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Torque » Torque OBD ECU Scanner » Torque Discussion / Ideas » LTFT

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Author Topic: LTFT
BadEend
Member
Posts: 2
Post LTFT
on: March 3, 2018 (GMT)

The value of the Long Term Fuel Trim is standing on 25%.
This is not correct, because my MIL is not on and the car is driving well.
How is that possible? (clio 3 1.6 16v 2007)

F-150Torqued
Member
Posts: 413
Post Re: LTFT
on: March 3, 2018 (GMT)

You can not say it isn’t correct – “because my MIL is not on” or because “the car is driving well.”

You are probably are just a short time away from a MIL, so I would suggest trusting Torque Pro.

STFT ‘reacts’ very quickly to changes in the presence of oxygen at the O2 sensor. If STFT is more positive than negative (or visa-versa) for a period of time (a moving average), the LTFT is adjusted in the direction of the STFT moving average. If LTFT is then sufficient to correct the oxygen level at the O2 sensor, STFT will again hover around ZERO % which doesn’t affect LTFT further.

That tells you there is a fuel/air ratio problem – but the system is able to correct for it. You won’t get an MIL until it is NO LONGER able to correct for the situation by increasing or decreasing LTFT. (parameters may differ on your vehicle, but my F150 5.4L Triton requires > 25% LTFT _AND_ > 10% STFT to set MIL)

If your LTFT is 25% ‘+’, you could have a small vacuum leak. If it’s ‘-‘ could have over active injector, or slightly high fuel rail pressure, or a stuck open EVAP purge valve drawing in fuel rich vapor from the fuel tank. Whatever – if it gets worse, you’ll have an MIL.

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54371019

BadEend
Member
Posts: 2
Post Re: LTFT
on: March 5, 2018 (GMT)

If the LTFT is on -25% or +25% the MIL is coming in!
That’s a rule in the EOBD.

Capp777
Member
Posts: 2742
Post Re: LTFT
on: March 5, 2018 (GMT)

May be thats true for your vehicle but
for my (our) Fords according to the
OBD Summary documents I’ve read:

Typical Malfunction thresholds…

Lean Threshold: LTFT > 25% and STFT > 5%. (Adding fuel).
Rich Threshold: LTFT < 25% and STFT < 10%. (Removing fuel -%). I believe calibrated thresholds are used internally.

moreause
Member
Posts: 435
Post Re: LTFT
on: March 6, 2018 (GMT)

on my last car it was trigering at 40% at idle …

was doing some sensor testing ..below was not giving check engine

And some newer car don’t even have ltft anymore
on my hyundai with the dealer software i could not see the ltft i had to go in obd to see it

i changed the injector for some 30% bigger one and the car didn’t even care …. ltft were 30% off but no check engine

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