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How much fuel trim is normal?
Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction. Long Term Fuel Trim (LTFT) is a longer term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time.
What causes positive fuel trim?
Here are some common causes of positive long-term fuel trim: Air or vacuum leaks in the intake manifold, throttle body or in a vacuum hose. Defective fuel pump not generating enough pressure or volume. Restricted fuel flow due to pinched hose or plugged filter.
Is positive fuel trim lean?
If the value is a positive number, it means that the ECU is adding more fuel to the mixture to enrich it because the information it’s receiving indicates that the air-fuel mixture is too lean.
Why is my long term fuel trim high?
Why are the fuel trim numbers high? 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.
What does a negative fuel trim indicate?
Negative fuel trim values If the displayed fuel trim value is a negative number, it means that the ECU is decreasing the injector pulse width to subtract fuel from the air/fuel mixture to lean out the air/fuel mixture because the input data it is receiving tells it that the mixture is too rich.
Can a bad catalytic converter cause negative fuel trim?
The downstream sensors only monitor the health of the catalytics. If the readings between the upstream and downstreams are too close or too far apart a code is thrown for catalyst inefficiency. The only way a catalytic will affect fuel trims is if it’s clogged, but under normal operation it has zero effect.
Is the oxygen sensor accurate for fuel trim?
Of course, accurate fuel trim values are based on a properly functioning oxygen sensor. So, when diagnosing any fuel injection malfunction, it’s important to make sure that the oxygen sensor is accurate and responsive to fuel trim changes.
How do you baseline the fuel trim data?
Baselining the fuel trim data is a simple four-step diagnostic process: 1. Connect an OBD II generic scan tool, preferably with recording capabilities. 2. Monitor and record the fuel trim values in the four common operating ranges—idle speed, light load (20 to 30 mph), moderate load (40 to 50 mph) and heavy load (60 to 70 mph). 3.
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.
What are the four ranges of fuel trim?
2. Monitor and record the fuel trim values in the four common operating ranges—idle speed, light load (20 to 30 mph), moderate load (40 to 50 mph) and heavy load (60 to 70 mph). 3. Analyze the collected data.