How do you diagnose fuel trim issues?
Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high as plus or minus 8, but closer to zero is best. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN.
What does positive long term fuel trim mean?
Positive fuel trim values If the displayed fuel trim value is a positive number, it means that the ECU is increasing the injector pulse width to add fuel to the air/fuel mixture to enrich the mixture because the input data it is receiving tells it that the mixture is too lean.
What affects long term fuel trim?
Long Term Fuel Trim 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.
What causes low long-term fuel trim?
During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This causes the fuel trim numbers to become negative during a boost situation.
When to check for a fuel trim problem?
A useful diagnostic tool is to determine if the fuel trim problem exists in more than one engine operating range. Fuel trim should be checked at idle, at 1,500 rpm and at 2,500 rpm. In addition to these no-load checks, drive the vehicle at various steady load conditions and watch for fuel trim variation.
What does a positive fuel trim percentage mean?
Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to richen the fuel mixture, to compensate for a perceived lean condition. Negative fuel trim percentages indicate the PCM is attempting to lean out the fuel mixture, to compensate for a perceived rich condition.
What causes long term fuel trim Bank 1?
In addition to these no-load checks, drive the vehicle at various steady load conditions and watch for fuel trim variation. 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.
When to use short term and long term fuel trims?
If you connect a scan tool to such a vehicle, both the short and long-term fuel trim data will be displayed as percentages. Ideally, both values should be at, or close to 0%, when the engine is running at a steady speed, such as when it is idling.