Fuel Trims: LTFT vs STFT vs Total Fuel Trim - whats going on???

[cwmoser]

My 2002 LX470 does not have Check Engine and has no DTC codes.
But I note that Tech Stream shows not only short and long term fuel trims but also Total.
My LTFT seems to always be negative and most time hovering in the -10 to -14 range.
I've read that LTFT should hover close to zero and remain steady but mine seems to vary a lot.

My Total fuel trim does seem to hover under 5.

So what should I make of this?

 

[jerryb]

 

[ramangain]

^ best explanation yet why vacuum leaks are no bueno

 

[jerryb]

Dude taught me something when I saw it the first time. Best I've seen, and I'm old.

Another good one is the young kid and the myth of lean equals hot.

 

[Live to Ski]

What a great lesson!

 

[cwmoser]

I've read a lot about basic short term and long term fuel trim, but what is Toyota's "Total Fuel Trim" as in Tech Stream.

 

[jerryb]

total in techstream isn't both combined? Do those numbers add up?
I do not know if they're stored for sure in volatile memory.
If you haven't zeroed those out it might be a good idea to get a good starting point.

 

[cwmoser]

I got my Tech Stream Interface and Windows Tablet hooked up to my LX470 and I think I solved my LTFT problem.
With the old MAF sensor I was getting high negative LTFT reading - as high as -15.
The first photo shows the LTFT's with the old "Ebay Chinese" MAF.
The second photo shows the LTFT's with the OEM Hitachi MAF.
I remember 4 years ago replacing the the MAF with the Ebay part but can't remember the circumstances.
Whats curious is that one consistantly gives negative trims and the other consistantly low positive trims.

What confused me was what was this TOTAL FT #1 in Tech Stream which looked like what a LTFT number should be.

Old Ebay Chinese made MAF Sensor: Suspicious Long Term Fuel Trims.

OEM Hitachi MAF Sensor: Long Term Fuel Trims now looking like they should be.

 

[ramangain]

Looks to me like TOTAL FT may not be a standard variable when it comes to "classic" understanding of STFT and LTFT.

Can TOTAL FT be reset?

 

[jerryb]

as much as I've messed with rear o2 voltage I could never be 100% certain they(FTs) were reset with a battery timeout.

I can verify swapping injectors and seeing the immediate change (FTs) without a battery timeout.
Plus there's so much lag in techstream ---> display, sometimes it's hard to narrow it down.

 

[shtbrwn86]

This is incredible. Guy really knows his stuff. Now I need to go outside and start looking into my fuel trims! At least I can see them on my phone now unlike when he made this video!

 

[cwmoser]

That is a very informative 2 part video.
I think though that one really needs not only the "book learning" from the video, but also "practical experience".
That video definitely though gives on a basis for learning.

 

[64fabius]

In addition to a new MAF, did you replace ignition coils?

 

[dwrecks]

Total novice on this topic, and havent finished the video yet; but comparing your MAF sensors, it seems the Ebay Chinese MAF is showing 10g/sec, double what the new one says. Could the faulty MAF sensor, thinking it had much more air then it did, cause the fuel trim to start pushing rich? Really cool to see that data comparison.

 

[Eicca]

Other things we can learn is that if both banks are reading equally wonky long term fuel trims, then the problem lies before the cylinder banks in the intake system somewhere. Positive numbers usually mean a vacuum leak, negative numbers usually mean a problem with the MAF or too much fuel delivery to both sides equally.

I mean, it is possible for both O2 sensors to fail in perfect sync I guess, but highly unlikely.

 

[cwmoser]

Other things we can learn is that if both banks are reading equally wonky long term fuel trims, then the problem lies before the cylinder banks in the intake system somewhere. Positive numbers usually mean a vacuum leak, negative numbers usually mean a problem with the MAF or too much fuel delivery to both sides equally.

I mean, it is possible for both O2 sensors to fail in perfect sync I guess, but highly unlikely.

This can be confusing. Make sure I am fully understanding all this:
Positive fuel trims mean that the ECU is adding more fuel to maintain 14.7:1 ratio because there more air sneaking into the engine because of a vacuum leak.
A positive voltage on the O2 sensor means it is sensing fuel in the exhaust, negative voltage that it is sensing oxygen in the exhaust.
????

Also, exactly how does one hook up a Smoke Machine to our Engines in order to find Vacuum Leaks?

 

[Eicca]

This can be confusing. Make sure I am fully understanding all this:
Positive fuel trims mean that the ECU is adding more fuel to maintain 14.7:1 ratio because there more air sneaking into the engine because of a vacuum leak.
A positive voltage on the O2 sensor means it is sensing fuel in the exhaust, negative voltage that it is sensing oxygen in the exhaust.
????

Honestly the actual voltage from the O2 sensors is still confusing to me, but yes, positive trim numbers on the scan tool means the computer is adding fuel because it sees more air than it expects.

Negative fuel trims mean it's pulling fuel because there is LESS air than it expects. Which, in reality, almost never physically happens because of an airflow problem. If the air isn't flowing, the sensors will see it. Unless the sensors are bad. Air never leaks OUT of a naturally-aspirated engine, always in.

 

[jerryb]

 

[cwmoser]

How reliable and long lived are O2 sensors?
If O2 sensors report back the wrong information to the ECU, it could cause the ECU to report LTFT up or down.
I'll admit that I did replace the right side O2 sensor with an Ebay likely Chinese part.
Hmmm.

 

[2001LC]

One test I like to do is in tech stream, by using active test mode. I bring up fuel injector controls and will take fuel to max - than to max +. Watch voltage of A/F (06-07 VVT) and O2 (98-07). Each bank should react same (Bk1S1 match to BK2S1, BK1S2 match to BH2S2. This give good indication of the senors health.

FT: I like logging while driving. I'm most interested in readings while cruising on HWY, which RPM need to be steady. Pressure on gas pedal or letting off, reads jump which I ignore for the most part. I also watch OAT, IAT and ECT. If air very cold air density is increased. This allow more air into intake. If a hot day hot air is less dense, which reduces intake air. Altitude matters as does barometric pressure. The ECU is constantly adjusting for these condition. So we may run - or + LT FT, something around 3% is great, up to 5% is in normal range.

Trick: When adding fuel cleaner like 44k. It's possible to increase effectiveness by take Fuel Injectors to max +, then shut engine down for the night. Aids in soaking the cylinders with fuel/44k mix. Than take on HWY for a high RPM burn out (7 minutes at near 5K rpm) heating the CAT and sensors. Like turning you oven to clean. Make sure you've good oil...