What is the maximum fuel trim adjustment for LTFT?
For example, if the maximum LTFT adjustment is +25% and the total fuel trim adjustment is +30%, then LTFT will report +25% and STFT will report +5%, for a total fuel trim value of +30%. The LTFT calculation is kept in memory on most vehicles, so the PCM does not need to relearn the fuel trim calculation the next time the vehicle is started.
How long does it take for fuel trim to change?
Fuel Trim (ming) Diagnostic Time. On some vehicles, if STFT has reached the specified limit, LTFT will change in a few seconds. On other vehicles it may take 15 to 20 seconds before a change occurs. The LTFT calculation is normally kept in memory, so the PCM is ready to use the last known injector pulse width following a restart.
What happens when fuel trim is greater than 10%?
If LTFT or total fuel trim is greater than plus 10%, the PCM thinks the air/fuel ratio is too lean and it’s adding fuel to bring STFT control to the correct range. This presents three possibilities: Unmeasured air is reaching the combustion chambers. Less than the commanded amount of fuel is reaching the combustion chambers.
What can cause a positive fuel trim number?
A dirty or faulty MAF sensor can also cause positive fuel trim numbers because it “under-reports” airflow, driving the base air/fuel calculation lean.
What causes the fuel trim readings to change?
Really, the oxygen sensors are what drive the fuel trim readings. Changes in o2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second.
For example, if the maximum LTFT adjustment is +25% and the total fuel trim adjustment is +30%, then LTFT will report +25% and STFT will report +5%, for a total fuel trim value of +30%. The LTFT calculation is kept in memory on most vehicles, so the PCM does not need to relearn the fuel trim calculation the next time the vehicle is started.
What causes a fuel trim sensor to fail?
If the fuel mixture is running rich because the O2 sensor is inoperative, or the feedback control system is staying in open loop, the firing voltages will likely be lower than normal with longer firing lines. Possible Causes of Fuel Trim Failures
What should the short term fuel trim range be?
Short Term Fuel Trim. • This is an immediate correction to the fuel systemto the fuel system • An acceptable range is +/- 10%. Long Term Fuel Trim. • This is how the system has a tendency to runtendency to run • An acceptable range is +/- 10%.