LTFT (Long-Term Fuel Trim)

LTFT (Long-Term Fuel Trim) is a learned fuel correction the DME stores in memory to compensate for sustained deviations from the ideal 14.7:1 air-fuel ratio. Unlike STFT, which reacts instantly to oxygen sensor feedback, LTFT accumulates gradually when STFT consistently trends in one direction and is retained across ignition cycles. Normal LTFT values stay within roughly ±5%, and values beyond ±10% typically indicate an underlying fueling or air metering issue.

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 inline-6 engines with split exhaust manifolds, LTFT is tracked independently for Bank 1 (cylinders 1–3) and Bank 2 (cylinders 4–6). A large LTFT on one bank with the other near zero is a strong indicator of a bank-specific issue — 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, commonly 2A70 (additive mixture adaptation) or 2A73 (multiplicative mixture adaptation). Clearing fault codes or resetting DME adaptations zeroes out LTFT, forcing the DME to relearn. This is why fuel trim values should always be read before clearing codes during diagnosis — the stored LTFT value itself is diagnostic information that points toward the root cause.