Tài liệu 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