When STFT fluctuates between -2% and +2%, with LTFT at -5%, what does this indicate about engine performance?

When Short-Term Fuel Trim (STFT) fluctuates between -2% and +2% and Long-Term Fuel Trim (LTFT) is at -5%, this indicates that the engine is performing normally. STFT values close to zero suggest that the engine management system is making minor adjustments to fuel delivery, which is a normal behavior during stable operating conditions. The small negative LTFT value means that over a longer period, the engine has learned to reduce fuel delivery slightly to maintain the proper air-fuel ratio, indicating that it is compensating for conditions that may make the mixture slightly rich.

A significant deviation in STFT or LTFT could suggest issues like a vacuum leak or fuel delivery problems, but the current values indicate that the adjustments are minor and well within acceptable limits. Therefore, the engine performance is deemed normal under these conditions.