Flashscan V7.2 Menu - LS1/LS6 Long Term Fuel Trims

[VetPet]

In order to access the LS1/LS6 LTFT's through the flashscan menu does the car need to be running or do you just have to have the key on and be connected to the PCM? Am I correct in assuming that if you access the PCM after you've driven the car that you'll get the latest LTFT readings? Would you use this information in place of the LTFT MAPs you'd create in scan tool or is this just a shortcut so you don't have to creat a MAP?

Thanks
8)

[Blacky]

Key on, engine off and you can read the LTFT matrix.

The LTFT PIDs' values are obtained by the PCM from the LTFT matrix.
If you log Fuel Trim Cell (FTC) and the two LTFT values, you should see the LTFT value changing each time the FTC changes. That's because the PCM is looking up a new LTFT value.
The LTFT cell values are updated when the STFT corrections become large, which means the LTFT PIDs' values will also be updated in that case - even when the FTC remains constant.

You may not be getting accurate correction information unless you combine the long and short term fuel trims. However, the STFT are only available when the engine is running. And the STFT are skewed by the PCM's closed loop method of maintaining stoich, where the PCM alternates between rich and lean to approximate stoich.

The LTFTs + STFT's will get you closer than guessing alone, but will not be as accurate as using a wide band O2 sensor.
Also the LTFTs and STFTs are not accurate at WOT in PE mode.

Regards
Paul

[VetPet]

Thanks Paul. Would I be correct in assuming that the LTFT table from the PCM would give the same numbers as the LTFT tables I create in scan tool for bank#1 and #2 or is there a difference?

Regards,
Tony

[Blacky]

There will be a difference. For example, just looking at one cell:
Assume it's value is +5 when you start logging and due to short term fuel activity:
- after 100 frames it is adusted by the PCM to +6
- after another 100 frames it is adusted by the PCM to +7
- after another 100 frames it is adusted by the PCM to +8
Then you stop logging - the value in your LTFT cell will be +8

EFILive will have logged the following values:
100 frames at +5
100 frames at +6
100 frames at +7
1 frame at +8
EFILive's cell average would be (5*100+6*100+7*100+8*1)/301=6.0066 which is not the same as 8.

Regards
Paul

[ToplessTexan]

Said another way, the PCM's LTFT values are averages. The values in map you've created are averages of averages.