LTFT (Long-Term Fuel Trim)
What is LTFT
LTFT (Long-Term Fuel Trim) is a learned fuel correction the engine's DME stores in memory, shown as a percentage. It compensates for sustained deviations from the stoichiometric air-fuel ratio (around 14.7:1 for gasoline). A positive LTFT means the DME has learned it must consistently add fuel; a negative LTFT means it has learned to consistently cut fuel.
Unlike STFT, which reacts instantly to oxygen sensor feedback, LTFT builds up gradually when STFT keeps correcting in the same direction, and it is retained across ignition cycles. Normal LTFT values stay within roughly ±5%, and values beyond ±10% typically point to an underlying fueling or air metering issue.
How it works in BMW systems
LTFT acts as the DME's long-term memory for fueling accuracy. When STFT repeatedly corrects in the same direction, for example consistently adding 8% fuel to compensate for a small vacuum leak, the DME shifts that offset into LTFT so that STFT can return closer to zero and retain its full correction range for transient changes. This learned value persists in the DME's adaptive memory even after the engine is shut off.
On BMW engines with split exhaust manifolds, LTFT is tracked independently per bank, with Bank 1 and Bank 2 each holding their own value. A large LTFT on one bank with the other near zero is a strong indicator of a bank-specific issue such as an injector problem, intake runner leak, or exhaust leak upstream of that bank's pre-cat oxygen sensor. When both banks show similar high LTFT, the cause is typically something affecting the entire engine: a dirty MAF sensor, fuel pressure issue, or large vacuum leak downstream of the throttle body.
When the combined STFT and LTFT correction exceeds the DME's allowable threshold, usually around ±25%, a fuel trim fault code is stored. BMW splits these into additive mixture adaptation (load-independent corrections, dominant at idle) and multiplicative mixture adaptation (load-proportional corrections, dominant under load), each with its own fault code. On BMW, LTFT is held in the DME's adaptation memory and is not cleared by simply erasing fault codes; resetting it requires a dedicated adaptation reset. Because the stored LTFT value is itself diagnostic information, fuel trim should always be read and recorded before any adaptation reset is performed.