Understanding O2 signals and Fuel trim

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.

 

Never mind I actually read this post and will calculate to see if it's the maf

 

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!

 

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.

 

You will find me patiently waiting here.

 

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.

 

Subbed. I currently have the p0172 code. Already changed out fuel injectors o2 sensors and maf. Maybe because of Maf sensor calibrator?

 

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.

 

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)

 

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.

 

AWESOME! thanks for the help, learn something new everyday.

 

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.

 

^^^^ good info, consider my version the Cliffs Notes!

 

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.

 

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.

 

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.

 

Hello,
I have a 2005 toyota tacoma 6 cylinder 4x4 that I just recently purchased off a private party. I am getting a P0158 code as well! I just replaced both downstream o2 sensors, ignition coils, and spark plugs (used all denso oem parts). I checked the resistance across the o2 sensors and they read around 16 ohms at 90 degrees Fahrenheit(means they're good). I logged both downstream o2 sensors and found out that sensor corresponding to bank one would have a voltage that varies. o2 sensor correspding to bank 2 holds at a constant voltage (around .9 volts)I will post pictures to show this.

I checked out the fuel trim on my truck and it looks like bank one is acting normal(according to what was specified in this post above). Bank two LT is indicating a 20%. This makes me think that it may be injectors.

Does anyone know where else I should go from here as far as diagnosing goes?

This is my original post.
https://www.tacomaworld.com/threads/p0158-check-engine-light.445114/#post-12982369