Why are my Short Term Fuel Trims Jumping?

[Two_Car_Garage]

So this morning I decided to log my Short and Long Term Fuel Trims just for fun and to learn more about my car on the way to work. I have done some reading on Fuel Trims and everything appears to be as expected. I have no codes. Car is 18 years old and 250k miles.

Log/Graph Attached.

Now my question is why why are my STFT jumping to around 15%? LTFT are ~ +4% and - 1%

Based on some reading I did, readings should be less than 10%. or is that a rule of thumb? I am thinking it could be a response to my spirited driving but not sure if this could also mean my Oxygen sensors are going bad. Too bad I did not log RPM's to see if it correlates with stop/starts.
Maybe a log for tomorrow.

In the meantime anyone has any ideas why the jump to 15% in STFT's?

 

[slow]

what kind of vehicle? any WOT runs in there? Ford vehicles the manufacture specific short term fuel trim will show the commanded lambda when in open loop, so that may be part of your issue.

 

[zkdiesel]

It’s adjusting for driving. Constantly adjusting fuel based on load and air

 

[vavet]

Unless your LTFTs are adjusting to keep the STFTs near zero, then it's a non-issue. It is only when the LTFTs get out of range that you need to start thinking about a problem.

 

[Two_Car_Garage]

what kind of vehicle? any WOT runs in there?

Mercedes Benz. No WOT's in the time interval logged.

Unless your LTFTs are adjusting to keep the STFTs near zero, then it's a non-issue. It is only when the LTFTs get out of range that you need to start thinking about a problem.

Like I said I don't think there is a problem just trying to understand the spikes. So how do we know the LTFT are adjusting? I am attaching the plot from the LTFT. The long term ones also show the spikes but the magnitude of the spike is much less percentage wise.

 

[theoldwizard1]

what kind of vehicle? any WOT runs in there? Ford vehicles the manufacture specific short term fuel trim will show the commanded lambda when in open loop, ...

It is not possible to meet any emission standards for the past 10 years if you are in open loop for more than about 1-3 minutes at very cold operations.

 

[DarrenF]

if your ltft are below 10% + or -, then nothing is wrong. the stft #'s dont really matter, for the most part, unless there is a drivability issue. The job of the long term fuel trims are to keep the short term fuel trims as close to zero as possible. When you see long term at + or - ~15% is when you probably have an issue devoloping, and would want to look at the other data pids to see if anything points to the cause (maf #'s, graph 02 sensors, etc etc)

 

[Two_Car_Garage]

if your ltft are below 10% + or -, then nothing is wrong. the stft #'s dont really matter, for the most part, unless there is a drivability issue. The job of the long term fuel trims are to keep the short term fuel trims as close to zero as possible. When you see long term at + or - ~15% is when you probably have an issue devoloping, and would want to look at the other data pids to see if anything points to the cause (maf #'s, graph 02 sensors, etc etc)

Thanks! Yeah that is what it looks like. I am just learning about this stuff.

 

[Walter_TA]

LTFT + STFT = the correction to the VE table by O2 sensor. VE table values comes from the MAF if it has one, if not, the MAP. If LTFT + STFT > than 25% you get a fault. O2 sensors do not work at WOT. The trims will lock.

 

[Two_Car_Garage]

LTFT + STFT = the correction to the VE table by O2 sensor. VE table values comes from the MAF if it has one, if not, the MAP. If LTFT + STFT > than 25% you get a fault. O2 sensors do not work at WOT. The trims will lock.

Hmm, just for giggles and to learn something what is VE Table. I assume is some lookup table the ECU uses for different loads and air/gas ratio mixes but have not heard that term before.