Fuel trim and O2 sensor values

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

96r50

96r50

10 Year 15 Year
Joined
Feb 25, 2007
Threads
60
Messages
2,750
So, disclaimer here, I'm not that familiar with engine management systems and fuel trims. With that in mind, I'm working on learning more :D

Last night I programmed my Scangauge to read the short term and long term fuel trims (STFT & LTFT). The Scangauge manual says the range is +/- 0 to 99. While driving around today I noticed each value fluctuating both positively and negatively. They both usually stayed within 5% of 0 on either side (+/-), and never (that I saw) passed 10% either way. As noted, I don't know much about fuel trims, so I have no idea what I'm really monitoring here. From reading today, 0 is the ideal fuel trim, and changes from there mean that the computer is either shortening or lengthening the injector duration to try and stay near/at 0. I'm wondering what the normal range for STFT and LTFTs are? I realize they probably vary with a bunch of external factors, but when should I start worrying about my engine? :eek:

The other value I was wondering about is on the second page of the link above. The Scangauge literature just says "O2 sensor values" and the range is 0-99. I programmed the Scangauge to read the values from each sensor and it seems that the front sensor is showing a limited range (0-25ish) whereas the rear sensor shows a larger range (0-80ish). Any ideas as to what this is?

Life was much simpler before I bought this thing! :doh:
 
I had the same questions, I have been considering starting a SCII FAQ type thread, but I don't know enough to contribute much. I am interested in the O2 sensor and fuel trim values, timing and fuel injector pulse, basically what are the optimal values, what are unsafe, etc...,
 
beno said:
Get rid of the Scam-gauge and get on with your life.
Click to expand...

I was expecting that! :p I'm more interested in using this to learn about fuel trims and such, I'm not worried about my truck blowing up. I'm still confident in the systems Mr T put in the truck to regulate everything, however I would like to know how they work and what the data means. I probably could get rid of the Scam-gauge now that I have fixed my P0401 and don't need to clear the code every week :D

As far as fuel trim, I read the diagnostic section of the engine chapter in the FSM last night and it gave me most of the answers I was looking for. The "normal" range for both STFT and LTFT according to Toyota is +/-20%. So that answers that question.


NLXTACY said:
Both of my sensors fluctuate wildly from 0 to about 80ish regardless of being at idle or WOT, hot or cold. Of course I am also throwing an O2 code
Click to expand...

Driving in to work today I noticed those values (O2 sensor values) using their full range (0-99), so I obviously didn't see all the fluctuations yesterday. I still would like to know what this data is for. Anyone have any ideas?
 
Landtank's MAF thread has some info on STFT and LTFT.
 
Beno's annoying outburst notwithstanding.... :p

I use mine for the digital water temp and intake temp, as well as voltage off the alternator. The 4th spot is open for other fun and games. Anyone program engine oil pressure or A/T temp?

Interesting to note when the alternator is "maintaining" and when it's recharging. Trying to monitor it vis-a-vis when I'm pulling a large voltage load (heater blowers, headlights, DS auto down window). I want to learn about whether or not my slow DS window is suffering from tired old wiring, or if it's all down to sticky run channels, etc.

Oh, and I'm clearing a P0401 almost daily now.

Thanks for your feedback.
 
Truckasaurus said:
Beno's annoying outburst notwithstanding.... :p
Click to expand...

The way I look at it if the truck is tuned right and everything is kosher, there should be no need for a Scam-gauge.

But hey, I drive a piece of s*** anyway, so what do I know?

:)

edit: I should clarify my position: if you are doing engine tuning, turbo/SCing it, doing other modifications that require precise measurements of engine performance and parameters, that I think it would be wise to get professional software that goes on to a laptop--sort of like what the pro's use. I have yet to see a pro. use the Scam-gauge.
 
Last edited:
the best way I've been able to monitor O2 sensors is to view it as a graph, the Scan gauge is pretty useless.

As far as fuel trim goes. Way too many environment variables such as air temp, altitude and humidity as well as system variables with dirt and age of the sensors and other components to have a common "normal" value.

But from my experience sea level is always a little positive, denser colder air means it's adding fuel over the base injector timing and high altitudes (6000 ft?) thinnier warmer air and the FT is -5 or 6%, subtracting fuel from the base injector timing.


disclaimer: as always this is just my opinion and understanding from data that I've collected.
 
beno said:
The way I look at it if the truck is tuned right and everything is kosher, there should be no need for a Scam-gauge.

But hey, I drive a piece of s*** anyway, so what do I know?

:)

edit: I should clarify my position: if you are doing engine tuning, turbo/SCing it, doing other modifications that require precise measurements of engine performance and parameters, that I think it would be wise to get professional software that goes on to a laptop--sort of like what the pro's use. I have yet to see a pro. use the Scam-gauge.
Click to expand...

I don't use mine for tuning, or doing modifications. I mainly use it to read codes, (only had to do that once), monitor my coolant temp, and use the trip computer functions for fuel consumption/use, I also like the ability to check other parameters just for fun. I like the fact that if my wifes car or a friends car throws a code, I can plug it in and read it, and will probably use it the next time I test drive a car, to see if codes have just been cleared, or if there are other obvious problems. I do not see it as a scam gauge at all, and am surprised you are so negative about it. For me, it is just a convenient, though not essential, tool.
 
STFT and LTFT Nerding

beno said:
Get rid of the Scam-gauge and get on with your life.
Click to expand...

Nice Beno! Absolutely the best quote I've read on scan-gauges in a while!

I have a few tools for OBDII and WB02 readings, as a rule, they are great diagnostic devices, and properly used can identify diagnostic trees to follow. Once done with them, put them away and get on with life?

WRT what Long Term Fuel Trim and Short Term Fuel Trim do. There are technical manuals available that explain them in minute detail. Applied to the 1FZFE 80 (including OBDI), here's my quick summary:

SHORT TERM FUEL TRIM
Short Term Fuel Trim (STFT) is instant reading and ECU compensation for trimming fuel injector duration. In narrow band 02 systems (including the FZJ80 series), they only indicate rich lean fuel trends... Whatever the Scanguage reads in 'value', the 02 inputs are ignored by the 80 ecu under (Open Loop=) WOT, decel, and O2 warm up modes. A value of +10 means 10% more fuel is added to the baseline fuel values, a value of -10 means 10% less fuel is subtracted from the ecu baseline fuel value. In the 80 a STFT value of over +/- 20% should throw a fault code. STFT has no 'memory', it is cleared when the engine is shut down.

LONG TERM FUEL TRIM
Long Term Fuel Trim (LTFT) is part of the ecu permanent memory, and is the basic injection duration calculation. This means it affects injector duration in open and closed loop = all running conditions. A LTFT value of 0 means no fuel is added to the permanent memory baseline, a value of +10 means 10% more fuel is added to baseline, a value of -10 means 10% less fuel is added to the baseline. LTFT values observed at higher altitudes should be negative, and LTFT values observed at lower altitudes should be closer to 0. Modified trucks should also observe higher LTFT values, since LTFT values are set for a stock truck at sea level.

Rear O2 sensors monitor the ecu corrrection of STFT, LTFT and catalytic efficiency. In the 80 OBDII Toyota also uses the rear O2 signal input to trim Baseline Fuel Values by about 2%. This is minimal, and common to account for reduced catalyst efficiency over the life of the cat.

ENTER THE SCANGUAGE
What do all these values mean? IMO/E without a CEL = not much, Beno is spot on. Both the LTFT and STFT values have a normal operating range. As an 02 wears/contaminates, the scanguage will show the 02 react slower and in larger values around 0. A new 02 will react faster, and in lower values around 0. Once the ecu preset values of STFT and LTFT (+/-20% on OBDII 80) are exceeded, the ecu throws a code.

I personally don't see much use in scanguage numbers of STFT or LTFT between engines. Too many variables to make comparisons valid. Scantools are good diagnostic tools, but really, for 02 readings, the wide band oxygen sensor is a better tool for the job, and can monitor well beyond the 'trends' of the narrow band oxygen sensors found in the 80.

HTH

Scott J
94 FZJ80 Supercharged
 
Last edited:
Nobody has made the claim that the scangauge is a replacement for a wideband O2 sensor or datalogging OBD program. I (and probably others) just wanna know what the heck STFT and LTFT are and what the numbers mean. Also, as to the SCII, what other useful or interesting things have people learned/used it for?
 
Thanks for the explanation Scott! It concurs with all the information I have read over the past few days and sums it up nicely.

As far as the usefulness of the Scangauge vis a vis fuel trim, I realize it is not the best tool for the job. However, I already own it and it performs a number of other functions that I find useful, and it is already in my truck. I didn't buy it with the goal of monitoring my fuel trims as I am aware that there are much better tools out there for that. However, I have it now, and it can show the current fuel trim data, so why not take a look at it? I also agree that a wideband sensor is a better tool for the job, but I don't have a requirement for one right now.

:cheers:
 
A OBDII scan tool can read the ecu STFT and LTFT values. The best way to look at these values is to think of LTFT values as the adjustment of your current motor to the conditions it operates in, vs Mr. T programming of a new engine from the factory. LTFT a memory imbedded constant "recalibration" from that factory 0 point. Then the STFT is the +/-20% from that recalibration value. Most EFI systems have had this STFT/LTFT capability since the mid 1980's.

The purpose of my post above was to explain how I understand these two values work in the 1FZFE engine ECU. The biggest problem with narrow band oxygen sensors is, they are narrow band oxygen sensors. Which specifically means, the scanguage values can see the 'trend' of the 02, but the absolute values don't really mean much.

IMO/E too much data of a properly functioning STFT/LTFT is just too much data. Since there is a bulb in the dash that identifies a a 'fault' value, why stare at it all the time? (insert Beno quote).

Scantools are great for those wanting to watch what the ECU 'sees' for specific values. For in depth diagnostics, they are usually calibrated against a more accurate tool (IR temp gun, wide band 02, voltage/resistance/Duty cycle metering). And/or, they are compared to some other scantool value (TPS, engine temp, IAT, etc) in a datalogging environment.

HTH

Scott J
94 FZJ80 Supercharged
 
You must log in or register to reply here.

Share this page

Back
Top Bottom