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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 01-15-2019, 05:49 PM   #4
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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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Old 01-26-2019, 01:26 AM   #5
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Default Re: Fuel trims questions

Thank you for your explanation. I agree it should be close to zero on long term trim. Based on the observation I am convinced that one side has leaking seals on the injectors or the injectors are broken. I showed it to a mechanic and he agreed with me that there is something wrong, but he disagreed about the seals. He said that seals would cause the pressure to drop immediately, but the system holds the pressure for like 30 seconds. He's a friend of mine, so not having any particular interest in diagnosing it. He said that injectors look worn out, whatever it means. He told me to replace the injectors with ones made by Delphi. He even found a part number for me. FJ10631

I am hesitating. This is an expensive repair. I mean he is probably right, but having a car with a value of 2000 bucks and then spend $600 on injectors replacement is somewhat hurtful. I don't know what to do. It sucks.

But the gas mileage sucks too. Damn. What a car. GM really screwed up with this.
They really made this car non-serviceable.

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Old 01-27-2019, 02:51 PM   #6
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Default Re: Fuel trims questions

Quote:
Originally Posted by Anjey View Post
But the gas mileage sucks too. Damn. What a car. GM really screwed up with this.
They really made this car non-serviceable.
These cars are great when taken care of. These vehicles are not non-serviceable. GM didn't screw it up. Some things may be poorly designed but pooching it, no. Man-made mechanical objects are prone to failure at some point when they are used/abused, subjected to harsh conditions and are given poor maintenance. Our cars don't hold value, that's a part of owning a vehicle that is discontinued that was short lived and not considered anything special to the market.

Your vehicle is aging and it is showing it by malfunctioning mechanically. I agree with your mechanic friend. You have a failing or plugged up injector or multiple injectors. Failure is not enough to set DTCs unfortunately. You can isolate which injector is failing by doing an injector cut out test that turns off each individual injector as the engine is running.

Injector failure is also caused by the kinds of fuel fed to it be it contaminated fuel, bad batch of fuel, bad mixture or cheaply made fuel.

If you're weighing the value over the repair and thinking it's too much money to put into it based on your budget, then you might be better getting rid of it for something else.

...
2008 Saturn Aura XE 3.5-Sport tuning T10 wheels-Dynomax bullet/Borla ProXS-eBay intake-Pioneer Premier 12 shallow mount/Kicker CXA600.1
Upgrades coming!

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Old 02-08-2019, 02:41 AM   #7
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Default Re: Fuel trims questions

"If you're weighing the value over the repair and thinking it's too much money to put into it based on your budget, then you might be better getting rid of it for something else."

It is even worse than that. Money is tight right now and I can't afford doing anything. I'm driving like this. Terrible.

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Old 02-11-2019, 07:47 PM   #8
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Default Re: Fuel trims questions

Quote:
Originally Posted by Anjey View Post
"If you're weighing the value over the repair and thinking it's too much money to put into it based on your budget, then you might be better getting rid of it for something else."

It is even worse than that. Money is tight right now and I can't afford doing anything. I'm driving like this. Terrible.
these cars are not great in stop and go drliving (3500+ lb car), either the 3.5 or 3.6 will get 12-16 mpg tops in city driving and I would not be worried by a 4% difference from one bank to another especially if LTFT's are less than 10%. That 4% difference could be attirbuted to how air and fuel mix one side vs. the other, or spark, or even deposits or compression. No matter what, that is a minor, if consistent difference. In fact, think about it-it's lean 2% on one side and rich 2% on the other-that is what the O2 sensors and the ECM are supposed to do-compensate for MINOR differences. If the range is outside the accepted <10%, you need to find the problem, you will get a CEL for that in most schemes.

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Old 02-12-2019, 01:33 AM   #9
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Default Re: Fuel trims questions

" If the range is outside the accepted <10%, you need to find the problem, you will get a CEL for that in most schemes. "


It is more than 10% sometimes. But not always. It is I think 6% always and then maybe 10-12% sometimes. You know - it would be better to plot these differences, but I can't. It is just me observing it. I have no means of recording the whole pattern. Those patterns change quite fast - and it is difficult to observe. Hard to say how long it gets 12% vs the total time, when it is just 6%.
What is CEL?

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Old 02-19-2019, 07:16 AM   #10
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Default Re: Fuel trims questions

CEL= check engine light.
Your LTFT's can occasionally go rich and over 10% during acceleration and other transitions from gas to brake, but should quickly respond in STFT to counter act the surplus. If it does not it is usually a vacuum leak, or poorly functioning fuel injector(s)/fuel filter/fuel pump and/or regulator performance. Compression differences also can cause fuel inbalances. O2 sensors get 'lazy' and switch slowly or incorrectly read differences in O2 levels between combustion gases and out side O2 levels. It is also not a bad idea to remove the neg cable from the battery overnight from time to time so the sensors all have to relearn baseline settings.

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Old 02-22-2019, 03:02 AM   #11
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Default Re: Fuel trims questions

Thanks.
I just got a CEL.
It says something about the catalyst inefficiency detected.
I realized that my car with its current book value is now a candidate for a trade in.

May GM go BK MoFos

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Old 03-11-2019, 06:51 PM   #12
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Default Re: Fuel trims questions

cat inefficency is usually a cat that is worn out or the O2 sensor behind the cat is bad. It may or may not have anything to do with your slightly out of spec fuel trims, they have to be pretty far out of range (like over 10% rich) to cause a P420. In most cases unless it is really bad and causing a plugged cat, it will not affect how the vehicle runs.

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