What causes high fuel trims?
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.
When to use LTFT or STFT fuel trim?
LTFT and STFT are added together to produce the total fuel trim, and neither should exceed ± 10% in normal operating conditions. Fuel trims (LTFT, STFT, or LTFT+STFT) exceeding ±10% may indicate a problem in the engine or the sensors.
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 do the trim levels on a Chevy truck mean?
Trim levels reflect the engine, interior, electronic, and other exterior design features that a specific truck model has. Chevy is known for having an extensive line of trim levels that can vary from year to year and truck to truck.
What’s the difference between short term and long term fuel trims?
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. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.
What’s the difference between STFT and LTFT fuel trims?
It’s similar with short and long-term fuel trims. STFT are immediate ups and downs in fuel, while LTFT are what is occurring over a longer period. A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second.
What should my long term fuel trim reading be?
Usually they’ll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added.
What does a positive number on fuel trim mean?
A positive number tells you it’s adding fuel. Since the trouble codes indicate a lean condition (P0171 and P0174) on both banks of cylinders, both Long Term Fuel Trim 1 and 2 should be in their maximum positive number range as the PCM adds fuel to compensate for the perceived vacuum leak or failing sensor (whichever it may be).
Is there such a thing as a 0% fuel trim?
While fuel trim values of 0% would be ideal, there is no such thing as a perfect engine, which means that in practice, achieving 0% fuel trim values consistently gets progressively more difficult as an engine ages.