Understanding fuel trim is an efficient in automotive emission troubleshooting
The oxygen sensor (o2 sensor) is the only detection for the PCM (onboard computer or power train
control module how rich or lean the vehicle is running. The PCM needs this information for adaptive
strategy known as Short Term fuel trim and Long Term Fuel strategy
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The close loop system is designed to rapidly change the air/fuel ratio from slightly rich to slightly lean of
14.7:1. By changing the air/fuel mixture from lean to rich is called close loop, this is when the PCM
responds to the O2 sensor to help balance the air/fuel mixture.
Anytime the PCM ignores the O2 sensor it's called open loop, in open loop the air/fuel mixture is based off
pre program information.
These values become the calculations for fuel
calculations. On the automotive scan tool has you
are looking at short term and long term strategy
you will see + or – percentage.
The (+) value indicates the computer is adding fuel to the system, this means for some reason the injectors are being held open for a longer period of time. If the value on the scan tool is (-) the computer is decreasing the pulse width for some reason.
Here is an example: if the vehicle has a vacuum leak the o2 sensor will produce a low voltage, (.1-.4v) indicating to the PCM that the vehicle is lean. The PCM strategy is to compensate for this, so it’s going to add fuel to the system. Your scan tool will show for example (+20), on the Short term or Long Term Fuel Trim.
This indicates that the computer is holding the injector’s open 20% more than usual to add fuel to the system.
Now if the O2 sensor is indicating that the vehicle
is rich by producing a high, voltage,(.5v – 1.0v) the
PCM will try to reduce the fuel delivery by closing
the injectors. On the automotive scan tool for
example you will see (-20%), on the Short term or
Long Term parameters. this means the PCM is
holding the injectors close 20% of the time.
(I hope that makes sense) other words the injectors
pulse width is smaller, (less fuel).
Precise Closed Loop control relies on a accurate feedback information from the O2 sensor.
The PCM uses the O2 sensor to claculate the fuel Injection pulse width.
Long term and Short term fuel trim should vary below and above “0” plus or minus 10% indicating a good fuel stragedy.
Now this is very important if it’s very high, ask yourself why is the vehicles computer trying to add fuel. If it’s low why is the computer trying to subtract fuel.
Here are some examples:
if the o2 sensor indicates a vacuum leak it will produce a low voltage to the computer, the computer will then turn around and add fuel by increasing the pulse width of the injectors. You will see this increase in the Short Term, (STFT) or long Term Fuel Trim above 10%.
One problem could be a vacuum leak in the intake manifold or a vacuum hose fell off, and the computer is trying to lengthen the pulse width to adjust (add fuel) for the extra air, (lean condition).
There reverse is also true if the O2 sensor is indicating a rich condition it will produce a high voltage, the computer will then turn around and subtract fuel by decreasing the pulse width of the injectors. You will see this decrease in the short or Long Term parameters.
The problem could be higher than normal fuel pressure or engine oil that’s diluted with gasoline. The computer sees this rich condition is trying to shorten the pulse width.
So let’s talk about Short Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) again.
The short term parameter is the computers direct response to the O2 sensor, when the o2 sensor goes rich, high voltage (.5v-1.ov) the STFT goes negative to compensate for a rich condition. When the O2 sensor goes lean, low voltage (.4v-.1v) the STFT will go positive to compensate for a lean condition.
Remember this only happens in close loop.
Long Term parameter is a learning ability on how lean or rich the computer needs to be at to establish the correct air/fuel mixture. In computer terms 14.7:1, that’s 14 pounds of air to one pound of fuel, this is a balance air/fuel mixture.
The computer system needs to see the O2 Sensor moving from .2v - .8v and averaging 450mv. Also the job of the LTFT is to keep the STFT moving above and below the “0” plus and minus 10%.
When the Short Term , (STFT) has been lean or rich to long the Long Term, (LTFT) will compensate for it. If the vehicle is rich LTFT will drive the system lean, to bring the STFT back towards “0” plus or minus 10%. When this happens the O2 sensor will move back and forth between .2 and .8v.
When the Short Term, (STFT) has been lean the Long Term , (LTFT) will drive the system rich, to bring the STFT back towards “0” plus or minus 10%. When this happens the o2 sensor will move back and forth between .2 and .8v.
So with that said, when you diagnose a vehicle you should use LTFT, this is a learn value and will tell you how the computer system is controlling the air/fuel mixture.
Short term is erase when the key is turn off, long term is not it’s a learn value. When a repair is made you should reset the fuel cells or adaptive fuel staragey so the computer can will not run off the old strategy.
Remember the goal of the computer is to see the 02 sensor voltage averaging .450mv, (moving between .2-.8v) and the STFT at “0” plus or minus 10%.
Also you should use (LTFT) Long Term Fuel Trim when troubleshooting emissions or drive ability problems.