Toyota training course t852 engine control systems sec06

Technician Handbook
874 Engine Control Systems II

Fuel Trim Overview

Fuel Trim Basics

The ECM must change the amount of fuel injected into the engine
based on operating conditions. At lower engine speeds and loads,
less fuel is needed to run the engine at a theoretical stoichiometric
mixture (14.7:1 for gasoline). As engine speed or load increases,
more fuel is needed. A basic fuel map is included in the ECM logic
and is based on a 0% fuel trim, or basic injection duration. As the
engine ages or when issues occur (vacuum leaks, misfires, etc.),
the ECM must adjust the amount of fuel injected to keep the engine
running at a theoretical stoichiometric mixture. The engine adjusts
the fuel injection duration based on the A/F or O2 (S1) sensor
signals (oxygen content of the exhaust gases). This adjustment is
known as fuel trim.
•  The ECM uses the MAF, Throttle Position and Crankshaft
Position (RPM) sensors to calculate basic injection duration.
•  The ECM uses the front (S1) A/F or O2 sensor(s) to make
corrections to injection duration in order to keep the engine
running at a theoretical 14.7:1 (for gasoline) (stoichometric) air/
fuel ratio during most engine operating conditions such as idle,
cruising, etc.
•  Feedback from the O2 (S1) or A/F sensor (S1) influences
short-term fuel trim, which influences long-term fuel trim.

Technical Training

91


Technician Handbook
874 Engine Control Systems II

Calculated Load
(Calc Load)

92

Calculated Load represents volumetric efficiency at a given throttle
position & RPM. It is a measurement of intake manifold pressure.
Some early model vehicles were equipped with a Manifold Air
Pressure (MAP) sensor, but today this value is calculated from
various other sensor inputs. This measurement is very important in
determining the amount of fuel needed to maintain a 14.7:1 air fuel
ratio at all operating conditions. This calculated value is compared
to the actual volume of air passing through the intake measured by
the Mass Air Flow (MAF) meter. Calculated Load is reported on
Techstream as Calc Load and can be very useful in diagnosis. It is
important to remember however, that Calc Load is calculated or
learned over time and must be referenced to a known good vehicle
under the same operating conditions.

Technical Training


Technician Handbook
874 Engine Control Systems II

Stoichiometric Air/
Fuel Ratio

Technical Training

The engine runs most efficiently and produces the least amount of
emissions when fuel and air are combined and combusted within
the combustion chamber at a stoichiometric ratio. This ratio is 14.7
parts air to 1 part fuel, by weight for gasoline. Using the A/F or O2
sensor(s) (S1) to measure exhaust oxygen content, the ECM will
adjust the amount of fuel injection duration (fuel trim) to meet this
stoichiometric mixture at all times in closed loop. This will ensure the
engine is running as efficiently as possible.

93


Technician Handbook
874 Engine Control Systems II

Fuel Trim

As an engine ages and operating conditions change, the ECM
needs to adjust the amount of fuel injected to keep the engine
operating at the theoretical 14.7:1 air/fuel ratio (for gasoline) for
peak catalytic converter efficiency and reduced tailpipe emissions.
To do this, the ECM uses a “feedback compensation value” —
otherwise known as “fuel trim” — to correct the injector duration.
The ECM determines how much air is entering the engine and then

injects the appropriate amount of fuel. By monitoring the exhaust
sensors [oxygen (O2) or air/fuel (A/F) sensors] the ECM can verify if
the correct amount of fuel was delivered. If the exhaust sensors
indicate either a lean or rich condition, the ECM will increase or
decrease injection duration accordingly. The TIS Techstream fuel
trim value is the percentage of correction the ECM made from its
initial estimate of injection duration.

Basic Injection Duration
and 0% Fuel Trim

The most important inputs to the ECM concerning the basic injection
duration are the engine RPM signal (NE) and the air quantity as
determined by the mass air flow (MAF) sensor. To help increase the
accuracy and “fine tune” its initial calculation, the ECM will use
injection corrections to accommodate for different operating
conditions. This makes up the initial injection duration and is
represented by a 0% fuel trim value on the TIS Techstream. If the
exhaust sensor voltages are what the ECM expects, the initial
injection duration will not change and fuel trim percentages will
remain at 0%.

94

Technical Training


Technician Handbook
874 Engine Control Systems II

Fuel Trim DTCs

•  P0171, P0174 — System Too Lean: When the A/F Ratio
feedback is stable after the engine is at operating temperature
and the fuel trim has reached a predetermined limit of
correction to the rich side. (Two trip logic)
•  P0172, P0175 — System Too Rich: When the A/F Ratio
feedback is stable after the engine is at operating temperature
and the fuel trim has reached a predetermined limit of
correction to the lean side. (Two trip logic)
When the sum of the short-term and long-term fuel trim
percentages exceeds a certain value, a Too Lean/Rich DTC will set.
Refer to the Repair Manual for more information on fuel trim DTC(s)
and malfunction thresholds.

Technical Training

95


Technician Handbook
874 Engine Control Systems II

Closed Loop and
Open Loop

When the ECM is correcting injection duration based on the exhaust
sensor response, it is known as “closed loop” fuel control. When the
ECM ignores the exhaust sensor information, it is known as “open
loop.” Loop status can change for many reasons and may be

displayed in different ways on the Techstream.
Under normal conditions, the ECM will be in open loop until the O2
or A/F sensor (S1) reaches operating temperature. Once the O2 or
A/F sensor (S1) reaches operating temperature, the ECM will go
into closed loop. A/F sensor equipped vehicles (S1) will enter into
closed loop much faster than O2 sensor (S1) equipped vehicles.

NOTE

96

Certain DTCs will suspend fuel trim control / monitor. Refer to the
Repair Manual for more information on what DTCs will suspend the
fuel trim monitor.

Technical Training


Technician Handbook
874 Engine Control Systems II

Positive or Negative
Fuel Trims

If the fuel trim value is positive, the ECM concluded that the engine
was running lean based on exhaust sensor readings and increased
the injection duration on the next injection/combustion cycle. If the
fuel trim value is negative, the ECM concluded that the engine was
running rich and decreased the injection duration on the next
injection/combustion cycle.

Short-Term and
Long-Term Fuel Trim

The short-term fuel trim value is the immediate amount of correction
the ECM is making when it is in closed-loop fuel control. The longterm fuel trim value is a learned value that is based on the shortterm trim correction. Long-term fuel trim is part of the engine’s basic
injection calculation and is applied during both open and closed
loop. Additionally, the engine may require a different amount of
correction depending on load and RPM. The ECM will learn different
long-term trim values for different engine conditions. The
Techstream will display the long-term trim percentage that is
currently being used. This is why, if you change the RPM, you may
see the long-term trim percentage change suddenly. Long-term trim
is stored in the ECM’s memory until power is disconnected from the
ECM for 60 seconds.
To determine the total amount of correction the ECM is making, add
both the short and long fuel trim percentages together. As an
example, if the short-term value is -10% and the long-term value is
+15%, the total correction is +5%. When the percentages exceed a
certain value, a Too Lean/Rich DTC will set.

NOTE

Technical Training

Abnormal fuel trim values do not necessarily mean the engine is
actually running rich or lean. Changes in fuel trim mean the ECM
had to make corrections to get the engine to run properly.

97

Technician Handbook
874 Engine Control Systems II

Feedback-Related
Issues

If there is a DTC without significant driveability issues, the problem
is not likely to be in the feedback system. If the ECM’s initial
injection duration calculation is not accurate, then based on the
exhaust sensor signal the ECM will judge the condition to be rich or
lean and correct the condition. An example is an incorrect air flow
measurement. Remember that when the ECM correctly detects a
lean or rich condition, it will make adjustments to the injector
duration, and therefore the spark plug readings, exhaust sensor
response, and engine performance will appear normal.
If the vehicle has a DTC and a noticeable driveability issue, a good
place to start is with the basics: clean fuel at the proper pressure,
good compression, and good spark.
Concerning the feedback system, if there is an exhaust sensor
problem, the ECM will receive incorrect information about how the
engine is running; therefore, the ECM will increase or decrease the
injection duration when the engine actually does not need it.
The actual engine running condition will be the opposite of what the
exhaust sensors and fuel trims are indicating, creating a driveability
issue.
Typically, an AF sensor stuck high (lean) will add excessive fuel to
its corresponding cylinders.
If the feedback system is operating correctly, then a severe sensor

or mechanical issue that the ECM cannot compensate for may be
the cause of a driveability issue. Some examples are lack of fuel
pressure or a heavily contaminated MAF sensor.

98

Technical Training


Technician Handbook
874 Engine Control Systems II

Mass Air Flow
(MAF) Sensor

The Mass Air Flow (MAF) sensor converts the volume of air drawn into
the engine into a voltage signal. This signal significantly impacts the
ECM’s calculation of engine load. Engine load determines how much
fuel to inject, when to ignite the air/fuel mixture, and when to shift the
transmission.
The MAF sensor is located directly in the intake air stream, between the
air cleaner and throttle body.

Operation

MAF sensors use the “hot wire” principle which depends in part on a
temperature sensor, also known as a thermistor, separate from the
intake air temperature sensor. The MAF sensor contains:
•  A thermistor, which changes its resistance in relationship to
temperature

•  A platinum hot wire
•  An electronic control unit
•  Intake air temperature (IAT) sensor
The thermistor measures the temperature of incoming air. The hot
wire is maintained at a constant temperature by the electronic
control circuit.
•  An increase in air flow will cause the hot wire to lose heat quickly.
•  The electronic control unit compensates by sending more current
through the hot wire.
•  The electronic control unit outputs a voltage signal (usually VG) in
proportion to hot wire current, allowing the ECM to determine
engine load (as a function of the volume of air entering the engine).

Technical Training

99


Technician Handbook
874 Engine Control Systems II

Airflow-Free VG Check

First check in fuel trim diagnosis is to determine proper MAF sensor
operation. A simple check to determine possible abnormal sensor
signal is the Airflow-free VG Check found under Mass Air Flow
Sensor component inspection section in the Repair Manual. The
test is used to judge baseline sensor output voltage with the engine
off. If the Techstream reports over the specification, swap out with a
known good sensor and retest.

Specification can be found in component section of repair manual.
Typical value is 0.46g/s.

CAUTION

100

Make sure the service bay exhaust hose is NOT connected to the
exhaust pipe.

Technical Training


Technician Handbook
874 Engine Control Systems II

MAF Sensor
Conditions

MAF sensor output signal can vary from normal output from a
variety of different conditions.
Using MAF sensor output signal and fuel trims can aid in the
diagnosis of DTCs P0171/172 and/or driveability concerns.
Very light dust adhesion with normal fuel trims is not a problem.
Fiber and dust that has adhered to the hot wire sensor can have the
following characteristics.
•  A higher than specified air-flow free VG signal.
•  Higher than normal negative fuel trims at idle.
•  Higher than normal positive fuel trims at high airflow
conditions.

The reason the VG signal in the air-flow free state is higher than
normal is that the fiber and dust will conduct a portion of the heat
away from the hot wire. This will cause the VG signal to be higher
than normal. The ECM will think that there is more air in the cylinder
than the actual amount of air that is in the cylinder. The injection
duration will increase creating a rich AF mixture. The AF sensor
voltage will go low, and the ECM will then adjust the fuel trim in the
negative direction at idle to correct AF ratio.

Technical Training

101


Technician Handbook
874 Engine Control Systems II
MAF Sensor
Conditions (cont’d)

As airflow increases, the dust and fibers act as an insulator keeping
heat in the hot wire sensor. The VG signal will now be less than
normal and the ECM will think there is less air in the cylinder than
the actual amount that is in the cylinder. The injection duration will
decrease creating a lean AF mixture. The AF sensor voltage will go
high, and the ECM will then adjust the fuel trim in the positive
direction at high airflow to get the correct AF ratio.
Example: A MAF has a specified value 0.46 g/s. During the VG airflow free check, 0.50g/s was reported. This is nearly a 10% change
from the specified output.
There are times when the contamination of MAF sensor hot wire
can result in high positive fuel trims in all ranges. Typically, the airflow free VG check will be lower than normal.

Any material blocking the airflow passage in the MAF will skew fuel
trims in the positive direction.
Wire harness issues can skew the MAF signal. Excessive
resistance on the ground side (E2G) will increase the VG (MAF
signal) voltage and that will cause high negative fuel trims in all
ranges. Excessive resistance on the VG (MAF signal) circuit will
decrease the VG (MAF signal) voltage and that will cause high
positive fuel trims in all ranges.

102

Technical Training


Technician Handbook
874 Engine Control Systems II

MAF Sensor or Intake
System Leak
Diagnosis Tips

Checking the TIS Techstream MAF value at idle and 2500 RPM is
one of the best ways to identify a contaminated MAF sensor or
intake system leak. If the MAF sensor values are lower than
expected, install a known good MAF sensor and retest. If the
original MAF sensor was contaminated, the grams per second (g/
sec) values and fuel trim values will immediately improve. If there is
no change, reinstall the original MAF sensor and inspect for an
intake leak. Keep in mind that temperature extremes and engine
loads can affect the readings. It is crucial for accuracy to use the

RPM reading on the TIS Techstream with the engine fully warm and
no accessory loads. MAF sensor readings increase with RPM or if
the engine is under higher loads such as with the A/C system ON or
if the engine is running poorly. This will prevent accurate diagnosis.
It will be helpful to make a reference chart, similar to the one in the
illustration, of approximate MAF sensor readings for your dealer’s
atmospheric conditions based on known good vehicles.

NOTE

Always refer to Service Bulletins (SB) and Repair Manuals (RM) for
information on specific vehicles and issues.

Technical Training

103


Technician Handbook
874 Engine Control Systems II

Relationship between
Fuel Injection
Duration and MAF
Malfunction or Air Leak

When the MAF signal does not accurately report the correct amount
of air entering the engine, the feedback system will attempt
compensate. Within, the constriction of feedback guard value most
injector pulsewidth readings will be similar to known good vehicles.

This is a sign of an air metering problem compensated by the
feedback system.
The MAF sensor or air leak may be off enough to cause a driveability
and/or starting problems. The fuel trim will attempt to adjust up its
guard limit.

Fuel Trim DTCs Without
Driveability Issues

If there is a DTC without significant driveability issues, the problem is
not likely to be in the feedback system. If the ECM’s initial injection
duration calculation is not accurate, then based on the exhaust
sensor signal the ECM will judge the condition to be rich or lean and
correct the condition. An example is an incorrect air flow
measurement. Remember that when the ECM correctly detects a lean
or rich condition, it will make adjustments to the injector duration, and
therefore the spark plug readings, exhaust sensor response, and
engine performance will appear normal.
There may be driveability problems if the feedback system cannot
compensate or compensate in time.

104

Technical Training


Technician Handbook
874 Engine Control Systems II

Relationship between

Fuel Injection
Duration and Fuel
System Malfunction

Technical Training

With fuel system problems, the injection duration will change
compared to the normal state or a known good vehicle. Making this
determination early, will focus your diagnosis and hopefully lead to a
faster resolution.

105


Technician Handbook
874 Engine Control Systems II

Notice how injector duration fluctuates to meet the needed fuel
demand determined by the feedback system. When compared to a
known good vehicle it is apparent that a fuel issue is present.

106

Technical Training


Technician Handbook
874 Engine Control Systems II

Where to Start

Diagnosis

When trying to determine the cause of a fuel trim issue, the most
important aspect of diagnosis is to match what the Techstream is
reporting to the engine’s actual running condition. For example,
black spark plugs coupled with positive fuel trims may indicate a
feedback issue. Verify that the fuel trims are currently abnormal.
When diagnosing an intermittent condition, check the Freeze Frame
data and duplicate the conditions. Investigate other DTCs and
troubleshoot them first. Use the Technical Information System (TIS)
for Repair Manual (RM) information, and look for applicable Service
Bulletins (SB).

Check the Basics

Abnormal cylinder compression, fuel pressure, wrong fuel, spark
plug condition, etc., will affect fuel trims. On multiple-bank engines, if
the fuel trims are significantly different between banks, concentrate
on things affecting only one bank, such as valve timing, exhaust
sensors, or mechanical issues. Most types of vacuum leaks or a
contaminated MAF sensor will not cause an issue on just one bank.

NOTE

Technical Training

Aftermarket intake systems, incorrect orientation of the MAF sensor
to the intake system, etc. can cause fuel trim conditions and DTCs.
Always visually inspect the engine for incorrect parts, disconnected
or incorrectly installed components or hoses, etc.

107


Technician Handbook
874 Engine Control Systems II

Injector Volume
Active Test

If you suspect the exhaust sensors are malfunctioning, verify their
response with the Techstream Injector Volume active test. This test
takes the ECM out of closed loop and allows you to drive the engine
rich or lean so you can verify all the exhaust sensors (O2 and A/F,
S1 and S2) are responding properly. Keep in mind that the ECM will
still use the long fuel trim values during this test. Clearing the fuel
trims before the active test forces the ECM to run only on the initial
injection duration. If the engine runs well during this test, it is
possible the DTC or driveability condition was caused by an exhaust
sensor feedback problem.
If it still runs poorly, the problem is not likely to be in the feedback
system. This test can also be helpful in isolating some fuel-trimrelated misfire conditions. Also keep in mind that if you find a
response problem with multiple exhaust sensors, confirm the
operation of the shared power and ground circuits. The heater
circuit must be operating for proper exhaust sensor response. Refer
to the Repair Manual for detailed O2 and A/F sensor diagnostic
procedures and specifications.
In the illustration, a large vacuum leak was present. While
performing an Injector Volume active test and driving the injection
volume rich (+24.8%) the A/F sensor (S1) responses were very

small (0.3V change) due to the exhaust stream extremely lean. The
O2 sensors (S2) stayed at 0.0V because of this extremely lean
condition.

108

Technical Training


Technician Handbook
874 Engine Control Systems II

Fuel Trim
Diagnosis Tips

A lot can be understood about the engine operating conditions by
observing fuel trim readings while increasing engine speed and load.
For instance:
•  If fuel trim (total) is normal at idle, but increases (+) with engine
speed and load, this may indicate a restricted fuel condition,
such as low fuel pressure, restricted injectors, restricted fuel
filter, etc. This may also be caused by a dirty MAF sensor hot
wire. These issues will usually affect both banks and may cause
driveability concerns, such as stalling or hesitation. If this
condition occurs on only one bank of a two-bank engine, this
may indicate restricted injector(s) on the affected bank.
•  If fuel trim (total) is high (+) at idle, but decreases (moves closer
to 0%) as engine speed and load increase, this may indicate an
un-metered air condition such as a vacuum leak. This issue will
usually affect both banks and may cause driveability concerns,

such as a high and/or rough idle. If this condition occurs on only
one bank of a two-bank engine, this may indicate a vacuum leak
or exhaust leak on the affected bank.

NOTE

Technical Training

•  If fuel trim (total) is fixed (+ or -) as engine speed and load
increase, this may indicate a stuck A/F or O2 sensor (S1) signal.
Not all A/F or O2 sensor malfunctions will show up this way. This
condition will usually only affect one bank and may cause
driveability concerns such as smoke from the exhaust pipe
(stuck lean), hesitation, etc.

109


Technician Handbook
874 Engine Control Systems II

•  If fuel trim (total) is high (+) at idle, but decreases (moves closer
to 0%) as engine speed and load increase, this may indicate an
un-metered air condition such as a vacuum leak. This issue will
usually affect both banks and may cause driveability concerns,
such as a high and/or rough idle. If this condition occurs on only
one bank of a two-bank engine, this may indicate a vacuum leak
or exhaust leak on the affected bank.

110

Technical Training


Technician Handbook
874 Engine Control Systems II

•  If fuel trim (total) is normal at idle, but increases (+) with engine
speed and load, this may indicate a restricted fuel condition,
such as low fuel pressure, restricted injectors, restricted fuel
filter, etc. This may also be caused by a dirty MAF sensor hot
wire. These issues will usually affect both banks and may
cause driveability concerns, such as stalling or hesitation. If
this condition occurs on only one bank of a two-bank engine,
this may indicate restricted injector(s) on the affected bank.

Technical Training

111


Technician Handbook
874 Engine Control Systems II

•  If fuel trim (total) is normal at idle, but increases (+) with engine
speed and load, this may indicate a restricted fuel condition,
such as low fuel pressure, restricted injectors, restricted fuel
filter, etc. This may also be caused by a dirty MAF sensor hot
wire. These issues will usually affect both banks and may
cause driveability concerns, such as stalling or hesitation. If

this condition occurs on only one bank of a two-bank engine,
this may indicate restricted injector(s) on the affected bank.

112

Technical Training


Technician Handbook
874 Engine Control Systems II

•  If fuel trim (total) is fixed (+ or -) as engine speed and load
increase, this may indicate a stuck A/F or O2 sensor (S1)
signal. Not all A/F or O2 sensor malfunctions will show up this
way. This condition will usually only affect one bank and may
cause driveability concerns such as smoke from the exhaust
pipe (stuck lean), hesitation, etc.

Technical Training

113


Technician Handbook
874 Engine Control Systems II

114

Technical Training

Technician Handbook
874 Engine Control Systems II

Technical Training

115