LTFT - How exactly is "long term" defined?

[inquirer]

LTFT is an average of STFT values, right? But how many of the STFT values does it take into account? Is the LTFT calculated by using the last driving cycle's STFTs only? Is it an average of more than one driving cycles or is it an average of all STFT values since the last (for whatever reason) reset?

I'm asking, because I was monitoring the LTFT of my car while driving in the city and it fluctuated widely between 4 or 5% to 12% repeatedly every few minutes minutes.

 

[mk378]

Fuel trim both short and long term is not stored as a single number. It will switch to different memories based on operating conditions (e.g. idle vs cruise) while driving.

 

[inquirer]

Fuel trim both short and long term is not stored as a single number. It will switch to different memories based on operating conditions (e.g. idle vs cruise) while driving.

Great! So is there a specific name for each of these separate types of STFTs and LTFTs? Can they be read separately by an OBD scanner or do ECUs only report a generic average value for STFT and LTFT?

 

[brianl703]

The STFT and LTFT reported by an OBD-II scanner are the values being used at that current moment.

From what I understand, STFT is based on what the oxygen sensor is reporting at that exact moment. LTFT is adjusted based on a rich or lean shift that is, for lack of a better word, chronic--for example, a slightly plugged injector or a vacuum leak will alter the LTFT, as the LTFT is shifted to keep the STFT cycling as it needs to from lean to rich and vice-versa to keep the 3-way catalytic converter operating correctly.

There are LTFT entries in a table in the ECU which is built upon input from the oxygen sensor, and the LTFT can vary depending on operating conditions as previously mentioned.

It isn't possible to see the entries in this table with a normal OBD-II scanner.

 

[brianl703]

So if something like a vacuum leak causes the oxygen sensor to read lean, the STFT will reach the limits of it's adaptation as the ECU adds fuel to compensate and then the LTFT will be adjusted to compensate to bring the STFT back where it needs to be, cycling nicely between rich and lean. If the LTFT reaches the limits of it's adaptation (25% from what I read) then a lean code will be set.

So STFT operates under LTFT. Think of STFT as the fining tuning knob and LTFT as the coarse tuning knob.

 

[inquirer]

The STFT and LTFT reported by an OBD-II scanner are the values being used at that current moment.

OK. In that case how can you use LTFT to diagnose how good does an engine run? In my case which of the LTFT values I mention in my first post should I take into consideration?

 

[brianl703]

OK. In that case how can you use LTFT to diagnose how good does an engine run? In my case which of the LTFT values I mention in my first post should I take into consideration?

Those values look fine. Not until LTFT reaches 25% do you get a trouble code. You might want to look for a problem if LTFT were approaching 25%. One article I read years ago, the author said he considers anything higher than about 15% to be indicative of a potential problem and suggested looking for the cause.

Years ago I had a car that had high LTFT. I think it was running at 20% at idle. (With a vacuum leak, LTFT will be highest at idle). There were no trouble codes. I inspected the vacuum and PVC hoses and found a deteriorated PVC hose. I replaced it and the LTFT went back down.

LTFT is so sensitive, you can often see an increase just by pulling the engine oil dipstick with the engine running (assuming that the engine has a MAF sensor).

 

[inquirer]

@brianl703

I read articles which state that values above or close to 10 indicate that the engine does not run under ideal conditions.

 

[brianl703]

@brianl703

I read articles which state that values above or close to 10 indicate that the engine does not run under ideal conditions.

The presence of ethanol, MTBE, or other oxygenates in the gasoline will increase the LTFT.

I would not be concerned about a value lower than 15.

 

[inquirer]

The presence of ethanol, MTBE, or other oxygenates in the gasoline will increase the LTFT.

I would not be concerned about a value lower than 15.

What would LPG do? Because I have recently converted my car to LPG.

 

[brianl703]

What would LPG do? Because I have recently converted my car to LPG.

It's entirely dependent upon the LPG conversion system/hardware. The fueling tables in the ECU are for gasoline. Assuming that there is an adjustable regulator, adjusting the pressure higher should reduce the LTFT. If there is more than one regulator you want to adjust the one closest to to the engine. Suggest contacting the supplier of your LPG conversion system for more details.

 

[wtd]

LTFT's are learned from the STFT's. The main job of the LTFT is to keep the STFT as close to 0% as possible. It is also used to keep the fuel trim corrections in memory and the PCM looks at LTFT's first to know where to start injector pulse width. A +/- 10% fuel trim is considered normal on most systems.

From my understanding, the LTFT retains its readings from previous drive cycles and does not start over on each new drive cycle.