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Understanding O2 signals and Fuel trim

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Old 03-27-2013, 08:01 AM   #21
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On the 1st gen tacoma, it shares 1 AF sensor between the 2 banks. This can be an issue for fuel consumption if/when it wears out. The newer tacomas have newer logic that has in turn created another code that will tell you if an AF is sticking lean for too long, and it has an additional code that can tell you if one bank or the other is lean. This is not possible on the first gen with only 1 AF.

My point is that maybe the AF data needs to be monitored to make sure it is ranging correctly and not staying close to 3.35 all the time, or not staying at 4.99 volts for extended periods.
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Old 03-27-2013, 09:56 AM   #22
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Quote:
Originally Posted by BamaToy1997 View Post
You will see STFT(Short term fuel trim) fluctuate a lot, as this is the computer fine tuning your fuel requirements. LTFT(long term fuel trim) is where the information is at. 15% LTFT is a typical indicator of a small vacuum leak, like a small hose, or a gasket. HOWEVER this can be misinterpreted if a MAF sensor is giving a faulty reading.

The formula used to check accuracy of your MAF sensor is complicated, but for those interested:

IMAP = RPM * MAP / IAT / 2 where RPM is engine speed in revolutions per minute, MAP (manifold absolute pressure) is measured in KPa, and IAT (intake air temperature) is measured in degrees Kelvin. This integated value can be converted into total air flow (grams) using the following formula: (grams of air) = (IMAP/60)*(Vol Eff/100)*(Eng Disp)*(MM Air)/(R) where R is 8.314 J/°K/mole and the average molecular mass of air is 28.97 g/mole. Note that in the above formula the volumetric efficiency of the engine is measured in percent and the displacement is in liters.
(Copied from http://www.lightner.net/obd2guru/IMAP_AFcalc.html)

You can use this formula to test if your MAF sensor is accurate. It can also be used to verify a vacuum leak, which can result in a lean condition, and P0171/P0172 codes. I have actually used this formula to diagnose an inaccurate MAF sensor in a Ford that would hesitate slightly, and show lean, but the customer had recently replaced all of their O2 sensors.
Never mind I actually read this post and will calculate to see if it's the maf
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Old 06-25-2013, 10:29 AM   #23
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Quote:
Originally Posted by BamaToy1997 View Post
Most welcome. I am working on a thread about the dreaded P0420 code that I will post soon.
Hey Bama are u still putting this together? My '95 is throwing P0420 but I'll wait for your write-up if it's forthcoming.

Thanks!
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Old 06-25-2013, 07:05 PM   #24
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To be honest, I had actually set that writeup aside since I had been busy with so many other things at the shop. I guess I need to get back on the ball here and take care of that.
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Old 06-25-2013, 09:50 PM   #25
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You will find me patiently waiting here.
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Old 06-26-2013, 01:29 PM   #26
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Will a "ScanGauge II" allow me to view the upstream air/fuel sensor and downstream O2 sensor data in real time - so maybe I can get a better idea of what is going on?
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Old 06-26-2013, 01:44 PM   #27
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Quote:
Originally Posted by scoostraw View Post
Will a "ScanGauge II" allow me to view the upstream air/fuel sensor and downstream O2 sensor data in real time - so maybe I can get a better idea of what is going on?
Yes, you need to add them via Xgauges, but the codes are there, at least for the A/F sensor. For the downstream sensors I have seen the codes, but haven't tried them.
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Old 09-16-2013, 09:26 AM   #28
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Subbed. I currently have the p0172 code. Already changed out fuel injectors o2 sensors and maf. Maybe because of Maf sensor calibrator?
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Old 01-19-2014, 11:29 PM   #29
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is +7 LTFT at idle normal? when on load it drops for +2 - +4 with AFR at 14.5 - 14.7.
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Old 01-20-2014, 06:48 AM   #30
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Quote:
Originally Posted by suhteevin View Post
is +7 LTFT at idle normal? when on load it drops for +2 - +4 with AFR at 14.5 - 14.7.
IMHO, I wouldn't worry about it as long as it stays consistent, but also this is where the ST trims come into play. Adding the 2 together gets you your actual fuel trims. Also check your open loop/WOT AFRs to ensure you aren't running too lean with that SC.
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Old 01-20-2014, 07:31 AM   #31
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Quote:
Originally Posted by Lurkin View Post
IMHO, I wouldn't worry about it as long as it stays consistent, but also this is where the ST trims come into play. Adding the 2 together gets you your actual fuel trims. Also check your open loop/WOT AFRs to ensure you aren't running too lean with that SC.
My STFT bounces between +5 to -5. AFR at WOT is at 12.0, Cruise is at 14.5 - 14.7, and Decel is a 17.9. Is this normal conditions?

MPG is great usually between 21 - 24 city and 18 - 19 Highway (cruising 75 mph)
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Old 01-20-2014, 09:52 AM   #32
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My opinion is that you are just fine. Again, the effective fuel trim is LT+ST, so if those are staying small single digits, I wouldn't give it a second thought.

On my 09, not SC'd though, cruise LT is 2-5, with ST bouncing around to equate to a +/-2 effective trim, although I do see my LT spike up to 6-8 occasionally if I am doing a fair amount of city driving. At idle I usually get a +/-1 effective trim.

BTW, decel afr goes high lean since the ECU cuts the injectors off on higher speed deceleration.
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Old 01-20-2014, 02:25 PM   #33
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AWESOME! thanks for the help, learn something new everyday.
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Old 01-22-2014, 02:26 PM   #34
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Quote:
Originally Posted by Lurkin View Post
IMHO, I wouldn't worry about it as long as it stays consistent, but also this is where the ST trims come into play. Adding the 2 together gets you your actual fuel trims. Also check your open loop/WOT AFRs to ensure you aren't running too lean with that SC.
There is a lot more detail on how that works. This is how the LTFT and STFT work:

When the engine is running in closed loop the computer makes fast, large changes in the injector pulse width based on your short term fuel trim. This is used for changes in your driving based on instant responses such as braking, accelerating, turning on the AC, etc.

Whenever the STFT goes either positive or negative more than about 10 points for a specific amount of time, the computer then changes the LTFT number by one point in the same direction, and locks that new injector pulse width as the baseline. Then it uses STFT numbers to try to balance out the instant changes again. If the STFT again stays too far the LTFT will again increase (or decrease) to compensate.

LTFT is the baseline adapting of change to the factory injector pulse width. Things that can affect this are dirty/clogged injectors, fuel composition, airflow restriction (A dirty air filter can and will affect your fuel trim number) etc.

STFT is the fast adaptation of change to the baseline injector pulse width. Things that affect that are: snapping open of throttle, AC compressor engaging/disengaging, alternator demand, etc.

When trying to diagnose fuel economy and other fuel related issues, including sensor codes, the primary number to look at is LTFT. If it is a high positive number (anything above 8) then you could be looking at a weak fuel pump, or clogged fuel filter or injectors. If it is a high negative number (anything above -8) then you most likely are looking at a dirty air filter, bad fuel pressure regulator, or bad EGR. LTFT Numbers in the extreme, like +25 or -25 are for gross errors: Vacuum line disconnected, injector stuck open, etc.
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Old 01-23-2014, 06:02 AM   #35
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^^^^ good info, consider my version the Cliffs Notes!
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Old 01-24-2014, 11:07 AM   #36
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Yeah, I do get carried away when it comes to O2 sensors and Fuel trim. I've worked performance for GM for so many years. lol.
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Old 03-24-2014, 10:43 AM   #37
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I'm starting to get bad gas mileage. It started after using the Techron concentrated fuel system cleaner 3 fill ups ago and the gas mileage has stood low since. I used to average 19mpg now it's 15mpg.

It's not the spark plugs because they were changed 3000 miles ago and mpg was still good after I changed them.

So I'm using the Scangauge to look at fuel trims.
Bank 1 shows +2 at idle and +3 at cruise for LTFT.
Bank 2 shows +4 at idle and +3 at cruise for LTFT.

However STFT for both banks range from -4 to +7 depending on the condition.

Could there be a vacuum leak?
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Old 03-25-2014, 07:33 AM   #38
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Quote:
Originally Posted by Hans Moleman View Post
I'm starting to get bad gas mileage. It started after using the Techron concentrated fuel system cleaner 3 fill ups ago and the gas mileage has stood low since. I used to average 19mpg now it's 15mpg.

It's not the spark plugs because they were changed 3000 miles ago and mpg was still good after I changed them.

So I'm using the Scangauge to look at fuel trims.
Bank 1 shows +2 at idle and +3 at cruise for LTFT.
Bank 2 shows +4 at idle and +3 at cruise for LTFT.

However STFT for both banks range from -4 to +7 depending on the condition.

Could there be a vacuum leak?
My opinion is that there isn't anything significantly wrong with those readings. Essentially you are still running single digit effective fuel trims. BTW, my 09's LTs hang around 0-4, with the STs running in the +/-4-5 range.
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Old 04-15-2014, 01:04 PM   #39
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Quote:
Originally Posted by Lurkin View Post
My opinion is that there isn't anything significantly wrong with those readings. Essentially you are still running single digit effective fuel trims. BTW, my 09's LTs hang around 0-4, with the STs running in the +/-4-5 range.
I found the cracks on the PCV hose on the driver side and breather hose on the passenger side so I replaced them.

Now bank 1 LTFT has been showing 0% for idle and cruise. But bank 2 LTFT is at 3% for idle and cruise. I wonder why I can't get bank 1 to match bank 2.
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