What is the normal range for long-term fuel trim data?
3 Answers
The long-term fuel trim range is ±10%, with a correction limit of ±25%. The long-term fuel trim value is referred to as the learned value. At idle, the normal range for long-term fuel trim is ±4%, and under partial load, the normal range is ±8%. The sign of the value indicates the correction direction of the current air-fuel mixture concentration: 1. Positive. This means enrichment correction, i.e., without correction, the current mixture would be lean, with an excess air coefficient greater than 1. 2. Negative. This indicates dilution correction, i.e., without correction, the current mixture would be rich, with an excess air coefficient less than 1. The long-term fuel trim value is referred to as the learned value. At idle, the normal range for long-term fuel trim is ±4%, and under partial load, the normal range is ±8%.
I've been playing with and fixing cars for over ten years, often helping friends check their car data. That long-term fuel trim number should normally be within ±10%, meaning from -10 to 10. It indicates the engine computer is automatically adjusting the fuel quantity. If the data falls within this range, it means the car is running smoothly with balanced fuel consumption. Once, my own car's value jumped to 15%, resulting in noticeably increased fuel consumption and a slight shake. After troubleshooting, I found a small leak in the intake manifold, and replacing a sealing ring fixed it. I recommend keeping an OBD scanner handy to check this number yourself. If it's out of range, it might be due to a clogged air filter or a faulty oxygen sensor—get it fixed promptly to avoid bigger issues. Regularly checking the fuel system and intake system can save you a lot on repairs and make driving more enjoyable.
As a mechanic, I deal with various fuel trim issues every day. The long-term fuel trim (LTFT) data should ideally be within the healthy range of -10% to +10%. If it exceeds +10%, it might indicate a lean mixture, possibly due to vacuum hose leaks or clogged fuel injectors; if it drops below -10%, it could suggest high fuel pressure or sensor malfunctions. These are common issues in practice, often triggering the check engine light, unstable fuel consumption, or sluggish acceleration. I've worked on many vehicles where abnormal values required time-consuming diagnostics, but once fixed, the data stabilizes. A reminder to car owners: don't just focus on surface-level symptoms. Regularly monitor this value using an OBD tool, especially after maintenance, to prolong engine life. Keeping it within 10% ensures your car runs efficiently, economically, and safely.
