Can long term fuel trim values be positive or negative?
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. A normal long-term fuel trim reading will appear to stay the same, giving a long-term average of fuel added.
What causes high long term fuel trim?
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 is the difference between long-term and short 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.
A high Long Term Fuel Trim is the result of an engine that is consistently running lean… OR because the computer THINKS it is running lean. The vast majority of the time, this is due to a vacuum leak. Do a visual inspection of all of the vacuum lines, PCV Valve , Brake Vacuum booster hose, and intake gaskets.
What does negative fuel trim mean?
NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing the pulse width or on-time of the fuel injectors) to reduce the amount of fuel injected into the engine. This is done to LEAN out the fuel mixture to compensate for what it perceives as a rich running condition.
What does long term fuel trim mean?
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 should short term fuel trim be?
The GM system assigns a number between 0 and 255 to define fuel trim, with 128 being the middle point. A short term fuel trim (integrator) reading of 118-138 is considered normal. Fuel trim can also be expressed as either a positive (+) or negative (-) percentage.