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Stft & Ltft Q?

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SvtTour98.5-3

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Aug 20, 2002
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Bloomfield Hills, MI
Ok, I read somewhere but now can't find it again. I saw an explanation on the signal polarity of the Short Term Fuel Trims and Long Term Fuel Trims. I remember it said they were the opposite as far rich or lean but don't remember which was which. Can someone please explain?

I think it said a positive STFT indicates lean (fuel enrichment) and a positive LTFT indicates rich (pulling fuel). Is that correct?


TIA,
Dale
 
A positive number means that it is adding fuel, a negative number means that it is removing fuel.

Neg = Running too Rich (Adjust Maf airflow negative)
Pos = Running too Lean (Adjust Maf airflow Positive)
 
Yeah, but I read that the polarity was different between short term and long term.

So what you're saying is that it doesn't matter which one it is.... neg is rich and pos is lean?
 
Off hand, I'm not sure which is which... but one does read lean as positive, the other reads lean as negative. It's in the SCT docs somewhere.
 
SvtTour98.5-3 said:
Yeah, but I read that the polarity was different between short term and long term.

So what you're saying is that it doesn't matter which one it is.... neg is rich and pos is lean?
Click to expand...

You are correct.

Long term is the the fuel that is added to satisfy the O2, the "trend" in fuel trims.

A long term fuel trim of 1.2 would have a STFT of .8 in many cases.

So if I don't get this backward, the long term tells you what the computer is trying to do, the short term tells you where it is at now. So in the above example the STFT is 20% lean. THe LTFT is showing the pcm is adding 20% fuel to compensate.
:eek: (if I didn't get it backward....since this is right from memory! lol)
 
Ok, there are no negative values then. Both are based from 1.0 as being 14.67 AFR then.

So, LTFT is sorta based off of lambda then and anything higher is lean, anything lower is rich? Or it's got nothing to do with lambda?
 
i've always know it to be short term is adjust the A/F ratio as its going in the cylinder to satisfy to o2 and longterm is after if has adjust for the current driving cycle and for the values neg is removing and pos is adding how they get the values is something i dont remember
 
SvtTour98.5-3 said:
Ok, there are no negative values then. Both are based from 1.0 as being 14.67 AFR then.

So, LTFT is sorta based off of lambda then and anything higher is lean, anything lower is rich? Or it's got nothing to do with lambda?
Click to expand...

Not exactly. It is based off of the commanded A/F at that particular point. So it could be lambda or it might not, althought it won't be really far off from it. e.g. 15:1, or 14:1

You would need to know the base fuel table setting commanded in percent lambda. It varies with the company, some are in A/F some are in lambda = 1.0 (like SCT) and so on.
 
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