Fuel Trims
The combined term for STFT and LTFT: how much the ECU adjusts fueling to hit target AFR.
Why it matters
Persistently high trims signal a boost leak or fuel system bottleneck. Combined high STFT + high LTFT = the ECU is working very hard to compensate.
What to look for
Total trim (STFT + LTFT) should be within +/-10% during normal driving. At WOT, STFT should be within +/-5%.
Related terms
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."
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."
Unintended air escape in the pressurized intake system. Symptoms: can't hit boost target, high WGDC, lean trims.
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