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Again, this is a 2002 Nissan direct injected engine (QR20DD). By judging spark plugs, I assume that the air / fuel mixture is not rich. May be some kind of a lean condition?

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But the issue is fuel trims. When it idles around 650RPM, the STFT is bouncing from -2% to -8% and the LTFT stays at -10%. STFT is 0% to 2% and LTFT is -5% at 2500RPM.

Whaty I don't understand is why the spark plugs indicating a lean mixture and fuel trims indicating a rich mixture?

I understand that the rich mixture at idle and fuel trim close to 0% at 2500RPM is an indication of a bad MAF sensor. But it reads pretty OK (1.25V - 1.32V) at idle. May be the intake valves are clogged or any other causes?

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    IMHO the spark plugs look quite good.
    – Martin
    Jan 18, 2019 at 8:20
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    If the intake valves were clogged, then the plugs would not look like that... Perhaps the idle speed is too low causing fluctuations in the trim readings, or perhaps there is nothing really wrong and you are worrying, or fiddling, needlessly.
    – Solar Mike
    Jan 18, 2019 at 9:05
  • @Solar Mike Idle is around 630RPM and 670RPM. It says normal is 650RMP in the tag. Jan 18, 2019 at 12:37
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    Rich on fuel trims will not show on the spark plus, rich to the computer is barely over the stoichiometric boundary. Negative fuel trims do not mean it is running rich, just adding a bit of fuel to keep a near perfect stoichiometric fuel mixture.
    – Moab
    Jan 19, 2019 at 1:04

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