What Causes Excessive Short-Term Fuel Trim Values?

[VonHadley]

Excessive short-term fuel trim values are caused by a lean air-fuel mixture. Here is additional information: 1. Lean mixture: When the air-fuel mixture is too lean, the oxygen sensor's output voltage decreases, causing the short-term fuel trim to increase. This means the fuel injection pulse width will lengthen. An increase in short-term fuel trim indicates that the values read on the diagnostic tool will be greater than 1. 2. Difference between short-term and long-term trim: Short-term fuel trim is a real-time correction control of the fuel injection quantity based on the oxygen sensor's feedback regarding the richness or leanness of the mixture in previous working cycles. On the other hand, changes in the long-term fuel trim coefficient are qualitative changes formed by the ECU based on quantitative changes from the continuous correct feedback of short-term fuel trim. This is the key difference.