This topic is still kicking me in my rear. What videos can you direct me in the premium channel where I can understand this topic better. Because when paul did the fuel trims with the honda for a rich condition, i tried following but got lost quickly. How do I read my own fuel trims in my car and decipher it?
I understand your confusion, and like you, I'm a student.
So, we'll let Paul and others chime in to provide feedback on how well I have absorbed the material.
How do you read the data?
The bottom line is you are looking for ZERO on both long and short term fuel trims. Positive numbers mean the engine is in a lean condition/rich command; in other words, there isn't enough fuel, and the computer is commanding the injectors to open for a longer period of time.
Negative numbers, mean the engine is in a rich condition, and the computer is commanding to take away fuel.
As long as you are within a range of -10 to positive 10, you are fine.
The fun comes with you have a high negative number or a high positive number.
For example, I had a Ford F150 6 cylinder come in with lean misfire codes; so I connected the scan tool and monitored fuel trims. As I increased the engine speed, (less vacuum), I noticed the short term fuel trim number drop. With less air going into the engine due do lower vacuum, the computer commanded less fuel. So, I smoke tested it, and found intake manifold leaks at the EGR valve, and two PCV hose leaks. Once I replaced the EGR valve and hoses (and cleared the DTCs), fuel trims went back to the normal range.
Still, I ran into a weird one today, so I'll be posting that as a separate question for the real experts to answer.
I was just having this conversation with the shop manager the other day...
Short term fuel trim (STFT) is immediate correction. There is a ratio imbalance detected by the oxygen sensor, and the PCM will add or subtract the appropriate amount of fuel to bring the ratio into correction.
Long term fuel trim (ltft) is used to keep short term fuel trim close to zero.
Think of it like playing catch with a little kid.
He throws the ball, it comes up short.
You need to walk 10 steps to pick up the ball. This happens 4-5 times, you decide to just close gap and stand closer.
That's LTFT. Instead of walking 10 steps forward to pick up the ball every time(STFT) you just stand in a range where he can hit you (LTFT). Because you adjusted your position, you no longer need to chase the ball. So your LTFT is 10 steps, but STFT is 0. If the ball comes up short again, you may have to move a step or two.
So the total fuel trim is the 10 steps you adjusted plus the one or two to you need to take to catch the ball.
Now his buddy shows up and over throws you. You walk 10 steps back to pick up the ball. (STFT) that gets old, so you stay the 10 steps back (LTFT)...
Get it?
That’s the best explanation I’ve heard to date. Noah, you knocked it out the ball park. Pun indented lol. Now when I’m driving my car and I have my scanner hooked up, I just have to slow down and make sense of the changing numbers. Thanks Noah.
Topic Author
