The car computer (ECU – Engine Control Unit) will continuously monitor several parameters to ensure optimal combustion. To ensure optimal combustion three elements must be delivered in exact measure: fuel, air and ignition. Optimal combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers. The stoichiometric air-fuel ratio is 14.7 parts of air to 1 part of fuel.
Since the driver controls airflow (load) with the accelerator pedal, the ECU can only control fuel. It uses sensors to measure or calculate airflow, consults an air/fuel ratio map in its permanent memory, then chooses the correct injector pulse width (the amount of time the fuel injector will stay open) to match that airflow. That pre-programmed injector pulse will provide exactly enough fuel to establish a stoichiometric air/fuel ratio, however the ECU will adjust injector pulse width to provide more or less fuel than specified in the map . 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. If the STFT shows an increasing trend, the LTFT will increase and allow the STFT to normalize and vice versa. The LTFT stored value in memory can be reset using a bi-directional scan tool or on some vehicles by removing the battery power for 15 minutes.
LTFT and STFT are percentages which represent the adjustment to the fuel injector pulse width. The LTFT and STFT are added together, so a LTFT of 10% and a STFT of -5% will equal 5%. A total fuel trim of 10% will increase fuel delivery with 10% and a total fuel trim of -10% will decrease the fuel delivery with 10%. If any of the value exceed 25% or beneath -25% then the check engine light is turned on with a number of error codes, which can be read using an appropriate scan tool.
The ECU reads parameters from MAF, MAP, Oxygen Sensor (O2) and A/F Sensor. They provide input on air entering the engine and the state of the exhaust gasses. These parameters will allow the ECU to adjust fuel delivery accordingly to ensure that the ratio of fuel and air is perfect and the ignition timing is perfect for maximum power.
LTFT and STFT will usually stay close to 0%, which equates to no adjustment. However as the engine wears, the adjustment will be necessary. A significant increase in STFT or LTFT from 0% means the ECU is detecting more air than expected and is adjusting fuel delivery with more fuel. A significant decrease in STFT or LTFT from 0% means the ECU is detecting less air than expected and is adjusting fuel delivery with less fuel.
A significant increase in STFT and LTFT can be caused by:
A significant decrease in STFT and LTFT can be caused by:
When using LTFT and STFT for troubleshooting, then the fuel trim must be checked in a number of conditions:
The fuel trims will alter during the above conditions and their altering will provide leads on what is malfunctioning.
Each bank has its own fuel trim reading. If your engine is a 4 cylinder, then it has only one bank, Bank 1. On a V-style engine you can isolate which bank is running rich or lean by watching that bank's fuel trims. If one bank is running properly, and another isn't you can narrow down a developing problem to one side of the engine or the other.