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Old 02-14-2020, 02:34 PM   #1
gagi
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The pesky 3% LT fuel trims

A soft failing MAF was most of my issue. However, it bugged me that my LTFTs were still centered around 3% and not 0%. Took apart everything on the intake side to double check. Sure enough, I missed something last time. The oil tube o-ring was crooked and sticking out. Put in a new o-ring and seated the tube properly. Now the LTFTs center around 0% and even go negative sometime.

Yap, the brighter light helped.
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Old 02-14-2020, 06:50 PM   #2
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Diligence pays off! Was it the oil dipstick tube, or some other pipe?
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Old 02-14-2020, 07:02 PM   #3
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It was the oil dipstick tube. The battery in my light was running low when I put it in last time. I missed that the o-ring was jammed and twisted. The oil dipstick tube wasn't fully seated.

If vacuum leak can be traced to the oil dipstick tube, one would think a really leaky oil pan can increase LTFT too?
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Old 02-15-2020, 03:22 PM   #4
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when you tracked down your soft failing maf what gave it away? from my understanding, a soft failing MAF will underreport the amount of air coming in, so if you were to watch the fuel trims, they would become notciably more positive the higher the RPM correct?
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Old 02-15-2020, 03:28 PM   #5
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when you tracked down your soft failing maf what gave it away? from my understanding, a soft failing MAF will underreport the amount of air coming in, so if you were to watch the fuel trims, they would become notciably more positive the higher the RPM correct?
Weve also seen a few over report. MAF value at warm idle is one data point, fuel trims another, swapping with a known working one is another data point that helps. Its tricky to isolate the problem to the MAF.
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Old 02-15-2020, 06:27 PM   #6
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when you tracked down your soft failing maf what gave it away? from my understanding, a soft failing MAF will underreport the amount of air coming in, so if you were to watch the fuel trims, they would become notciably more positive the higher the RPM correct?
My soft failing MAF was under reporting by about 10%. During idle, the new MAF reports 3.5 to 3.8 g/s but the old MAF was at 3.0 to 3.3. Cleaning it with MAF cleaner didn't do anything. Fuel trim was around 10%. +- 2%. Service Engine Light would have been tripped if staying above 12% for a period of time.

With a soft failing MAF, the RPM wasn't higher. The engine just runs richer than it needs to be. You might smell it at the tail pipe.
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