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Old 11-11-2009, 01:29 PM   #11
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If you're tuning on the ragged edge then simple changes in temperature can cause your AFR to swing a couple of points.

Your BFT is nothing more than a target its the MTF thats sets the AFR.
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Old 11-11-2009, 01:35 PM   #12
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Let me try one more time.

The following statements are true, and the three of us have all mentioned them at some point in this thread:

1. my base table says .882.
2. stft logs commanded lambda in open loop.
3. my stft should (and has been for a very long time) log .882.

_______

All of a sudden one day, stft started logging as .844, without any changes to the tune at all. This disagrees with #3.

I want to know what is causing STFT (commanded lambda) to log a value that is different from the base table, even though fuel source says it is pulling it from there.
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Old 11-11-2009, 01:42 PM   #13
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YOu could be to the point that your injectors are going wide open and you're just dumping fuel.
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Old 11-11-2009, 01:43 PM   #14
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I have SVT injectors and pulled the values for them straight from my friend's SVT tune.

The car has been running great and with stft logging .882 wideband logging 12.9 for at least a month before this randomly started happening.
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Old 11-11-2009, 01:47 PM   #15
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hasnt it gotten colder lately??

colder more dense air with more flow could have been just enough to make the tune change.
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Old 11-11-2009, 01:47 PM   #16
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I am unaware of any adders to the base table due to ACT

Also it happens no matter what the ACT, anywhere from 50 to 90 it logs .844 in stft despite .882 being in the base table
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Old 11-11-2009, 01:49 PM   #17
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I can watch my cars AFR change as the weather changes, and I think there "may" be an adder there somewhere.
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Old 11-11-2009, 01:51 PM   #18
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.5 afr?

thats a big swing.

and besides, like I just said it does it anywhere from 50-90 ACT, the ecu doesnt care about ambient temp it only uses ACT.
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Old 11-11-2009, 01:53 PM   #19
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And you ACT will be ambient........
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Old 11-11-2009, 03:34 PM   #20
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Just in case anyone but Tom, John, Pete, and I are reading this, I'll spell out a few things that have been discussed here; please correct or clarify if necessary.

CLOSED LOOP (aka CL): ECU starts by commanding stoichiometric AFR (14.64:1 air/fuel ratio). ECU then repeatedly uses the narrowband O2 sensor to detect if AFR is above or below stoichiometric, then "trims" fuel towards stoich. STFT (Short Term Fuel Trim) constantly shows amount of trimming. A tune with a good MTF (Mass-air-flow Transfer Function) won't need to trim more than 5% (if you take the time to do it right, you will only trim 1-2% ).

OPEN LOOP (aka OL): ECU "blindly" commands AFR by reading some fuel table (fuel source tells you which table). STFT shows the AFR (in lambda) that the ECU is commanding.

Pete's problem is not that his MTF is off; his CL STFTs are within a few % of 1.00 lambda, and his wideband matches his OL STFTs (Commanded AFR). Pete's problem is that his OL STFTs do not match his base fuel table. He has already checked that the ECU is using the correct source. He wants to know if anyone has any idea what could be causing this to happen. He does not think it is based on temperature, because he observes the phenomenon across a wide range of ambient and charge temperatures.
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