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fuel trim low/high with low/high ambient temperatures

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This topic contains 0 replies, has 1 voice, and was last updated by plat plat 1 year, 1 month ago.

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    It’s more complicated than this.

    Issues started with bogging on acceleration a few months ago. After ruling out everything else, Mechanic replacement MAF and O2 sensors, which seemed to fix it for a little while.

    Mechanic checked all the other possible causes, vacuum leak, fuel pump/pressure, etc.

    Couple months later, the bog returns in addition to pinging(spark knock) when the engine is hot and hot outside, on acceleration. Bog usually happens at a specific RPM, 2700, but sometimes as low as 2000. It feels like the engine simply cuts out for a fraction of a second when you hit the gas.

    I buy a scan tool at this point and scan while driving for a week. I see overall fuel trims are low (-20) when it is cool outside, and high (up to +20) when hot in the afternoon. Trims are off consistently at all engine speeds, loads and engine temperatures. Only ambient temperature affects it. I also see that the MAF reads far higher when it’s cold outside than hot, under the same conditions. For example, at idle with a/c and lights off in neutral, MAF reads up to 0.57 lbs/min when cold, and down as low as 0.41 lbs/min under same conditions when hot. That difference seems too large to be just a result of different air density.

    At this point I am thinking the new MAF is defective and take it back. Mechanic agrees and replaced it under warranty.

    However on the second new MAF, the exact same behavior is still happening.

    The car has never bad any codes or other known issues throughout this.

    Mechanic has given up diagnosing it and just wants to throw parts at it with my money. Please can anyone help or suggest something.

    Car is a 97 Maxima with stick shift, 130k miles.

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