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Fuel Trim Chart Positive or Negative Numbers

  • randy61
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11 Sep 2019 19:28 - 11 Sep 2019 22:12 #33548 by randy61
According to the chart 128 is neutral. I have a snapon MT2500. When my o2 is unplugged the ST and LT Fuel Trims stay on 128. When o2 is plugged in my STFT bounces between 0 and 40 give or take some. But the numbers do not show a plus or a minus next to the numbers. My LTFT has gotten down to 62. Again no plus or minus sign by the number.

QUESTION:
Because my numbers are below 128 on a scale from 0 to 255 does that mean my Fuel Trim numbers are considered Negative or Positive according to the chart?

(Note) As described above when my STFT hits 0 and bounces up to 40, where in the chart is 0?

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Last edit: 11 Sep 2019 22:12 by randy61.

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11 Sep 2019 21:01 - 11 Sep 2019 21:17 #33554 by John Curtis
Replied by John Curtis on topic Fuel Trim Chart Positive or Negative Numbers
Below 128 is subtracting fuel, above 128 is adding fuel.
62 LTFT would indicate the 02 sensor is leaning out meaning you have a rich condition. and the computer compensated for this be subtracting fuel.

Time to find the cause of the rich condition.

Thinking out loud always helps me in the process.
Last edit: 11 Sep 2019 21:17 by John Curtis.
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11 Sep 2019 22:43 - 11 Sep 2019 22:44 #33556 by randy61
Replied by randy61 on topic Fuel Trim Chart Positive or Negative Numbers
The only factor I didn't mention was that I made some repairs and the LTFT has been decreasing, I think due to ECU relearning.

I don't suppose that because the LTFT has been decreasing that it makes any difference regarding a rich condition, as far as it correcting itself. Would that be incorrect to make that assumption?

Thanks again John.
Last edit: 11 Sep 2019 22:44 by randy61.

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11 Sep 2019 23:26 #33557 by John Curtis
Replied by John Curtis on topic Fuel Trim Chart Positive or Negative Numbers
After a repair your short term fuel trim should be on the opposite side of the long term to get the value as close to 0 or 128 as possible 118 to 138 is a good range.


Example
Before repair of a lean condition it might look like
LTFT +17
STFT + 3
Total fuel trim = +20

After repair it would be something like this
LTFT +17
STFT -15
Total fuel trim +2 (+/- 10) on a good fix

Thinking out loud always helps me in the process.
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11 Sep 2019 23:28 #33558 by John Curtis
Replied by John Curtis on topic Fuel Trim Chart Positive or Negative Numbers

randy61 wrote: The only factor I didn't mention was that I made some repairs and the LTFT has been decreasing, I think due to ECU relearning.

I don't suppose that because the LTFT has been decreasing that it makes any difference regarding a rich condition, as far as it correcting itself. Would that be incorrect to make that assumption?

Thanks again John.



The LTFT will take some time to get back into range. But total fuel trim is the key indicator and confirmation of a fix

Thinking out loud always helps me in the process.
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