Adjusting Fuel Trim: A Step-By-Step Guide To Fine-Tuning Your Engine

Resetting the Long-Term Fuel Trim (LTFT) is an important step in maintaining optimal engine performance. The LTFT adjusts the base fuel map (BFM) to compensate for engine wear, sensor drift, and fuel quality, ensuring the engine runs smoothly. Failing to reset the LTFT after maintenance work can lead to unstable fuel maps, resulting in engine surges and hesitations. Resetting the LTFT involves discharging the system by disconnecting the battery or turning the ignition key. This process erases the computer's memory, allowing it to relearn fuel adjustments and ensure efficient combustion.

Resetting the LTFT

The process of resetting the LTFT involves the following steps:

• Start the car and let it run until it reaches its normal operating temperature.
• Turn off the ignition.
• Locate the ECM (engine control module) main fuse in the fuse box and remove it carefully.
• Turn the ignition to the "on" position without starting the engine and wait for the system check to complete. Errors may be detected during this step due to the missing ECM main fuse.
• Turn off the ignition again.
• Reinsert the ECM main fuse into its designated slot in the fuse box.
• Turn the ignition back to the "on" position and wait for another system check to complete.
• Start the car and ensure that everything is functioning properly.

It is worth noting that resetting the LTFT is crucial after performing maintenance work that may impact the engine's fuel and ignition systems. This ensures a smooth and stable adaptive fuel map, preventing engine surges and hesitations, and saving car owners time and money.

Additionally, if your vehicle or scan tool does not support resetting the fuel trim (also known as Keep Alive Memory or KAM), there is an alternative method. You can remove the key from the ignition, disconnect the battery terminals, and then connect a one-ohm/one-watt resistor to discharge the PCM's capacitors/memory. This will effectively reset the fuel trim, optimising your vehicle's performance.

Disconnecting the battery

To reset the fuel trim, first, locate the battery in your car. It is usually in the engine bay and may be secured by a clamp or bracket. Once you have located the battery, follow these steps:

• Disconnect the negative terminal first: loosen the nut or bolt on the negative cable (usually black) with a wrench, then detach the cable from the battery post.
• If desired, also disconnect the positive terminal: loosen the nut or bolt on the positive cable (usually red) and detach it from the battery.
• Wait for about 10 minutes. This allows the capacitors in the system to discharge.
• Reconnect the battery: attach the positive cable first, then the negative cable. Ensure both connections are secure.

Note: Some vehicles may require turning the ignition key to the starter position to fully discharge the system. Additionally, disconnecting the battery may not reset the fuel trim in some cars, such as certain BMW models, as their fuel trims are stored in non-volatile memory.

Oxygen sensor feedback

The oxygen sensor provides feedback to the Powertrain Control Module (PCM) by reporting the exhaust stream as rich or lean with a stoichiometric switch point of 450mV. A voltage above 650mV represents a rich (low oxygen and/or high hydrocarbon content) air/fuel mixture in the exhaust, while a voltage below 350mV represents a lean (high oxygen and/or low hydrocarbon content) mixture. A voltage reading between 350 and 650mV indicates a stoichiometric (14.7:1) air/fuel mixture.

The PCM uses the oxygen sensor's input signal to adjust fuel injector pulse-width values. If a longer-than-normal injector pulse width is required to maintain the desired oxygen sensor signal, the ECM/PCM will display an increased fuel trim value in its datastream. Therefore, accurate fuel trim values depend on a properly functioning oxygen sensor.

The performance of the oxygen sensor can be measured using a scan tool, a DVOM, or a labscope. To check its performance, you can monitor whether the sensor responds correctly to rich and lean fuel conditions. A rich condition can be temporarily created with a propane enrichment tool, while a lean condition can be simulated by removing a vacuum line. If the engine control system is in closed loop, you can monitor the injector pulse-width changes in response to variations in the oxygen sensor readings. If these values are normal, it can be assumed that the feedback fuel control system is functioning as intended.

It is important to reset the Long-Term Fuel Trim (LTFT) after performing any maintenance work that may impact the engine's fuel and ignition system, including replacing or adjusting the oxygen sensor. Failure to reset the fuel trims can lead to unstable or non-converging adaptive fuel maps, resulting in engine surges and hesitations. Resetting the LTFT involves discharging the system by disconnecting the battery for a few minutes or turning the ignition key.

Fuel map continuity

The base fuel map is hard-coded into the ECM (Engine Control Module), and the Long-Term Fuel Trim (LTFT) adjusts it for engine wear, sensor drift, and fuel quality. The LTFT values are adapted to specific RPM and load combinations, and they may not adapt at the same rate, leading to diverging and discontinuous fuel maps. This can cause engine surges and hesitations.

To maintain a smooth and continuous fuel map, it is crucial to reset the LTFT after performing any maintenance work that may impact the engine's fuel and ignition systems. Resetting the LTFT can be done by disconnecting the battery or removing the ECM main fuse for a few minutes, allowing the system to discharge. This will help ensure a stable fuel map, preventing engine performance issues and saving car owners time and money.

Additionally, verifying the Short-Term Fuel Trim (STFT) and LTFT while reading live data is important for diagnosing lean and rich conditions. The optimal reading for both STFT and LTFT is between 5% and 10%. Deviations from this range can indicate issues such as a loose or faulty gas cap, vacuum leaks, or faulty sensors.

Fuel trim diagnostics

Understanding Fuel Trims

Fuel trim refers to the adjustment made by the engine computer to the fuel mixture, ensuring a balanced air-fuel ratio for efficient combustion. The engine computer relies on data from the oxygen sensor and the mass airflow (MAF) sensor to calculate the required fuel trim. A positive fuel trim indicates that the engine control module (ECM) is adding fuel, while a negative fuel trim means fuel is being taken away. The fuel trim value is typically displayed as a percentage or a positive/negative number within a fixed range on a scan tool.

Factors Affecting Fuel Trims

As engine conditions change, fuel trims adjust accordingly. Various factors influence fuel trims, including throttle angles, loads, and temperatures. These inputs can modify the length of the fuel injector's pulse, impacting the amount of fuel injected into the cylinder. Additionally, the presence of unmetered air past the MAF sensor or a clogged air filter can affect fuel trims.

Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT)

Fuel trims can be categorised into two types: STFT and LTFT. STFT values fluctuate rapidly in response to engine load, speed, temperature, and other operating conditions. On the other hand, LTFT changes slowly and provides a longer-term average of the engine computer's adjustments to the fuel mixture over a predetermined interval. LTFT values typically range from -10% to +10% and are considered a more accurate indicator of fuel mixture corrections.

Diagnosing Fuel Trim Issues

When diagnosing fuel trim issues, it's crucial to start with an engine that is warmed up and in closed-loop operation. This ensures that the oxygen sensor is active and providing accurate data. Here are some steps to diagnose fuel trim problems:

• Inspect the engine bay for any visible issues, such as disconnected vacuum hoses or a damaged air filter housing.
• Check for vacuum leaks, as they can add extra unmetered air and affect fuel trims.
• Evaluate the oxygen sensors' performance using a scan tool, DVOM, or labscope. Ensure they respond correctly to rich and lean fuel conditions.
• Monitor the injector pulse width changes in response to variations in oxygen sensor readings when the engine control system is in closed-loop operation.
• Refer to the fuel trim values in the scan tool's live data. Analyse the STFT and LTFT values to identify any abnormalities.
• Consider other factors that may impact fuel trims, such as a clogged fuel injector, low fuel pressure, or a restricted exhaust.
• Reset the LTFT if necessary, especially after maintenance work that impacts the engine's fuel or ignition systems.

Resetting Long-Term Fuel Trim (LTFT)

Resetting the LTFT is crucial to maintain stable fuel maps and prevent engine performance issues. Here's a step-by-step guide to resetting the LTFT:

• Start the car and let it reach its normal operating temperature.
• Turn off the ignition and locate the ECM (engine control module) main fuse in the fuse box.
• Remove the ECM main fuse.
• Turn the ignition to the "on" position without starting the car and wait for the system check to complete.
• Turn the ignition off again.
• Reinsert the ECM main fuse into its original slot.
• Turn the ignition back to the "on" position and wait for the system check to complete again.
• Start the car and ensure everything is functioning properly.

By following these steps, you can effectively perform fuel trim diagnostics and reset the LTFT when necessary, ensuring optimal engine performance and fuel efficiency.

Frequently asked questions