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+/- LTFT+STFT AFR RICH OR LEAN

Understanding AFR

Discussion and questions related to the course Understand AFR


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I'm still confused a bit on what the scanner is representing when the LTFT/STFT are reading -7 vs 7. When its at -7 is the engine actually running 7% rich and removing fuel on its own or is it running lean and adding fuel?

The trim is positive when the ECU/PCM is adding fuel. So -7 is removing 7% fuel, so whatever table is providing the information (VE, fuel table, MAF) is requesting an amount that is too rich, with the LTFT/STFT added, the engine should be running close to the commanded AFR, but without the trims it would be rich.

- LTFT is rich condition .

+ LTFT is lean condition .

- LTFT is rich condition .

+ LTFT is lean condition .