How does short term and long term fuel trim work?
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’s the difference between STFT and long term fuel trim?
This adjustment is called fuel trim and can be read using a scan tool. There are two types of fuel trim – short term (STFT) and long term (LTFT). STFT is used for immediate adjustments based on the parameters, whereas the LTFT is a slower adjustment. The LFTF value is stored in memory and “learns” from the STFT.
What should the fuel trim be on a V style engine?
On V style engines there will be a total fuel trim for each operating bank of the engine. Total fuel trim spread should also be looked at and should not exceed 10% (-5% STFT plus +5% LTFT= fuel spread of 10%).
When to turn off long term fuel trims?
However, when long-term fuel trims exceed about 10% or so to either side of 0%, there is a problem that needs to be investigated, but note that codes indicating rich or lean running conditions will generally only be set when the deviation reaches about 25% or so.
What should total fuel trim be for a car?
Total fuel trim spread should also be looked at and should not exceed 10% (-5% STFT plus +5% LTFT= fuel spread of 10%). For many years the rule of thumb was +/-10% total fuel trim for most North American vehicles (European vehicles with traditional O2 sensors were lower at +/-5%).
What does a negative long term fuel trim mean?
LTFT (long term fuel trim) should not change instantly, it is a learned response that happens over time. If the STFT stays consistently negative or positive for a long period of time. That will show up in the LTFT. If the LTFT is negative, it means the ECU has been reducing fuel to maintain an ideal fuel mix.
What is the adaptive strategy for fuel trim?
Fuel trim’s adaptive strategy consists of two components: short-term fuel trim (STFT) and long-term fuel trim (LTFT). In the European world, these terms are known as additive for STFT and multiplicative for LTFT.
When to use short term fuel trim ( STFT )?
Once the engine is up to operating temperature, the PCM/ECM employ a short term fuel trim (STFT) strategy. But as the PCM/ECM begins to notice a pattern of adjustments, it stores those adjustments in a cell similar to an Excel spreadsheet.
On V style engines there will be a total fuel trim for each operating bank of the engine. Total fuel trim spread should also be looked at and should not exceed 10% (-5% STFT plus +5% LTFT= fuel spread of 10%).