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Old 05-16-2012, 05:18 PM   #1
silver2ssrs10
 
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Long term fuel trims question

When datalogging LTFTs what do the values mean in relation to your air/fuel ratio? Is a ltft value of 0.90 rich or lean? Also will the long term fuel trim value modify your commanded pe air fuel ratio? Like say I am commanding the engine to run at 12.5:1, but at the time of WOT PE my long term fuel trim was at 0.90. Would I still be seeing 12.5:1 when monitoring with a wideband or would my air/fuel be off because the ltft is off? Thanks.
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Old 05-16-2012, 07:28 PM   #2
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If the fuel system is operating rich, long term fuel trim will compensate for the rich condition by decreasing the injector pulse width, and a negative long term fuel trim is indicated.

If the control module determines that the fuel system is operating lean, long term fuel trim will compensate for the lean condition by increasing the injector pulse width and a positive long term fuel trim value is indicated
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Old 05-16-2012, 07:47 PM   #3
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So greater than 1 is positive (lean and adding fuel) and less than 1, which in my case is .90 is rich and subtracting fuel?
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Old 05-16-2012, 07:59 PM   #4
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I wouldnt worry about under or over positive 3 %.

That number will change slightly from day to day, fuel, weather, altitude, the type of driving you did that day, etc.

if your swinging 7-10% differance as the norm, that should be corrected.

You should be very happy with 1% because the only time its at zero in WOT.

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Old 05-16-2012, 08:15 PM   #5
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Ok I will keep an eye on it, but just to be clear 0.90 is off by 10% right?
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Old 05-16-2012, 10:28 PM   #6
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Ok I will keep an eye on it, but just to be clear 0.90 is off by 10% right?
No, 0.90 is 9 tenths of 1 percent.

1.0 would be 1% and 0.90 is 0.90%. If you look at the description it should be something like LTFT(%) so it is indicating it is already in percent format. Under normal circumstances the decimal 0.90 would be 90% but in your log it is already a percent value.

You are under 1% so you are golden.
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Old 05-17-2012, 12:49 AM   #7
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Also will the long term fuel trim value modify your commanded pe air fuel ratio? Like say I am commanding the engine to run at 12.5:1, but at the time of WOT PE my long term fuel trim was at 0.90. Would I still be seeing 12.5:1 when monitoring with a wideband or would my air/fuel be off because the ltft is off? Thanks.
Fuel trims do nothing when the computer goes into open loop. The computer does not compensate anything when doing a WOT pass. Thats why its important to dial in WOT because the computer doesnt care what it sees, its only going to do what it is commanded. The oxygen sensors turn off when in open loop.
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Old 05-17-2012, 05:58 AM   #8
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Fuel trims do nothing when the computer goes into open loop. The computer does not compensate anything when doing a WOT pass. Thats why its important to dial in WOT because the computer doesnt care what it sees, its only going to do what it is commanded. The oxygen sensors turn off when in open loop.
Ok, I just heard if ltft was lean it would add that extra fuel on top of pe enrichment in open loop, so I just wanted to verify. Thanks.
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Old 05-17-2012, 10:04 PM   #9
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Ya, that's a common misconception. The factory o2 sensors are actually narrowbands, which mean they can only read from like 13.5 to 15.5 afrs, so when the computer commands 12.x at wot, the sensors can't read it because that is out of their range, so the computer ignores them in PE.
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Old 05-19-2012, 04:03 AM   #10
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Fuel trims do nothing when the computer goes into open loop. The computer does not compensate anything when doing a WOT pass.
That's not correct.

If there's a positive trim in the fuel trim cell prior to entering PE mode, the PCM will carry that positive trim over into PE as a protective measure.
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Old 05-19-2012, 07:34 AM   #11
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They can go negative too.

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Old 05-19-2012, 07:44 AM   #12
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Originally Posted by silver2ssrs10 View Post
When datalogging LTFTs what do the values mean in relation to your air/fuel ratio? Is a ltft value of 0.90 rich or lean? Also will the long term fuel trim value modify your commanded pe air fuel ratio? Like say I am commanding the engine to run at 12.5:1, but at the time of WOT PE my long term fuel trim was at 0.90. Would I still be seeing 12.5:1 when monitoring with a wideband or would my air/fuel be off because the ltft is off? Thanks.
Send me your logs I will have a look.

If your logging with SCT .90 LTFT means the MAF Curve is a little Heavy and can be adjusted to get closer to our target of .96-1.0

Ted.
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Old 05-19-2012, 10:27 AM   #13
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Send me your logs I will have a look.

If your logging with SCT .90 LTFT means the MAF Curve is a little Heavy and can be adjusted to get closer to our target of .96-1.0

Ted.
Ok I will send you the log Ted, thanks.
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Old 05-19-2012, 06:30 PM   #14
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That's not correct.

If there's a positive trim in the fuel trim cell prior to entering PE mode, the PCM will carry that positive trim over into PE as a protective measure.
Please correct me if I'm wrong but is not a positive fuel trim a condition where the PCM interprets the fuel flow to be RICHER than commanded and reacts by reducing the fuel flow accordingly which effectively leans the AFR? If so, how can reducing fuel flow when entering power enrichment mode be a PROTECTIVE MEASURE?
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