Short Term Fuel Trim
The ECU's real-time, moment-to-moment adjustment to fueling. Think of it as the ECU saying "right now I need to add a little more fuel" or "right now I need to pull back a little."
Why it matters
Large swings during wide-open throttle mean the ECU is scrambling to compensate for something unexpected: a failing sensor, a boost leak, injectors that can't keep up, or a fueling calibration issue.
What to look for
During WOT, STFT should stay within +/-5%. Warning at +/-10%. Critical above +/-10%.
Related terms
The ECU's permanent, learned correction to fueling. Unlike STFT which changes moment-to-moment, LTFT is the ECU saying "over time, I've learned I consistently need to add or remove this much fuel."
The ratio of air to fuel in the combustion chamber. Lower number = richer (more fuel). Higher number = leaner (less fuel).
Another way to measure air-fuel ratio, expressed as a ratio where 1.0 = perfect stoichiometric combustion. Below 1.0 = rich. Above 1.0 = lean.
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