What causes low short-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.
What is short-term fuel trim 1?
In short, fuel trim describes the adjustments that a vehicle’s engine control unit (ECU) makes to the fuel delivery system to keep the air-fuel mixture as close to its ideal ratios as possible.
What is short term and long-term fuel trim?
The short-term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long-term fuel trims (LTFT) are driven by the short-term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time.
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.
Why do I need to use fuel trim?
If the condition exists in all rpm ranges, the cause is more likely a fuel supply issue caused by low fuel pressure, a restricted injector or a defective sensor input from a MAF or engine temperature sensor. Fuel trim can also be used to identify which bank of cylinders is causing a problem.
Can a low pressure fuel pump increase fuel trim?
The answer is yes. The low-pressure fuel pump volume discussion is basically the same—idle requires less volume to the fuel injectors and higher loads require greater volume. Depending on the condition of the low-pressure fuel pump, the fuel trim values may be only slightly elevated at idle, but get worse with greater load.
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.
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.
What is the formula for negative fuel trim?
Too little oxygen in the exhaust indicates a rich mixture, causing the PCM to reduce injector pulse width (negative fuel trim). The formula for fuel trim calculation is: Fuel mass = Air mass x (short-term fuel trim x long-term fuel trim) divided by (equivalence ratio x 14.64)
What should my LTFT be for fuel trim?
Normally LTFT should be between -10 to +10 percent. But the closer it is to zero, the more ideal the combustion mixture. In my case -33% means the ECU is reducing fuel by 31% to maintain the ideal fuel mix. STFT (short term fuel trim) is the instantaneous response of the ECU to reduce or add fuel to maintain an ideal combustion mixture.
Can a false rich code be used on fuel trim?
If you use only the false rich codes to diagnose and don’t observe all the fuel trim and o2 sensor data, you may make a false diagnosis. Also, each bank has it’s own fuel trim reading. If your engine is a 4 cylinder, then it has only one bank, Bank 1.