What Is a Normal Reading for Long Term Fuel Trim Reading on a Scan Tool
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What Is Fuel Trim?
past Larry Carley copyright 2019 AA1Car.comFuel Trim is the adjustment the engine computer (PCM) makes to the fuel mixture to maintain a balanced air/fuel ratio. Fuel trim is usually displayed as a PERCENTAGE reading on a browse tool.
For lowest emissions, the engine computer tries to go along the fuel mixture counterbalanced around fourteen.vii to ane (14.7 parts of air to one function fuel). If the air/fuel ratio is less than 14.7 to one (say 12 to 1), the fuel mixture is RICH. A rich fuel mixture tin can produce more power (up to a bespeak) but it also increases fuel consumption and emissions. Conversely, if the fuel mixture is greater than xiv.seven to i (say 16 to ane), information technology is LEAN. A lean fuel mixture reduces fuel consumption only can also increase emissions if the air/fuel mixture is so lean that information technology fails to ignite and causes lean misfire.
The engine calculator monitors the air/fuel ratio via the oxygen sensor(s) in the frazzle manifold(s). An oxygen sensor is essentially a RICH or LEAN indicator. When the engine is running lean (too much air and non plenty fuel), the O2 sensor generates a low voltage signal that tells the engine computer more than fuel is needed. When the engine is running rich (too much fuel and not enough air), the O2 sensor produces a higher voltage signal that tells the engine computer the engine is getting too much fuel and to cut back the fuel delivery. On vehicles that have an Wide Ratio Air/Fuel sensor (WRAF) or A/F sensor, the sensor tells the calculator the verbal air fuel sensor so the computer can increase or decrease the fuel delivery as needed.
Accurate fuel trim values require an accurate feedback betoken from the Oxygen sensor, otherwise the engine reckoner has no way of knowing whether the fuel mixture is running rich or lean.
When a common cold engine is outset started, it may take 10 to xxx seconds or more for the heaters inside the oxygen sensors to warms the sensors upwards to operating temperature. Until that point is reached and the fuel feedback control organisation goes into "closed loop", the fuel mixture is stock-still at a predetermined value so no fuel trim adjustments are made. Just once the Oxygen sensors are hot and the coolant temperature is loftier enough for the figurer to go into closed loop, the figurer starts to generate fuel trim values and make adjustments in the fuel mixture.
When the engine is close off, the fuel trim values are retained in the calculator's memory so the adjacent time the vehicle is driven information technology tin choice up where information technology left off. Erasing the computer's memory with a scan tool or by disconnecting the battery or the PCM power supply to clear codes besides wipes the fuel trim values, which means the calculator has to outset learning the fuel adjustments all over once again the adjacent time the engine runs.
How to Read Fuel Trim
The fuel trim value is read past plugging a scan tool into the OBD 2 diagnostic connector located under the musical instrument panel (on the drivers side near the steering column). When the central is turned on, the browse tool will initialize and start to communicate with the vehicle's onboard computer. Depending on the tool and the vehicle, it may be necessary to enter the vehicle year, make, model and engine VIN code before the browse tool tin can read the data.
The engine must exist started and running to read the fuel trim information. Depending on the scan tool and how its menu options are ready, you cull the selection that allows you to read system alive data. This will display a long list of sensor outputs and other readings called PIDs (Parameter IDs). On this list will be 2 fuel trim values for inline four and six cylinder engines, and four fuel trim values for V6 and V8 engines (one pair for each cylinder banking concern).
There are two types of fuel trim values shown:
Brusk Term Fuel Trim (STFT) is what the engine reckoner is doing to the fuel mixture correct now.
This value changes rapidly and tin bounce around quite a bit depending on engine load, speed, temperature and other operating atmospheric condition).
Values normally range from negative 10 percent to positive 10 per centum, though the readings may bound every bit much equally 25 percentage or more in either direction.
Long Term Fuel Trim (LTFT) is a longer term average of what the engine reckoner has been doing to balance the fuel mixture over a predetermined interval of fourth dimension.
This value is a more than accurate indicator of how the fuel mixture is beingness corrected to recoup for changes in the air/fuel ratio that are occurring inside the engine.
STFT B1 is Short Term Fuel Trim engine cylinder Bank 1
STFT B2 is Short Term Fuel Trim engine cylinder Depository financial institution 2
LTFT B1 is Long Term Fuel Trim engine cylinder Bank 1
LTFT B2 is Long Term Fuel Trim engine cylinder Banking concern 2
What Fuel Trim Values Mean
POSITIVE fuel trim values mean the engine calculator is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add together more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine's air/fuel mixture is running too lean.
NEGATIVE (-) fuel trim values mean the engine figurer is subtracting fuel (decreasing the pulse width or on-fourth dimension of the fuel injectors) to reduce the amount of fuel injected into the engine. This is washed to LEAN out the fuel mixture to recoup for what it perceives every bit a rich running condition.
Think, all this is based on what the oxygen sensors are telling the engine computer. If the O2 sensors indicate LEAN, the figurer adds fuel and generates a POSITIVE fuel trim value. If the O2 sensors are reading RICH, the estimator compensates by subtracting fuel and generates a NEGATIVE fuel trim value.
By reading the STFT and LTFT fuel trim values on a scan tool while your engine is running, y'all can tell if the air/fuel mixture is running rich (negative fuel trim percentages) or lean (positive fuel trim percentages).
What Fuel Trim Values Should Be
Ideally, the STFT and LTFT should be inside a few pct points of zero when the engine is idling or being held at a steady RPM. Recollect, STFT tin bounce around quite a chip equally when you suddenly snap open the throttle or decelerate. Just LTFT tin can tell you if the average fuel/mixture is running rich or lean.
Proficient LTFT values should be as close to zippo as possible, though they can range from 5 to 8 percentage depending on the condition of the engine. If the LTFT is getting upward around 10 per centum or higher, it usually indicates a problem that needs to be diagnosed.
LTFT values that go up effectually 20 to 25 percent will commonly set a P0171 or P0174 lean code.
LTFT values that drop downwards to negative 20 to 25 will usually set a P0172 or P0175 rich code. 
This scantool is displaying a STFT value of 25 per centum. Normally that would bespeak a problem,
merely in this case the engine is not running (Engine RPM is cypher). As shortly as the engine starts
and goes into closed loop, the fuel trim readings will begin to change.
How Fuel, Ignition and Engine Bug Touch on Fuel Trim
Lean fuel mixtures are a more common problem than rich fuel mixtures, though either tin happen depending on the cause.
LEAN fuel mixtures will generate higher than normal POSITIVE fuel trim readings on your browse tool.
RICH fuel mixtures will generate NEGATIVE fuel trim values.
Possible causes of LEAN fuel mixtures include:
Air or vacuum leaks in the intake manifold, near the throttle body or at vacuum hose connections.
Weak fuel pump that is not generating plenty pressure or volume
Fuel line restrictions (like a pinches hose or plugged filter)
A weak fuel pressure level regulator that is not maintaining adequate fuel pressure level
Air leaks in the PCV plumbing
Dirty MAF (Mass Airflow) sensor that is nether reading airflow into the engine
Muddied or dead fuel injectors
Ignition misfire (a fouled spark plug, weak ignition gyre or bad plug wire that causes a misfire allows unburned oxygen to pass into the exhaust and fool the O2 sensors)
Compression leaks (bad exhaust valve that allows unburned oxygen into frazzle and fools O2 sensors)
Exhaust manifold crack or gasket leak (allows unburned air into exhaust and fools O2 sensors)
Bad O2 sensor (point shorted to ground and so the sensor reads lean all the time)
Possible causes of RICH fuel mixtures include:
Leaky fuel injector
Excessive fuel pressure due to bad fuel pressure regulator or restricted fuel render line
Extremely dirty air filter or restrictions in air intake system
Exhaust restrictions (chock-full converter, crushed frazzle piping or plugged muffler)
Bad O2 sensor (output shorted to voltage and so it reads RICH all the time)
Using Fuel Trim to Diagnose Problems
Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high every bit plus or minus 8, but closer to cypher is all-time. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN. Rev the engine to 1500 to 2000 RPM and concord information technology steady for one-half a minute or so. If the fuel trim numbers drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. This is considering vacuum leaks have less of a leaning effect on the fuel mixture as engine speed and load increase.
If the fuel trim readings do not change much, the lean fuel status is more likely due to a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak.
LTFT fuel trim readings that are trending high might too exist the result of a slight ignition misfire that is slap-up enough even so to set a misfire code but is bad enough to cause a drop in fuel economy. Ane or more fouled spark plugs that are misfiring occasionally, or a weak ignition roll or bad plug wire that is allowing some occasional misfires could exist the cause. For more than information on misfire diagnose, Click Here.
You tin can apply fuel trim to identify muddied fuel injectors. If the LTFT fuel trim readings are trending up (POSITIVE), it means the fuel feedback command arrangement is compensating for an air/fuel mixture that is becoming progressively leaner over time. The almost likely cause would be muddy fuel injectors. Fuel delivery tin be restricted by the accumulation of varnish deposits inside the injector nozzles. The fix hither is to clean the injectors. If the fuel trim values return to normal later on the injectors have been cleaned, it verifies you accept solved the problem. If the fuel trim values don't alter afterwards cleaning the injectors, the lean fuel status may be due to depression fuel force per unit area or air/vacuum leaks.
You can apply fuel trim readings to check the response of the oxygen sensors and engine computer to changes you lot make in the fuel mixture. While the engine is idling, temporarily disconnect a vacuum hose. You lot should meet the STFT fuel trim readings leap immediately and become POSITIVE, and the LTFT should offset to pitter-patter up in response to the artificial lean fuel mixture you have just created by disconnecting the vacuum hose.
To test a rich response, you tin feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a driblet in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.
No change in fuel trim readings when yous create an artificial lean or rich fuel mixture would tell you the engine calculator is NOT operating in closes loop, or that the oxygen sensor(s) are not responding to changes in the fuel mixture.
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More than Fuel Organisation Manufactures:
Air/Fuel Ratios
More than Fuel Organisation Manufactures:Fuel Trim by Wells Manufacturing (PDF file).
Ford P0171 & P0174 Lean Codes
How Fuel Injection Affects Emissions
Most Common Trouble Codes (and what causes them)
Mass Airflow Sensors
Oxygen Sensors
Wide Ratio Air Fuel (WRAF) Sensors
More on Bank check Engine Lights & Fault Code Diagnostics
Scan Tool Diagnostics
Decoding Onboard Diagnostics
TROUBLE CODES Help
OBD II Diagnostic Tips
Help with DTC P0300 Random Misfire Codes
Troubleshoot Fuel Injectors
Troubleshoot Electronic Fuel Injection & Fuel Pump Diagnosis
Be sure to visit our other websites:
Auto Repair Yourself
CarleySoftware
OBD2HELP.com
Random-Misfire.com
Scan Tool Assistance
TROUBLE-CODES.com
Source: https://www.aa1car.com/library/what_is_fuel_trim.htm
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