Understanding fuel trims some more

[wirelessF]

I have a computer based OBD-II software that reads fuel trims and a Scanguage that is programmed to read fuel trims. I understand some of the basics of fuel trims like positive % means lean or adding more fuel and negative % means rich or adding less fuel. Having 0% in LTFT and STFT is perfect.

I've tried reading the fuel trims on various vehicles to find the "normal operating limits". Do manufacturers have "normal" fuel trims values to ensure that the engine is running normal? In my Tundra, the LTFT is near zero percent and the STFT fluctuates between -3% and 3% in idle and fully warmed up. However in my Ford the LTFT are -11% and STFT is at near zero when at idle and fully warmed up.

 

[brianl703]

Anything exceeding 25% correction should set a code. Outside of that there is no definition of normal.

 

[BuickGN]

The way it is, at least in my old Gn is when short term hits it's max, the long term will change a little and short term goes back to 0.

In fact the way it's displayed is with cells with each cell representing a long term fuel value and the number inside of the cell representing the short term value.

In short, I wouldn't worry if the short term is out of whack as long as the long term is ok.

 

[Gary Allan]

My numb speed/density system appears to work as BuickGN states ..sorta anyway. If I read the description correctly in the FSM, the short term adaptive cells alter the long term adaptive cells if the change last too long. The short term adaptive cells read the closed loop data on (what it called) valid trips. That's used to select the fuel mapping during open loop operation. If the pre-mapped fuel curve is too out of whack, for too many instances, with the current state of the pulse width generated as indicated by the short term adaptive cells ..the long term adaptive cell increments or decrements accordingly.

Essentially, too many LEANS it increments a correction factor toward rich. Too many RICH .it decrements a correction factor.

I imagine that a brief bout at higher altitude would not alter long term adaptive cells, but would surely alter short term adaptive cells. If you moved from sea level to 5000 ft, your long term trims would surely be altered. It also accounts for long term degradation of the fuel system.

 

[brianl703]

My experience with long term fuel trim is that it changes within seconds. You can pull the dipstick on a car and watch the long term fuel trim increase within a few seconds.

 

[SHOZ]

+ - 5% is normal