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Old 01-04-2019, 10:47 PM   #1
Anjey
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Default Fuel trims questions

Guys, how should I interpret fuel trims readout on my scanner?
There are 4 fuel trims shown - short and long term, banks 1 and 2.

Which ones are more diagnostically important? The long trim or short?
I find it difficult to watch the short trims, because they change too fast.
Are the short trims used for practical diagnostics?

I mostly watch the long term trims and now I am concerned.
If my long term trims are uneven - what does it say really?
My bank 2 long term trim is generally 4 percent points higher than bank 1.
Is that significant?
(For instance bank 1 says -2% and the bank 2 is 2%, it's generally higher, so when trims drop, they tend to keep the difference - so when b1 says -4.7% then b2 is about -0.7%)

Also - when the engine is rev'ed 2000 RPM in park (no load) - the fuel trim is even more uneven - the difference is 6 percent points!
Again - for example - it would go up to 4% on bank 2, while bank 1 stays at -2%

What is the significance of these measurements? What does it mean on the Aura 3.5?

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Old 01-10-2019, 06:51 PM   #2
mmtphoto
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Default Re: Fuel trims questions

Quote:
Originally Posted by Anjey View Post
Guys, how should I interpret fuel trims readout on my scanner?
There are 4 fuel trims shown - short and long term, banks 1 and 2.

Which ones are more diagnostically important? The long trim or short?
I find it difficult to watch the short trims, because they change too fast.
Are the short trims used for practical diagnostics?

I mostly watch the long term trims and now I am concerned.
If my long term trims are uneven - what does it say really?
My bank 2 long term trim is generally 4 percent points higher than bank 1.
Is that significant?
(For instance bank 1 says -2% and the bank 2 is 2%, it's generally higher, so when trims drop, they tend to keep the difference - so when b1 says -4.7% then b2 is about -0.7%)

Also - when the engine is rev'ed 2000 RPM in park (no load) - the fuel trim is even more uneven - the difference is 6 percent points!
Again - for example - it would go up to 4% on bank 2, while bank 1 stays at -2%

What is the significance of these measurements? What does it mean on the Aura 3.5?
Generally speaking, if the LTFT is less than 10% (positive or negative) there is nothing wrong with the fuel management, it is when it is more than 10% that something is not right. STFT's history determines the LTFT readings, so it is within what is considered normal. If the differences are only a couple of percentage points from one side to another, that could be atributed to minor differences in injector flow, ignition output variation from cylinder to cylinder,
or actual differences in O2 sensor capabilities.

Side 2 readings should not vary (switch) rapidly but tend to stay close to one setting because it really is used as a diagnostic for catalytic converter health.
From my experience, readings for S2 sensors tend to settle around .6-.7 volts, S1 switch rapidly from around .1 to .9 volts but should average .45 volts and this can be seen in a graphing scanner with a smooth waveform with .9 volts at top and .1 at the bottom.

Last edited by mmtphoto; 01-10-2019 at 07:02 PM..

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Old 01-11-2019, 12:59 AM   #3
Anjey
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Default Re: Fuel trims questions

Thank you for your answer. I read more about it online - and I noticed there exist no comprehensive method of looking at it. It looks like it is a matter of experience of a tech. I watched this many times now on my car now and I see that the LTFT tends to go up to 10%. It also sums to over 10% on my car.
It happens asymmetrically. One bank does that. Then another. Not simultaneously. This is really interesting. It is not happening always, but tends to happen under specific loads and speed conditions. Obviously there is something wrong but I can't say I understand the cause.

One side is definitely richer at times. Sometimes a lot richer. But while it looks so often but not always. Sometimes the total trim sums to -12% on bank 1 and sometimes the values go opposite way, so the total trim is small. I find it very difficult to observe. My tool is not allowing me to observe that easily due to simple outlay of graph. I can only see 4 traces. I wish I had access to professional tool to learn more about it. It would be better to look at O2 trace with throttle angle.

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Old 01-11-2019, 04:20 AM   #4
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Default Re: Fuel trims questions

Wow, it is a very interesting forum.

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Old 01-15-2019, 05:49 PM   #5
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Default Re: Fuel trims questions

When I look at fuel trims I look at STFT first to see if the readings are over 10% at idle, then watch the LTFT-they should be as close to 0 as possible. I do use a graphing scanner so I can see O2 sensor switching to determine if .45 is a realistic mean point, then look at S2 sensor voltage to see cat health -
(.7 ideal).

If you are seeing 10% or more consistently something is wrong-although I have seen engines in the plus 20 range still running, but poorly. If pulling back or adding more than 10% fuel, most ECM's would, after a predetermined set of variables, set an CEL.

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