem somewhere (though perhaps not depending on the map and setup). The short term correction usually doesn't tell you much so focus on the long term correction as it can still adjust pretty quickly to changes.

if the car is a hatch then they can go as far as +/- 20 and will do so on the regular but still you'd be looking to see as low as possible on the long term trims. Anywhere from +5 to -5 would be ideal but +/- 10 will run without issue.
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Old 13 February 2018, 12:25 PM   #3
LiamB
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Originally Posted by bludgod View Post
if your car is a bug/blob/hawk then +/- 15 is the correction maxed out and would usually indicate there's a problem somewhere (though perhaps not depending on the map and setup). The short term correction usually doesn't tell you much so focus on the long term correction as it can still adjust pretty quickly to changes.

if the car is a hatch then they can go as far as +/- 20 and will do so on the regular but still you'd be looking to see as low as possible on the long term trims. Anywhere from +5 to -5 would be ideal but +/- 10 will run without issue.
Thanks for the reply. The car's a 2005 WRX. Am I right in thinking the total fuel correction at a given time is long term + short term?

The car has been running smoothly, no hesitation or misfiring in the closed loop, no CEL. I've not had it on the open loop since noticing these fuel trims but it was smooth as butter.

I've just had a small vacuum leak repaired and wondered if it was present on the car during mapping or whether it appeared afterwards. I'll ask the mapper if an adjustment is required.
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Old 13 February 2018, 12:42 PM   #4
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yes that's correct as far as I know they are both added together along with any IAT/ECT based comps to injector duty/airflow readings resulting in the total injector pulsewidth for that cycle. However the short term trim will be added into the long term trim if its needed for multiple cycles (so if LTFT is +10 and STFT is -2 then you'll see the LTFT reduce to +8 and hopefully the STFT will remain closer to zero).

The long term trims are then broken up into 4 zones generally they match up with your warm idle, cold idle/startup, light cruise and just before you get into boost (labelled A/B/C/D or 1/2/3/4 depending on what software your using). The ranges can be adjusted in the map so if the cars been mapped before the new ranges might not line up as I've said above.

If you can start the car from cold, warm it up, idle and go for a little cruise with your long term trim staying between +/- 5 then it shouldn't be anything to worry about, though if you've fixed a leak somewhere that was present when mapping it's always good to get a quick check done on the AFR's to make sure they are still in line.
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