MarineBlue03GMC

I have noticed that when monitoring my 03 5.3L sierra with my predator there are measures of the LFT's (I think thats what they were called) short term and long term.

In fact there are 4 total measurements. Does anyone know why there are 4 measures?

I know that these LFTs and LTRMs have something to do with fuel.

From what I read on a forum post you want them to be at 0 or less right? What numbers are best?

IF my LFTs are higher then they should be how can I lean out the fuel with my predator?

Can I DECREASE the fuel enrichment? Would I do this till I see knock retard?

My truck is an 03 and only has about 5800 miles. I only have an air filter and exhaust with AS&M headers soon to come. I wouldn't think the truck would be running to rich or lean BUT what do you all think? What about when I get headers? Do you guys think this would call for some fuel ENRICHMENT?

I would appreciate any info.

jimmyblue

iTrader: (11)

LTFT = Long Term Fuel Trim (aka LTerms etc.).
For closed loop operation, the PCM keeps track
of how much additional fuel is needed, on a
rolling average, to satisfy the mixture=14.7
(average) requirement (as 50% duty cycle on the
O2 sensor about 0.5V) for whatever "cell" the
engine is currently operting in. This, based
on RPM, MAP, load calculations, MAF airflow.
If the "open-loop" mixture calculation produces
a lean result, the O2 feedback loop will correct
by adding fuel (positive LTFT). If the calculation
is producing a richer-than-stoichemetric result
then you will have fuel subtracted (negative
LTFT).

Now, how this matters is, when you leave closed-
loop for WOT (an open-loop region of operation),
you are left with one of two possibilities. If
your LTFTs were zero or negative, the engine is
tuned open-loop-rich and the PCM is OK with that,
and just sets the LTFTs to zero. So you "run what
you brung", mixture-wise, and hopefully it's
something like 12.8:1 AFR so you get near max
power output.

Now, if you had positive LTFTs, the PCM knows you
tended to run lean without closed loop correction
and you need help. So it helps you by adding the
last-known-LTFT, and you will see this value at
WOT. Thing is, you needed this just to get to
14.7:1, which is well down from power peak and
more prone to ping than a nice fat mix. And, the
LTFT changes day by day, and if you tuned it up
with all of your bolt-on fiddling Friday you
might well be a few points north or south come
Saturday. That's no way to dial in any consistency
and you can chase your tail over pinging forever.
You don't need that kind of help, you need the
kind that gives you a naturally, slightly rich
mixture.

There are several ways to arrive at this, but I
will talk only about the non-software side. The
"mechanical" approach uses the PCM's "natural"
computation but fools (or not) some aspect of
its input. For example, the MAFT convinces the
PCM that there is more airflow than actual, the
PCM computes more fuel, the O2 sez "Look, man"
and after learning starts to subtract fuel. LTFT
less than zero, richer than natural open-loop.

Or, you install 28# injectors instead of the 26#
ones. 8% additional fuel flow per unit pulse
width; the PCM computes the same fuel demand as
before, but gets that much more; LTFT would move
negative by about that 8%.

When you have these sorts of consistent scaling
applied, externally, it's a pretty good bet that
the (purposeful) error they introduce, will also
apply at WOT. So you can backyard-tune the car to
a pretty decent mixture by simple means and some
small cost, without a computer (as long as you
have some sort of quantitative performance
feedback to illuminate your cut-and-try).

Now, if you stick your fingers in the software
guts and start individually fiddling MAF or
injector scaling values, the LTFTs are no longer
a solid predictor of what your WOT mixture will
be. Point-specific changes just don't have the
general influence over far-away regions of
operation. If you scale the whole table uniformly,
then, yeah. Still need to get the LTFT to be a
non-actor so you can tune against a consistent
basis, so figuring the LTFT might wander around
by a couple of points, you need to bias it below
zero just so you don't have to play "Whack-a-Mole"
with it later.

A LTFT of only a couple of points negative is fine
in that respect, but it does not predict a happy
WOT mixture. Peak power comes in at 12-18% rich
(14.7/12.8=1.148). Your -5% LTFT, if everything
holds true, only puts you to about 14:1 AFR. Not
rich enough for maximum smiles per gallon. So you
want either a second, WOT "****" to tune up the
open loop to suit, or you want to bias the mixture
so far down that you see -15% LTFTs (so that, when
you get down to it, you have the mixture you like).
This will probably cost you some mileage, since
you get this richness during every throttle
transient (possibly good, it will for sure cover
any transient lean-out from airflow step, but you
will burn more than you need in light mixed
driving).

STFT is short term fuel trim, and it's just the
momentary O2 correction term that jitters around
based on last reading to make up fuel and, when
averaged up, produces the LTFT.

The LS-1 PCM controls cylinder banks separately,
so you have two LTFTs, two STFTs (just like you
have two pairs of O2 sensors).

MarineBlue03GMC

Wow thanks for the SUPER in depth answer!

Now a few more questions. I only have two engine bolt ons, a corsa 3" exh. and a UPD intake. I will be getting a set of AS&M's within the next few weeks. Anyway do you think there is ANY way I would need to richen up the fuel mixture with these mods? I have an 03 so technically everything should be running alright.

I remember NEVER seeing anything over 0 (zero) on the LFTRM when at WOT, so is this cool?

Since I have a predator tuner I can basically subtract or add to the fuel enrichment measures, on a -25% to +25%.

I am pretty familiar with fuel systems, but mainly on Imports, like my Accel DFI equipped turbo honda. I KNOW I dont need a new fuel pump and since I dont really want void my warranty fuel injectors are probably not a good question.
So my options are pretty much limited to enrichment with the Predator. So what should I do?