Is this valid, the idea that you take Lt and St fuel trim levels and add them together to get the true number ect???????????


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When adding the long term trim to the short term trim, you obtain a basic “total trim number”. This number should be very close to zero, never exceeding 10% positive or negative.

For example, A LTFT of 3% and a STFT of 1% would give a total trim of 4% which is within the normal range.

Negative numbers can work in your favor as well; for instance, the STFT is 4% and the LTFT is -2%, the total trim number would be 2% which is near perfect. The same thing if the total trim number wound up in the negative.

A number close to zero is fine, but 10% negative or positive is out of limits.

Now, besides the total trim value being well below the 10% threshold, the spread between the long term and short term fuel trim values must be as well.

As noted above, adding a negative number to a positive number can get you closer to zero which is a good thing, but if the spread is too large, you need to find out why.

For example, LTFT is 7% and STFT is -7%, well that would give a total trim number of zero which is “perfect”. However, the spread between the numbers is 14% which exceeds the target “10%”. While it is true the total trim number is zero, it’s a bad way of getting there. It is an indication that the PCM (power train control module) is not in complete control of the fuel injection, it’s scrambling to make up for an issue that is nearly out of its control. If the vehicle utilizes mass air flow (MAF), this condition STRONGLY suggests Mass Air Sensor issues. If the vehicle utilizes MAP (manifold absolute pressure sensor), check the vacuum at the MAP sensor.

Suppose that the numbers are all well within the target 10% limits at idle (remember, that doesn’t mean 9% is good, just that the numbers may not have exceeded 10% yet). You should now make certain that there is no fuel starvation issues occurring.

Hold the engine RPMs up around 2500 for a minute and observe the trim data once more. (Hint if the numbers don’t change, that would be very suspect. Short term trim will usually be moving all the time, but if your Long term trim doesn’t change from idle to 2500, you need to investigate further.


Tom in Dallas/Plano
2005 S-type 3.0 84k

 

That is similar to averaging the averages. They are separate and distinct readings that provide information on mixture adjustment.

 

That I understand. However I generally look at only LT- is that correct or incorrect or should I ADD the 2??
Thanks

Tom

 

The sum of the two gives you the actual fueling adjustment at any given time.

LTFT is the long term afar rage adjustment. If you were to do a hard reset of the ECU the LTFT would be zero and the STFT would be high. As you drive the car the LTFT will store the average STFT and the STFT values will approach zero.

What you should end up with is a LTFT of some number and a STFT that oscillates a little above and below zero.

Some care needs to be taken in judgement of the values because all kinds of things change them. Differences on engine load. A/C compressor. Air temperature. Whether the car is warmed up or not. Mass air flow (ie how fast you are going).

The 10% rule is mostly about the LTFT but you can use the behavior of the STFT to diagnose a problem... E.g. If it is suddenly very high and always positive you might have a vacuum leak.

 

It's a bit half-right e.g. there's no reason to say that LTFT shouldn't (or should) change from idle to 2500.

Similarly, if LTFT was +7 and STFT -7 or the other way round you'd wonder why it's not learned so that LTFT is about 0 and STFT swinging around (or steady) close to 0. Engage brain to answer.....

There are good & detailed articles on the net about this stuff if you want to know. Try to figure LOAD as well

Just on this site, look for articles by xjrguy (& Brutal but I don't think he's posted many on trims).

 

Thanks.

It was something I stumbled across as I was looking at my friend's mother's Taurus and was actually doing some research on that when I found this. I had always and generally looked at LT and while I have no problem on my own car- I was trying to gain insight for what might be ultimate usage.

Tom in Dallas