Mercedes Mullins
Fuel trim levels are adjustments made by a vehicle's engine control unit (ECU) to the fuel delivery system to maintain the ideal air-fuel ratio. The air-fuel ratio is the specific balance of air and fuel required for a car's engine to run properly. The ideal ratio is 14.7:1. Fuel trim levels are reported in two forms: short-term fuel trim (STFT) and long-term fuel trim (LTFT). STFT refers to immediate changes in fuel occurring several times per second, while LTFT refers to changes in STFT averaged over a longer period. Positive fuel trim percentages indicate the addition of fuel, while negative percentages indicate the removal of fuel.
| Characteristics | Values |
|---|---|
| Definition | Fuel trim describes the adjustments that a vehicle's engine control unit (ECU) makes to the fuel delivery system to keep the air-fuel mixture as close to its ideal ratio as possible. |
| Short-term Fuel Trim (STFT) | Immediate changes in fuel occurring several times per second. |
| Long-term Fuel Trim (LTFT) | Changes in STFT but averaged over a longer period of time. |
| Positive Fuel Trim | Indicates that fuel is being added. |
| Negative Fuel Trim | Indicates that fuel is being taken away. |
| Normal STFT Reading | Fluctuates between negative and positive single digits 2-3 times per second. |
| Normal LTFT Reading | Should be close to zero, positive or negative single digits. |
| Abnormal Readings | ST or LT fuel trims in double-digit positives or negatives indicate abnormal adding or lessening of fuel. |
| Causes of Abnormal Readings | Leaking fuel injectors, an unmetered air leak, or similar issues. |
| Fuel Trim Values | Generated by the computer when oxygen sensors are hot and the coolant temperature is high enough for the computer to go into closed loop. |
| Fuel Trim Value Range | Normally range from -10% to +10%, but can jump as much as 25% or more in either direction. |
| Total Fuel Trim | LTFT and STFT are added together to produce the total fuel trim, which should not exceed ±10% in normal operating conditions. |
| Engine Control | The engine control module (ECM) or powertrain control module (PCM) uses sensors to determine airflow, calculate fuel injection, and adjust fuel quantities based on actual airflow. |
Explore related products
What You'll Learn
Short-term fuel trim (STFT)
STFT is a result of changes in the oxygen levels of exhaust gases. An oxygen sensor monitors the flow and generates a signal that is proportional to the oxygen levels in the gases. If the o2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening the injector pulse or "on-time" to compensate. Conversely, if the o2 sensors inform the computer that the exhaust is rich, the computer shortens the injector pulse, adding less fuel to compensate. This change in fuel being added or taken away is called Fuel Trim.
A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually, they'll stay within %5 +/- of zero. On an older engine, they may go up towards 10% at times. If you experience STFT in the double digits, positive or negative, this would indicate an abnormal adding or lessening of fuel. This could be due to leaking fuel injectors, an unmetered air leak, or something similar. For example, if the o2 sensors are reading lean due to, say, a vacuum leak, the engine computer will compensate by adding fuel.
To retrieve STFT information, the engine must be operating in closed-loop mode, which means running it for a few minutes until the oxygen sensor reaches normal operating temperature. It is usually best to monitor the fuel trim under a few different load conditions.
Toyota's Most Fuel-Efficient Cars: Top Models Reviewed
You may want to see also
Explore related products
Long-term fuel trim (LTFT)
LTFT refers to changes in STFT but averaged over a longer period of time. It is the percentage (%) the PCM/ECM adjusts the calculated quantity of fuel injected into the cylinders to compensate for changes over long periods of time, like clogged fuel injectors. LTFT is driven by STFT. A normal long-term fuel trim reading will appear to stay the same, giving a long-term average of fuel added. It, too, should be close to zero, with positive or negative single digits under normal circumstances.
Positive numbers for LTFT indicate a lean condition/rich command, meaning there isn't enough fuel, and the computer is commanding the injectors to open for a longer period of time. On the other hand, negative numbers indicate a rich condition, meaning the computer is commanding to take away fuel. If LTFT gets too large (approximately ±25%), the PCM/ECM will trigger a "MIL" warning and set one or more Data Troubles Codes (DTC) to help diagnose the problem.
Troubleshooting Car Start Issues: Fuel Return Pinch
You may want to see also
Explore related products
Oxygen sensors
The oxygen sensor is typically located in the exhaust manifold, where it can be susceptible to contamination from soot and other debris. It is a small metal tube that is heated to a very high temperature. As the exhaust gas passes over the heated sensor, it causes the sensor to change colour. The computer interprets this colour change to determine the oxygen levels in the exhaust gas.
Maintaining the oxygen sensor is essential for optimal engine performance. A malfunctioning or faulty sensor can lead to incorrect air-fuel mixtures, resulting in the engine running too rich or too lean. This, in turn, can cause issues such as poor fuel economy, reduced engine performance, and even engine damage. Regular cleaning and maintenance of the sensor are crucial to prevent such problems and ensure the vehicle operates efficiently.
The performance of oxygen sensors can be measured using a scan tool, a DVOM, or a labscope. These tools help diagnose any potential issues with the sensor's functionality. It is important to ensure that the oxygen sensor is accurate and responsive to fuel trim changes when addressing any fuel injection malfunctions.
In addition, the number of oxygen sensors in a car may vary depending on the make and model. Most newer vehicles have at least two oxygen sensors: one upstream and one downstream of the catalytic converter. However, some vehicles, such as diesel cars, may have additional sensors to comply with emissions control configurations.
The Evolution of Oil: Powering Cars from Past to Present
You may want to see also
Explore related products
Engine control unit (ECU)
Fuel trim levels are the percentage of change in fuel over time. For an engine to operate properly, the air-fuel ratio needs to stay within a small window of 14.7:1. The engine computer is constantly fine-tuning the amount of fuel going into the engine to maintain this ratio. This process is called short-term and long-term fuel trim.
The Engine Control Unit (ECU) is the brain that controls the air-fuel mixture and makes changes to the delivery of either component, based on its assessments. The ECU monitors the flow of both air and fuel into the engine and directs changes to either part to ensure the proper mixture. This process, known as both short- and long-term fuel trim, is how the ECU keeps track of how much fuel is flowing in.
The ECU uses a few sensors to determine how much air is flowing into the engine. It then uses the airflow data, along with a target air-fuel ratio, to calculate how much fuel it should inject. The ECU uses oxygen sensor readings to find out what the actual air-fuel ratio is, and 'trims' the fuel quantities based on this 'real' air-fuel ratio. If the ECU detects that the calculated quantity of fuel needed is too much or too little, it will add or subtract fuel to 'trim' the quantity of fuel and optimise the combustion.
Oxygen sensors are critical to this process. When the engine is running rich (too much fuel and not enough air), the O2 sensor produces a higher voltage signal that tells the ECU that the engine is getting too much fuel and to cut back on the fuel delivery. Conversely, if the O2 sensors inform the ECU that the exhaust mixture is lean, the ECU will add fuel by lengthening the injector pulse, or "on-time", to compensate.
If an oxygen sensor fails, the ECU might not be able to make the correct decisions on the air-fuel mixture. Sensor failure could lead to the wrong fuel mixtures from the ECU. If the system is expecting a certain level of fuel or air and does not receive it, the air-fuel mixture could be disrupted.
Nitro Fuel for RC Cars: Expensive Hobby or Affordable Fun?
You may want to see also
Explore related products
$54.81
Air-fuel mixture
The air-fuel mixture is a delicate balance that is critical to the performance of an engine. The ideal ratio of air to fuel for an engine to operate properly is 14.7:1. This ratio must be maintained across various conditions, including cold start-up, idling in heavy traffic, and cruising on the highway.
The air-fuel mixture is monitored by various sensors, including oxygen sensors and a mass airflow sensor, which communicate their readings to the car's electronic control unit (ECU). The ECU acts as the brain of the engine, controlling the mixture and making adjustments to the delivery of air and fuel based on its assessments.
Oxygen sensors play a crucial role in determining the air-fuel mixture. When the engine is running rich (excess fuel) or lean (excess air), the oxygen sensor produces a signal that instructs the engine computer to adjust the fuel delivery accordingly. This change in fuel delivery is known as fuel trim. Fuel trim values can be either positive (indicating an addition of fuel) or negative (indicating a reduction of fuel).
Short-term fuel trim (STFT) refers to immediate changes in fuel delivery that occur several times per second. It is influenced by factors such as engine load, speed, temperature, and other operating conditions. On the other hand, long-term fuel trim (LTFT) provides a longer-term average of the adjustments made to the fuel mixture over time.
It is important to monitor both STFT and LTFT to ensure the engine is functioning optimally. Normal STFT readings typically fluctuate between positive and negative single digits a few times per second, while LTFT readings should remain close to zero under normal circumstances. Deviations from these norms could indicate potential issues with the engine or sensors.
What Fuel Does My Car Take?
You may want to see also
Frequently asked questions
Fuel trim levels refer to the adjustments that a vehicle's engine control unit (ECU) makes to the fuel delivery system to keep the air-fuel mixture as close to its ideal ratio as possible.
The ideal air-fuel ratio is 14.7:1.
There are two types of fuel trim levels: short-term fuel trim (STFT) and long-term fuel trim (LTFT).
Short-term fuel trim levels refer to immediate changes in fuel occurring several times per second. Long-term fuel trim levels are driven by short-term fuel trim levels but are averaged over a longer period of time.
You can monitor the fuel trim levels in your car by using a scan tool connected to the OBDII connector, usually found on the driver's side under the instrument panel.
