What does "fuel trim" refer to in an EFI system?

Fuel trim in an EFI (Electronic Fuel Injection) system refers to the adjustments made by the Engine Control Unit (ECU) to the duration of fuel injection in order to maintain optimal air-fuel ratios across different operating conditions. The ECU constantly monitors the air-fuel mixture through feedback systems, such as oxygen sensors, and makes calculations to determine the necessary adjustments to fuel delivery.

These adjustments are vital because they help optimize engine performance, reduce emissions, and improve fuel efficiency. If the engine is running too rich (too much fuel), the ECU trims back the fuel delivery to achieve the desired ratio. Conversely, if the engine is running too lean (too little fuel), the ECU increases fuel delivery. This dynamic response ensures that the engine runs smoothly and efficiently under varying conditions, such as changes in load, temperature, and altitude.

The other options relate to different aspects of the fuel system but do not accurately describe the concept of fuel trim. For instance, the total fuel stored in the tank pertains to fuel capacity, factory settings relate to manufacturer default parameters, and temperature adjustments concern engine cooling and efficiency rather than direct fuel delivery adjustments.