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What is a normal long term fuel trim reading?
Long-term fuel trim values Ideally, long-term fuel trims should be at, or close to 0% when the engine is running at a steady speed. However, while changes to the engine speed will (and must) produce changes in the long-term fuel trim value, this value should return to a point close to 0% when the engine speed steadies.
What causes high positive long term fuel trim?
If the engine uses a mass airflow sensor (MAF), high fuel trims at idle are a classic symptom of a vacuum leak, especially if LTFT decreases at higher engine speeds. 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.
How does long term fuel trim work?
Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter. While the sensors’ signal will fluctuate frequently upstream, where short-term fuel trim is measured, the signal at the sensors further down should be fairly constant.
How does the ECM work to calculate fuel trim?
To better understand how fuel trim numbers are generated, the ECM/PCM uses the oxygen sensor or air/fuel sensor input signal to adjust fuel injector pulse-width values. If a longer-than-normal injector pulse width is required to maintain the desired oxygen sensor signal, the ECM/PCM displays an increased fuel trim value in its datastream.
When to use fuel trim for a problem?
Fuel trim can also be used to identify which bank of cylinders is causing a problem. If the LTFT for bank 1 is 20% and the LTFT for bank 2 is 3%, the source of the problem is associated with bank 1 cylinders only. The diagnosis should focus on factors related to bank 1.
What should bank 1 fuel trim be at idle?
If long-term fuel trim (LTFT) for bank 1 is 25% at idle, but corrects to 4% at 1,500 and 2,500 rpm, focus on factors that cause a lean condition at idle, such as a vacuum leak.