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Old 07-16-2014, 01:43 PM   #1
-voided
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Default Help understanding AFL numbers (Cobb AP)

I've got a stage 2 TP map running on my WRX, mods are just a Cobb catted downpipe, turbosmart BPV, and upgraded IC/TB hose. I've had a few revisions done on the map based on my AFL numbers being too high in the past, but now they're starting to go low. It's averaging around -7 to -4 while accelerating, and seems to hit 0 when the engine has no load.

I noticed a change pretty much around the time when the weather changed - here in Tucson, the monsoons have started and the weather got a lot more humid in the past few weeks.

I understand that AFL1 is actually an average of 3 different AFL numbers, but the main question is this: What does it mean when the number is positive? What does it mean when it's negative?


I appreciate your time.
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Old 07-16-2014, 03:18 PM   #2
Gallo2011
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Spend some time searching for the answer. If you have still not found it, post your question here.

http://forums.nasioc.com/forums/forumdisplay.php?f=140
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Old 07-16-2014, 03:21 PM   #3
-voided
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Ah, NASIOC. I've spent a fair amount of time searching - I'm not "one of those newbies," however I'm clearly not asking the right questions to Google. That's why I'm here.
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Old 07-16-2014, 03:28 PM   #4
dpenn
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From Bill@COBB:
"A/F Learning 1" is simply the currently applied long-term fuel trim, which would be one of the A/F Learning 1 A, B, C, or D values. Those values are determined and applied across different Mass Airflow ranges. So, for the 02-05 WRX, it would be something like:
0 - <5.6 g/s = "A" Range
5.6 - <10 g/s = "B" Range
10 - <50 g/s = "C" Range
50+ g/s = "D" Range
So, if Mass Airflow shows 5 g/s, given the above example, "A/F Learning 1" would be applying (and show in log) the values stored in A/F Learning 1 A. If Mass Airflow was 8 g/s, then it would be showing A/F Learning 1 B, and so on. The advantage of looking at A/F Learning 1 A,B,C,D, say via live data, is that you can quickly check all 4 without a data log that would requiring you hitting all these Mass Airflow ranges (which would be the case if you were just looking at "A/F Learning 1"). In fact, you can check these A,B,C,D values with the engine OFF ignition ON. So, if you suspect a fueling issue, you can drive around until A/F Learning 1 values populate accurately, and then simply check the A/F Learning 1 A,B,C,D via live data without worrying about a driving data log with just "A/F Learning 1". Keep in mind that the A/F Learning 1 values are all cleared when you install/uninstall AccessPORT, reflash a map, reset the ECU or disconnect the car's battery. They will then need to drive a bit for those to populate more accurately (usually drive through about a full tank of gas).
Most WOT data logs will hit A,B,C,D, so it usually isn't necessary to look at A,B,C,D individually. A WOT pull will definitely hit "D" and when they let off (or before the run), you'll likely see a progression up or down through A, B, C. "A" can be the trickiest, but if you lift off at the end of the WOT run and data log that for a little bit, you'll usually catch it.
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Old 07-16-2014, 03:28 PM   #5
dpenn
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There are different mass airflow ranges, so A is at idle, while C and D are learned more cruise-type driving (D is also applied in open loop at WOT but not learned there). For 08+ STi, trims should be within around +/- 8% - sometimes a touch worse (due to the factory FPR setup). All others should be within +/- 5% of so.

Boost leaks tend to show up as extreme negative values, especially in C and D (sometimes D is disabled by map and will be always zero). A bad MAF (esp. with older cars) tends to show extreme positive values. Front O2 can be erratic - hard to predict based on the type of trims. Vacuum leaks will show as extreme positive in lower ranges generally.
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Old 07-16-2014, 03:42 PM   #6
-voided
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That was extremely helpful, thank you dpenn.
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Old 07-16-2014, 03:49 PM   #7
nmlittlebigman
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Weather can definitely affect fuel trims. Humid air has less oxygen and therefore your ecu may begin to pull a little fuel (negative fuel trims) to meet the same targets. Ethanol content in your fuel can also affect trims. So can bad/dirty MAF and O2 Sensors (Not likely on a new car though) Don't worry about it unless you see you fuel trims (A,B,C,D) get outside of the +/- 8% and stay there.
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