Stft & Ltft Q?

[SvtTour98.5-3]

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

[Barge]

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)

[SvtTour98.5-3]

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?

[Barge]

Unless something's changed from when I was tuning they are both the same.

[morbid]

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.

[Barge]

Well that's just stupid.

[warmonger]

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?

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.
(if I didn't get it backward....since this is right from memory! lol)

[warmonger]

Well that's just stupid.

Tut tut....there is a method to the madness...

[Barge]

Tut tut....there is a method to the madness...

Yeah.. to confuse everyone.

I'm pretty sure with the TwEECer they were both the same direction.

[SvtTour98.5-3]

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?