Toyota training course t852 engine control systems sec06

874 Engine Control Systems II Technician Handbook Fuel Trim Overview The ECM must change the amount of fuel injected into the engine based on operating conditions.. The engine adjusts

874 Engine Control Systems II

Technician Handbook

Fuel Trim Overview 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

Fuel Trim Basics

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

Calculated Load

(Calc Load)

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

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%

Basic Injection Duration

and 0% Fuel Trim

874 Engine Control Systems II

Technician Handbook

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

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

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

Closed Loop and

Open Loop

NOTE

874 Engine Control Systems II

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 long-term fuel trim value is a learned value that is based on the short-term 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

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

Short-Term and

Long-Term Fuel Trim

NOTE

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

Feedback-Related

Issues

874 Engine Control Systems II

Technician Handbook

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

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)

Mass Air Flow

(MAF) Sensor

Operation

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

874 Engine Control Systems II

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

MAF Sensor Conditions

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 flow free check, 0.50g/s was reported This is nearly a 10% change from the specified output

air-There are times when the contamination of MAF sensor hot wire can result in high positive fuel trims in all ranges Typically, the air-flow 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

MAF Sensor Conditions (cont’d)

874 Engine Control Systems II

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

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

NOTE

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

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

Relationship between

Fuel Injection

Duration and MAF

Malfunction or Air Leak

Fuel Trim DTCs Without

Driveability Issues

874 Engine Control Systems II

Technician Handbook

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

Relationship between

Fuel Injection

Duration and Fuel

System Malfunction

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