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I have seen many people on the forum saying good things about new O2 sensors...which O2 sensor are they talking about? is it the 1 after the turbo in the exhaust manifold? or those wideband O2 sensors? I thought it only effect low speed/idle and not at WOT.

anyone care to explain?

Jonas
 

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They mainly effect cruising and part throttle. The 2nd O2 sensor in the DP isn't really used, its a mystery to me as to why OBD1s have it. Its the first sensor that the ECU uses.

The follow text is quoted from Toyota about how the sensor works in effecting part throttle AND wot a/f:

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
input.

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.

LEARNED VALUE (Vf) OR FUEL TRIM

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.
 
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