Fuel Trim

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How quickly does the ECM adjust LTFT in response to STFT? Eg, assume I have a lean condition causing LTFT to be +10%. I correct the problem so my STFT is right on fluctuating around 0. Will my LTFT correct to zero in a few seconds, a few minutes, a few hours, etc? Thanks.
 
It does not work that way. If you correct the problem which caused LTFT to be in +10%; your STFT will immediately go to -10% and then both will go slowly towards 0. The actual fuel provided by ECM is the sum of STFT and LTFT.

The time delay will depend upon manufacturer but the "slow" is with respect to the oscilloscope! If your engine is in that "cell" for a minute or so, it LTFT will definitely be back to normal.

Since you are asking this questions, I am assuming you are familiar with the basics and I am not explaining the "cell" concept.

Few Key ideas in understanding fuel trims:-

- There is no single STFT/LTFT value
- Engine has lots and lots of STFT and LTFT based upon different operating conditions.
- Regardless of lean or rich as indicated by trim, the ECM is always trying to keep the engine running in the stoich mode (except when it is not!)
- Since there are lots of cells, you could get both rich and lean codes and will drive your brain in to somersault!
- When it hits the pre-programmed limit, it lights up the code

scannerdanner and schroidingercats are two youtube channel that you want to subscribe to if you are interested in understanding this.

Short answer "a minute or two" :)
 
I am see something weird on my scanner. My Bank 1 AF sensor is shot with both sensor and heater circuits open. STFT stays flat at zero independent of RPM. Bank 2 AF sensor is responding as expected but something very odd is happening with Bank 2 fuel trims. LTFT stays at +10.9% at idle. It will drop as engine is revved up but always goes back to +10.9% at idle. Bank 2 STFT at idle hovers around -10 apparently trying to offset LTFT. As engine revs up, Bank 2 STFT gets less negative as LTFT gets less positive appearing like it is still trying to compensate.

Two questions. Is it possible to diagnosis Bank 2 without a functioning Bank 1 AF sensor? What would prevent my Bank 2 LTFT from moving to zero when the STFT is trying so hard to get it there? Thanks.
 
You still don't have a code for the bank 1 sensor? That would be very strange if your claim of Bank1 wideband sensor is dead. Is there a pending code? If so, then it would be possible that LT is not getting updated.
 
Originally Posted By: Vikas
You still don't have a code for the bank 1 sensor? That would be very strange if your claim of Bank1 wideband sensor is dead. Is there a pending code? If so, then it would be possible that LT is not getting updated.


I threw two codes yesterday, P0125 and P0171. Both consistent with a non-responding AF sensor. I cleared the codes and they did not come back for the remaining one mile trip home. Did not check for pending codes. I assume the sensor and heater circuit codes will show the next time or two I drive the car.

How does a failed Bank 1 AF sensor affect Bank 2 LTFT?
 
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It is certainly possible that once there is a pending code in the fuel delivery portion, ECM can put bunch of things on hold. For example, many of the test will no longer be attempted. Similarly, using the ST to bring the LT down periodically may be suspended. I vaguely recall something like watching on a real live case on youtube.

Another explanation is that the engine has not been at idle for long enough time for ECM to take the updates from ST in to LT but I suspect my earlier guess is more probable than this.
 
Originally Posted By: Vikas
You still don't have a code for the bank 1 sensor? That would be very strange if your claim of Bank1 wideband sensor is dead. Is there a pending code? If so, then it would be possible that LT is not getting updated.


Just threw P1130 after about 1 mile. Only 43K on a Denso AF sensor.
 
Originally Posted By: Trav
What kind of car is this?


1999 Toyota Avalon XLS with 3.0 V6 1mz-fe engine. Replaced original Bank 1 AF sensor about 7 years/43k miles ago. Original Bank 2 AF sensor is still performing ok at almost 16 years/123k miles.
 
Originally Posted By: artbuc
I am see something weird on my scanner. My Bank 1 AF sensor is shot with both sensor and heater circuits open. STFT stays flat at zero independent of RPM. Bank 2 AF sensor is responding as expected but something very odd is happening with Bank 2 fuel trims. LTFT stays at +10.9% at idle. It will drop as engine is revved up but always goes back to +10.9% at idle. Bank 2 STFT at idle hovers around -10 apparently trying to offset LTFT. As engine revs up, Bank 2 STFT gets less negative as LTFT gets less positive appearing like it is still trying to compensate.

Two questions. Is it possible to diagnosis Bank 2 without a functioning Bank 1 AF sensor? What would prevent my Bank 2 LTFT from moving to zero when the STFT is trying so hard to get it there? Thanks.


Its possible the bank one sensor you replaced just died (it does happen even with Deso or NGK), replace it and see how the fuel trims respond.
Bank 2 quite possibly has a exhaust leak between the A/F monitor and rear O2, this will really scramble the ECM's eggs and cause the symptoms you describe.
If it is a leak it will be a very small one, check any flanges/gaskets and hanger mounting brackets for cracked welds.

You may need to get the car on ramps and actually feel around the weld when the engine is cold started to feel it because you may not hear it. pull down on the pipe and see if a crack opens.
 
Originally Posted By: Trav
Originally Posted By: artbuc
I am see something weird on my scanner. My Bank 1 AF sensor is shot with both sensor and heater circuits open. STFT stays flat at zero independent of RPM. Bank 2 AF sensor is responding as expected but something very odd is happening with Bank 2 fuel trims. LTFT stays at +10.9% at idle. It will drop as engine is revved up but always goes back to +10.9% at idle. Bank 2 STFT at idle hovers around -10 apparently trying to offset LTFT. As engine revs up, Bank 2 STFT gets less negative as LTFT gets less positive appearing like it is still trying to compensate.

Two questions. Is it possible to diagnosis Bank 2 without a functioning Bank 1 AF sensor? What would prevent my Bank 2 LTFT from moving to zero when the STFT is trying so hard to get it there? Thanks.


Its possible the bank one sensor you replaced just died (it does happen even with Deso or NGK), replace it and see how the fuel trims respond.
Bank 2 quite possibly has a exhaust leak between the A/F monitor and rear O2, this will really scramble the ECM's eggs and cause the symptoms you describe.
If it is a leak it will be a very small one, check any flanges/gaskets and hanger mounting brackets for cracked welds.

You may need to get the car on ramps and actually feel around the weld when the engine is cold started to feel it because you may not hear it. pull down on the pipe and see if a crack opens.


Installed a new Denso Bank 1 AF sensor. New sensor is working great but it did not change the high LTFT at idle. Still wants to settle at -10.2%. Rev her up and LT trim goes positive as the engine demands more fuel but goes right back to -10.2 at idle. STFT stays +/- around zero as does both LT and ST trims on Bank 2.

In addition to the possible exhaust leak, I wonder if I have one or more Bank 1 injectors delivering too much fuel or perhaps just an erroneous signal from the ECM?
 
Originally Posted By: Trav
Bank 2 quite possibly has a exhaust leak between the A/F monitor and rear O2, this will really scramble the ECM's eggs and cause the symptoms you describe.


Trav, could you say more about this? I have searched for a detailed explanation and can not find one. An exhaust leak is listed as a possible cause for a rich DTC but I do not understand how. As I understand it, the upstream AF sensor determines fuel trim and thus a lean or rich condition. The downstream O2 sensor just measures the efficiency of the cat converter, right? Thanks.
 
The rear O2 is generally considered just a cat monitor but thats not always the case.
It is an O2 sensor so it has capabilities other that just looking for a difference between the front O2 or A/F monitor.

When you have a leak between the front O2/A/F monitor and the rear O2 it allows more oxygen to the measured by the rear O2 than the ECM thinks it should have based on front O2 readings and in end effect get confused.
In fact most manufacturers have a manufacturers code for this which basically means the ECM knows something is wrong but cant determine exactly what it is.

In your case you may well have a slightly leaking injector on bank one (i doubt its issue with the ECM, it is sending out a signal based on input of sensors) and a completely different issue on bank 2.
When i have a problem like this with no common cause that could effect both banks i treat each bank as individuals and attack the diagnosing that way.
.
It might also have a weak coil, wire/boot or plug on bank one or another issue like moisture in a spark plug tube, anything that could raise the misfire count but not enough to trigger a misfire code.
Anything causing unburned fuel to be dumped int the exhaust will show up as negative fuel trim, in this case this is more noticeable at idle or low RPM.

http://www.autoserviceprofessional.com/a...iagnosis?Page=1

https://books.google.com/books?id=zst2aO...0o2&f=false
 
I've never seen the second 02 to be anything but a monitor for the cat. You can run without it if you want you will get codes but no performance hit. If the B1O2 sensor is bad or the heater is bad i would assume the ECU puts the car in open loop and just goes off the fixed tables in the ECU map.

I prefer to look at the O2 voltage. It is more informative and immediate that the STFT or LTFT.

Some cars have true wideband O2s for the primary one. It will have 5 wires.
 
Originally Posted By: Trav
The rear O2 is generally considered just a cat monitor but thats not always the case.
It is an O2 sensor so it has capabilities other that just looking for a difference between the front O2 or A/F monitor.

When you have a leak between the front O2/A/F monitor and the rear O2 it allows more oxygen to the measured by the rear O2 than the ECM thinks it should have based on front O2 readings and in end effect get confused.
In fact most manufacturers have a manufacturers code for this which basically means the ECM knows something is wrong but cant determine exactly what it is.

In your case you may well have a slightly leaking injector on bank one (i doubt its issue with the ECM, it is sending out a signal based on input of sensors) and a completely different issue on bank 2.
When i have a problem like this with no common cause that could effect both banks i treat each bank as individuals and attack the diagnosing that way.
.
It might also have a weak coil, wire/boot or plug on bank one or another issue like moisture in a spark plug tube, anything that could raise the misfire count but not enough to trigger a misfire code.
Anything causing unburned fuel to be dumped int the exhaust will show up as negative fuel trim, in this case this is more noticeable at idle or low RPM.

http://www.autoserviceprofessional.com/a...iagnosis?Page=1

https://books.google.com/books?id=zst2aO...0o2&f=false


Thank-you...very helpful. Maybe I need to drive a few cycles with the new sensor before I comclude anything.
 
Originally Posted By: SHOZ
You should disconnect the battery for a few minutes and clear out your stored fuel trims.


Thought that happened when I erased the codes?
 
Originally Posted By: artbuc
Originally Posted By: SHOZ
You should disconnect the battery for a few minutes and clear out your stored fuel trims.


Thought that happened when I erased the codes?


May have but to be sure.
 
Originally Posted By: SHOZ
You should disconnect the battery for a few minutes and clear out your stored fuel trims.


Looks like you are right. I found a Toyota TSB which gives two options to reset ECM. You can use a Toyota Diagnostic Tool or you can disconnect the battery ground for 5 minutes. I will try the latter and report back.
 
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