Fuel Trim

[LvR]

Originally Posted By: artbuc
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?


IMO the fact that the STFT varies around 0 is a clear indication of the ECU being in closed loop mode and in control while ensuring the LTFT gets maintained - the fact that the LTFT is not closer to 0, is simply an indication of the ECU realizing that the engine operating conditions dictate a different fuelling map ....................... now the cause for that particular need on that particular bank needs to be found to maybe (!) move the LTFT back closer to 0.

The fact that the LTFT changes with throttle variations, is imo no indication of "Rev her up and LT trim goes positive as the engine demands more fuel" - the change in LTFT is simply an indication of the ECU dictating a fuelling map other than the factory std lab optimized fuelling map that would have the engine always running at LTFT=0 if every parameter on the engine and vehicle is exactly as designed for in the manufacturers lab

Could be a leaky injector, misfiring cylinder on that bank or any one of a host/combination of other things ..........

Would be interesting to see the effect on both LTFT and STFT after a cold/hot/cold/hot drive cycle

 

[artbuc]

Successfully reset the ECM. Started the engine and both LT trims were at 0% and both ST trims were about -5%. As car warmed up (I did not drive it), both ST trims worked their way up to about -1.5%. The downstream O2 sensor was responding in range from .2 to .8% when I snapped the throttle. However, it settled out in the high .7's indicating a rich condition. The first LT trim correction showed up as -6% on Bank 1 and -1.3% on Bank 2.

Why would the downstream sensor show a rich condition when the AF sensors are working and controlling upstream stoichiometry to the correct ratio? Could I possibly have a bad downstream sensor? Is it possible that even though the downstream sensor is responding as expected to lean/rich conditions, it is reading high? I have watched and read many tutorials and they all say the downstream sensor is ok if it is responding. But it seems possible or even likely that after 123k miles, the downstream sensor is no longer accurate, even though it is responding and showing relative changes.

 

[artbuc]

Quick update: i just found an article that said O2 sensors normally fail by showing a rich bias. I think that is what mine is doing. I will inspect Bank 1 plugs and see if the tailpipe is sooty.

Correction: Mileage is 128, not 123k.

 

[LvR]

Originally Posted By: artbuc
Quick update: i just found an article that said O2 sensors normally fail by showing a rich bias. I think that is what mine is doing. I will inspect Bank 1 plugs and see if the tailpipe is sooty.

Correction: Mileage is 128, not 123k.
So you then never replaced the O2?

Quote:
O2 sensors normally fail by showing a rich bias. I think that is what mine is doing. I will inspect Bank 1 plugs and see if the tailpipe is sooty.
If it reads rich when failing your exhaust will probably not be sooty because the engine is effectively running lean.

 

[SHOZ]

Originally Posted By: artbuc
Successfully reset the ECM. Started the engine and both LT trims were at 0% and both ST trims were about -5%. As car warmed up (I did not drive it), both ST trims worked their way up to about -1.5%. The downstream O2 sensor was responding in range from .2 to .8% when I snapped the throttle. However, it settled out in the high .7's indicating a rich condition. The first LT trim correction showed up as -6% on Bank 1 and -1.3% on Bank 2.

Why would the downstream sensor show a rich condition when the AF sensors are working and controlling upstream stoichiometry to the correct ratio? Could I possibly have a bad downstream sensor? Is it possible that even though the downstream sensor is responding as expected to lean/rich conditions, it is reading high? I have watched and read many tutorials and they all say the downstream sensor is ok if it is responding. But it seems possible or even likely that after 123k miles, the downstream sensor is no longer accurate, even though it is responding and showing relative changes.


The downstream O2 will normally settle in around the high point voltage wise. Usually .7v or so when cruising. This shows the ECU the capacity of the cat. The only time the post cat O2 really changes is at idle where it will drop to a leaner voltage.

If the post cat O2 were to follow the primary O2 it would indicate the cat is no good and set a code.

I really advise you to look at the O2 voltage and not the fuel trims. It is much more informative. The primary O2 should run from about .05v to .850v and cycle. If it is cycling it is in closed loop. If it is at 0.00V it is in open loop.

The Ultra-Gauge is a great inexpensive OBDII monitor that you can leave plugged in all the time and stick it to the dash somewhere.

 

[SHOZ]

If you have a smart phone you can get the app Torque. Along with a Bluetooth dongle the total cost is about $40. Torque also has some car manufactures PIDs in a separate plug-in app for a few dollars more that shows much more than the generic PIDs.

But the great thing about Torque is you can set up your own screen and you can easily log anything that it reads.

This is the way to study your O2 voltage and see if it is reading on the high voltage (rich) side. It will graph it for you right on the phone.

Here's a screen shot of what I usually monitor with Torque.

You can get Torque for an Iphone but you need a wifi OBDII dongle.

 

[artbuc]

Originally Posted By: SHOZ
I really advise you to look at the O2 voltage and not the fuel trims. It is much more informative. The primary O2 should run from about .05v to .850v and cycle. If it is cycling it is in closed loop. If it is at 0.00V it is in open loop.


Yes, both upstream A/F sensors read 0.660 volts and are cycling. Operation is closed loop.

 

[artbuc]

LvR, I replaced Bank 1 upstream AF sensor. Bank 2 upstream AF and downstream O2 sensors are original with 128k miles. Yes, I suspect my car is not running rich which is why I want to check plugs and tailpipe for evidence of a rich condition. Perhaps, 0.79 is a normal idle reading for my downstream O2 sensor.

 

[LvR]

Originally Posted By: SHOZ
If you have a smart phone you can get the app Torque. Along with a Bluetooth dongle the total cost is about $40. Torque also has some car manufactures PIDs in a separate plug-in app for a few dollars more that shows much more than the generic PIDs.

But the great thing about Torque is you can set up your own screen and you can easily log anything that it reads.

This is the way to study your O2 voltage and see if it is reading on the high voltage (rich) side. It will graph it for you right on the phone.

Here's a screen shot of what I usually monitor with Torque.

You can get Torque for an Iphone but you need a wifi OBDII dongle.

Have to agree with SHOZ here - cant tell you how many vehicles have been diagnozed in short order using Torque - you have to look at the bigger picture and all the major parameters to draw a conclusion
Quote:
Yes, both upstream A/F sensors read 0.660 volts and are cycling. Operation is closed loop.
Not following you there:

You have to get the motor to operating temperature and then, after the ECU has actually seen both a cold and warm engine condition start looking at the O2 voltages, after the cat reached operating temp and after the vehicle has been allowed to idle for about 2 minutes - I don’t know what you mean by "both upstream A/F sensors read 0.660 volts and are cycling" - what you want to see is a proper fast cyclic fluctuating upstream O2 voltage signal between about .1 and .8V (the STFT will only vary with small single digit values around 0 in sympathy only once the LTFT has been properly updated after the ECU has seen stable operating conditions and not before) .

Quote:
The first LT trim correction showed up as -6% on Bank 1 and -1.3% on Bank 2.
IMO you already have your answer right there - you have a ECU running in closed loop telling you it needs to decrease fuelling wrt factory standard fuelling map - thats all.

Now sure, that may be an indication of a possible "problem" on bank 1, but it may also simply be the ECU telling you its still happy to adjust fuelling for some drifted/changed bank 1 engine condition (relative to bank2 anyway) and that as long as no codes are generated/triggered the ECU is able to ensure the proper combustion conditions for efficient and satisfactorily engine operation.

I would totally ignore the downstream O2s till you have at least done a few cold/hot/cold/hot cycles on the motor ............

IMO single digit upstream O2 LTFT values not triggering codes, could simply be a normal wear and tear compensation mechanism if you are not able to see anything obviously wrong when looking under the bonnet ............................

 

[artbuc]

LvR, my upstream sensors are wide band A/F sensors, not O2 sensors. The base line output voltage is 3.3 (typical Toyota) and shows on a scanner as 0.660 (3.3/5). My downstream sensor is a regular O2 with a 0-1 volt output.

 

[SHOZ]

Originally Posted By: artbuc
LvR, my upstream sensors are wide band A/F sensors, not O2 sensors. The base line output voltage is 3.3 (typical Toyota) and shows on a scanner as 0.660 (3.3/5). My downstream sensor is a regular O2 with a 0-1 volt output.


Many cars now have the 5 wire WBO2s as the primary sensor now. Torque can read the actual A/F ratio from them and see the commanded ECU ratio too.

 

[artbuc]

Looks like problem solved. After a nice spin where cat converter got fully up to temp, Bank 1 fuel trims were both less than 2% (LT - and ST +). Bank 2 LT trim surprised me at +8.6% but that may settle down after another drive cycle or two. Downstream O2 voltage was less than 0.6 indicating all is well. I would say all looks pretty good now. Thanks to all for your help. I have learned a lot.

 

[Vikas]

Yet another case of "New Scanner Owner Syndrome" here

 

[artbuc]

Originally Posted By: Vikas
Yet another case of "New Scanner Owner Syndrome" here


LOL. Actually I have had my Actron CP 9575 for several years but only break it out when my CE lights up. Fortunately that has not happened in a long time.

 

[LvR]

Originally Posted By: artbuc
Looks like problem solved. After a nice spin where cat converter got fully up to temp, Bank 1 fuel trims were both less than 2% (LT - and ST +). Bank 2 LT trim surprised me at +8.6% but that may settle down after another drive cycle or two. Downstream O2 voltage was less than 0.6 indicating all is well. I would say all looks pretty good now. Thanks to all for your help. I have learned a lot.
Sorry for not realizing you had A/F sensors iso O2 upstream and posting some clutter, but then luckily ito STFT and LTFT values the fault-finding principles are exactly the same.

I would still be interested to see where that LTFT ends up after the replacement sensor took over operations and you did a few cold/hot/cold engine cycles under varying road conditions