Long Term Fuel Trim (LTFT)
Gradual, persistent adjustments to the fuel delivery map to compensate for aging components or vacuum leaks.
Long Term Fuel Trim (LTFT) represents the persistent, learned corrections applied by the Engine Control Unit (ECU) to the engine's fuel map. Unlike the volatile Short Term Fuel Trim, LTFT changes slowly and is stored in the computer's non-volatile memory (KAM - Keep Alive Memory), remaining saved even after the engine is turned off.
LTFT is calculated based on historical trends of Short Term Fuel Trim. If the STFT is constantly adding fuel (positive value) to correct a lean condition over time, the ECU will eventually transfer that correction to the LTFT. For example, an LTFT of +15% means the computer has permanently increased the baseline fuel delivery by 15% to compensate for an ongoing issue, such as a vacuum leak or a weak fuel pump.
High positive LTFT values are classic indicators of unmetered air entering the engine, while high negative LTFT values often point to leaky fuel injectors, excessive fuel pressure, or EVAP purge system faults. Disconnecting the vehicle's battery or clearing codes with a scan tool resets the LTFT values to 0%, forcing the computer to relearn the engine's operational characteristics.