Supra Forums banner
1 - 3 of 3 Posts

I searched the archives and came up with the following discussion.

From: "Tommy Banh" <[email protected]>
Date: Thu Mar 8, 2001 4:33 pm
Subject: RE: [mkiv] s-afc throttle correction

I will try that.

>From: "Steve Vache" <[email protected]>
>Reply-To: [email protected]
>To: <[email protected]>
>Subject: RE: [mkiv] s-afc throttle correction
>Date: Wed, 7 Mar 2001 11:38:49 -0600
>Mohd is able to control the fuel curve. Just not richen it above stock
>levels. It's doubtful that his is a fuel controller problem.
>50 percent throttle is probably too low. Bryce guessed that the transition
>point from closed loop to open loop mode was somewhere in the 80 percent
>range. That's why I set mine even higher. You only want to modify the
>curve in open loop mode and if it takes an extra couple milliseconds to go
>from 50 - 95 percent throttle who cares? After that you're at 100 percent
>throttle anyway for the rest of whatever run you're making (ignoring the
>quick dips for shifts.)
>Check to see that the throttle percentage varies from ~0 - 100 on the S-AFC
>as it should.
>Assuming so, verify the wiring makes good connections.
>And if all that works, if you have access to another s-afc, connect it into
>your harness (make sure you record their settings and then change them back
>to 0's first.)
>Good luck,
>Steve Vache | [email protected] | 1997 TT Auto BPU+ |
> > -----Original Message-----
> > From: [email protected] [mailto:eek:[email protected]]On Behalf Of Tommy
> > Banh
> > Sent: Wednesday, March 07, 2001 1:20 AM
> > To: [email protected]
> > Subject: Re: [mkiv] s-afc throttle correction
> >
> >
> > Steve,
> > I been haveing problem with my afc , can't control the fuel curve
> > i set the throttle at 40-50 low- high res
> > soI thought that could fix my problem to set it at 49-50 to
> > simple it but it
> > does not help.
> > there is a post about S-afc that doesnot work when dyno ( can't
> > adjust the
> > curve) so I have been playing with the afc and spent a LITTLE of
> > money on
> > dynojet every time i thought that i would fix the problem.
> > Turn out today i still didnot fix it and find out my friend car
> > that have a
> > HFC (Field) fuel controler and it doesnot work either.
> > for those that have a fuel controler get on the dyno to see if your fuel
> > controler work???? we thought it work by using the voltmeter.
> > So far there are three people my friend , Mohd, and myself are
> > having this
> > problem and many more that have not dyno go dyno..
> > ATT: Mohd find out yet????
> > Tommy
> >
> >
> > >From: "Steve Vache" <[email protected]>
> > >Reply-To: [email protected]
> > >To: "MKIV List" <[email protected]>
> > >Subject: [mkiv] s-afc throttle correction
> > >Date: Tue, 6 Mar 2001 15:25:28 -0600
> > >
> > >See the S-AFC manual page 20 for details
> > >
> > >
> > >I recently realized that I had been setting the S-AFC
> > incorrectly and think
> > >that others were probably making the same mistake when they
> > ignored the low
> > >throttle settings. The unit interpolates values between the high and
> > >throttle thresholds. So to avoid affecting low throttle values (and
> > >thereby
> > >fighting the ecu and eventually affecting long term fuel mappings,) one
> > >should set the low throttle threshold to just under the high throttle
> > >threshold.
> > >
> > >I now have my throttle values set to 94 95 (low and high respectively.)
> > >
> > >To recap., set the S-AFC low threshold just under the high threshold
> > >maintain the low ne-point correction values at 0's.)
> > >
> > >Steve
> > >
> > >----
> > >Steve Vache | [email protected] | 1997 TT Auto BPU+ |
> > >
> >
> > _________________________________________________________________
> > Get your FREE download of MSN Explorer at
> >

Get your FREE download of MSN Explorer at

Found this too.

From: "lance" <[email protected]>
Date: Wed Jan 31, 2001 7:30 am
Subject: [mkiv] ECM fuel trim rehash from years ago (VERY TECH, VERY LONG)

Reg Riemer posted on this subject a while back and even linked a few pages
from the Toyota Technical Training Manual regarding long and short term fuel
trim. The following text is quoted from the manual.

Fuel Trim

Fuel trim is a term used to describe the percentage of correction to basic
injection duration based on oxygen feedback. There are two different fuel
trim values which affect final injection duration; long fuel trim (Long FT)
and short fuel trim (Short FT).

Long FT is after the basic injection duration calculation. It is determined
by how closely the fuel system achieves the theoretical air/fuel ratio.

Long FT is a learned value which gradually changes in response to factors
beyond the control of system design. For example, fuel oxygen content,
engine wear, air leaks, variations in fuel pressure, and so forth.

Short FT is an addition to (or subtraction from) basic injection duration
and fuel trim calculation. Oxygen sensor information tells the ECM how
close it comes to the ideal air/fuel ratio and the Short FT corrects for any
deviation from this value.

How Short FT Works

Short FT is a temporary correction to fuel delivery which changes with every
cycle of the oxygen sensor input. Under normal conditions, it fluctuates
rapidly around it ideal value of 0% correction and is only functional during
closed loop operation.

Short FT is a parameter on the OBD-II data stream, that can be displayed in
the Diagnostic Tester. Its normal range is +/- 20%, but under normal
operating conditions rarely goes beyond +/- 10%.

Short FT responds to changes in oxygen sensor input. If basic injection
duration + Long FT results in a lean air/fuel ratio, then Short FT responds
with positive corrections (+1% to +20%) to add fuel or enrich the mixture.
If basic injection + Long FT is too rich, then Short FT responds with
negative corrections (-1% to –20%) to subtract fuel or lean the mixture.

When Short FT is varying close to +/- 0%, this indicates a neutral condition
where the basic injection duration calculation is very close to
stoichiometry, without any significant correction from the oxygen sensor

How Long FT Works

Long FT is a data parameter on the OBD and OBD-II data streams. It is a
more permanent correction to fuel delivery because it is the correction
right after basic injection duration calculation. Long FT changes slowly,
in response to Short FT. Its normal range is +/- 20%, positive values
indicating rich correction and negative values indicating lean correction.

If Short FT deviates significantly from the +/- 10% for too long, the Long
FT shifts, changing the basic injection duration. This shift in basic
injection duration should bring Short FT back to the +/- 10% range.

Unlike Short FT which affects injection duration calculation in closed loop
only, the Long FT correction factor affects the basic injection duration
calculation in both open and closed loop. Because Long FT is stored in
NVRAM and is not erased when the ignition is switched off, the fuel system
is able to correct for variances in engine and fuel conditions even during
warm-up and wide open throttle conditions.


At the same time the ECM is controlling the fuel injection duration using
input of feedback from the oxygen sensor, it is also “learning” about the
amount of fuel correction that is taking place. This learned value (Vf) is
used by the ECM during both open and closed loop operation to help fine tune
the fuel control in relation to engine wear, sensor wear, and air leaks.

Basic Injection Duration and Oxygen Sensor Input

The basic injection duration value is the ECM’s best guess at the actual
injection time necessary to achieve an ideal fuel/air ratio. Generally this
basic injection calculation is very accurate, typically within +/-20% of
what is needed. Once within this range, the ECM can trim the air/fuel ratio
to stoichiometry based on oxygen sensor input. Depending on many different
factors, the amount of correction required for Ox feedback will vary. If
the amount of necessary correction remains relatively small, for example
less than 10%, the ECM can easily adjust the mixture. As the Ox feedback
correction reaches the +/- 20% limit, the ECM fuel correction range becomes
limited. The ECM then uses its “learned memory” to adjust or trim the basic
injection calculation. By increasing or decreasing the basic injection
duration, Ox correction can be held within an acceptable range, maintaining
the ECM’s ability to correct over a wide air/fuel ratio range.

Please don't argue about the content here. It is directly from the horse's
mouth, word for word, I just retyped it because the original jpegs were all
but illegible. Many thanks to Reg for posting it a long time ago, it
certainly helped me understand what is going on inside the ECM and helped me
to develop the great respect that I have for the Toyota engineers that
designed our cars.

'93 TT 6 speed Coupe
1 - 3 of 3 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.