Fuel Trim Won't Adjust!
 

My LTFT numbers are a +8, in the IFR table I took out -.02 to start and it droped only slighty. Than another -.02 than -.04. My trim would not adjust past 6.3. Than I took out -.2 and still no change in FT at WOT, idle and cruse changed. I replace the fuel filter and cleaned the injectors, still no difference!
What am I doing wrong?

Also with another persons tune (LTFT was 0) about 2 months ago under WOT @ the track the trans would go into limp mode and shift 1 2 3 real fast, take the tune out, it work fine, what gives? I realy don't know all of his changes.

Jason

I think the injector flow table wants multiplicative scaling
(*0.98, not -0.02) since the flows are maybe going to
vary a lot and you are trying fix a scaling error (LTFT
is expressed as +8, but this is really saying you need
1.08X the fuel that the base computation delivers, so
-dividing- the table by 1.08 would put it roughly to zero
LTFT (1.00X).

Check it: What does (orig value / 1.08) give you, relative
to the values there from your series of changes? A lower
value should produce a richer result.

But, not knowing the particulars, IFR might or might not
be the right thing to be messing with. If you messed
with the MAF then maybe it's the MAF table.

Just trying other peoples' tunes w/o any knowledge is
like borrowing someone else's used rubber.

scale your MAF table by 92-94% or so. that will put you where you need to be.

If no changes were made to the MAF I wouldnt mess with that table. Use your IFR table to scale back your fuel trims. IE 93% and click the mult button. The MAF table does nothing but skew and trick the computer into thinking more or less air is entering the engine. I definately wouldnt mess with it unless you ahve changes the properties of your maf ie (port or bigger MAF)

ok. can someone clarify things here? i have heard of changing your LTRIMS both ways, MAF and IFR. now, why would you change your IFR tables if your injectors/pressure haven't changed? the flow rate of an injector at a given pressure should be a constant across the board unless you are using a FMU. check this out. would this not be true: when you change a certain part in the engine ie. camshaft, heads, internals, even headers, you are certainly changing the amount of air that the engine uses/requires. so, why would you not make air/fuel adjustments in your MAF tables? another example. i now have a 408. the old motor was a 4.8. slight difference! you mean to tell me that the same amount of air is flowing into the 408 as the old 4.8 and i should tell the ECM that it has smaller injectors instead? i don't buy it.

anybody else? not try to start !*#!$#, just get solid answers.

MAF data is used by other things (like transmission
shifting and pressure control). So, changing the MAF
affects this, and the changes people usually make are
ones that make for softer line pressure and slower apply
under load - bad.

MAF also affects spark timing. This is where the "seat of
the pants power gains" come from - at least at part
throttle, at least until you start to pick up KR.

Getting the MAF to match the GM frequency/flow curve
makes your transmission shift like stock, which is at least
better than sanding the clutches on every half-throttle
upshift.

IFR table is a single-target "knob". It only affects mixture
and this makes it a friendlier tuning method (no side
effects).

But, if the MAF is miscalibrated (or "calibrated to improve
performance" as the people who sell them like to say)
then fixing it with the IFR table leaves the other
components working from bent data. Not much of an
issue for an M6,but can be life-shortening for an A4.

Flow rate across the injector depends on the pressure
drop across it. Line pressure fades with RPM / delivered
flow on these cars due to the regulator being 10 feet
from point-of-delivery, wiring drops. The PCM has no
direct fuel pressure measurement and embodies some
assumed pressure/flow estimates. To any extent that
your reality disagrees with GM's ideality, you have a
fuel pressure error (varying) in the fuel shot calculations.
IFR is your direct handle on this.

When you change an air-path component, the MAF just
measures it like before and gives a different result. This
is why MAF setups are bolt-on friendly - speed-density
mode needs the volumetric efficiency map updated for
any significant intake or exhaust, cam or head change.
Note that the LS1 still needs this too in the operating
regions where SD still rules (idle, cold start). The MAF
table only needs updating when the MAF is not the same
as was delivered with the car.

THANKS! i'll buy that now. so then the stock MAF table shouldn't change if you are still using the stock MAF? what kind of changes should be made to the VE tables to compensate for various mods? such as my 408 w/blower/cam/headers/etc? that would be a great explaniation too if you have any strength left in your fingers.

I'm not any kind of expert on that, but the VE of the
motor at low speeds really decreases when you have
a high-overlap cam because you can have both valves
open at once. I know I've seen some rules of thumb
for scaling the bottom couple of VE cells posted, but
paid them little mind (since no heads/cam/headers are
in my near term plan). Once the motor is up past 1K
the VE gets pretty back to normal I reckon. You might
find more about this in the Internal section since it goes
mostly along with cam swaps.

As for the blower, you may have other interesting issues
like the MAP sensor only working up to 1BAR
(atmospheric?) and maybe making the top end S/D
fuel calcs suspect, while the MAF also has an upper
frequency limit. No idea whether any of this was addressed in the blower install. But if you had enough
boost to go "over the top" on the MAF, I'd expect it
ain't good for fuel management. Seen discussions of
this sometimes in the Forced Induction forum. Most of
that kind of stuff (mod-specific tuning) seems to stay
on the mod-related forums, looks like to me.

Its a A4 SY4000 3.42 stock internal bolt on car. The MAF is stock and the table has not been changed.
(*.98 is to add or -.98) is what I need to do?

And no, I ain't using no ones used rubber !! lol

Thanks Jason

Another quick thing to try is to clean your MAF wires.

* = multiply. But, if your LTFTs are +8 (consistently
across cells) then I would divide by 1.08, or multiply
by 0.92. The 0.98 was just a for-instance. That will
tell the PCM that the injectors deliver 8% less fuel
than it thought before, so it will increase pulse widths
the same amount (roughly) to make it back up and
the result will be a richer delivery.

But, having messed around with it a few times, you
might want to go back to the stock tables first and
go from there. If you have a logger that will give you
LTFTs by cell, you could do a less shotgun adjustment
but for now, I'd go back to the start and start scaling
by multiplier. Maybe by 0.90 (-10%), to get you a little
negative margin.

The MAF was SLIGHTLY ported! Put a stock end on and down went the LTFT! Thank you guys very much, jimmyblue it took me a min. Not a math wiz!!
Having fun learning.

Jason