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Open- and Closed-Loop Fuel Management

Fuel injection systems use oxygen sensors, or O₂ sensors, in the exhaust stream to manage fuel. These sensors only work once they have reached their operating temperature, at which time the system is said to be in closed-loop. The term closed-loop refers to the relationship between the fuel being delivered to the engine and the data feedback from the exhaust-monitoring O₂ sensors. Before the O₂ sensors reach operating temperature, the fuel is managed by a preset strategy based on information from other sensors in the fuel injection system. See the Electronic Fuel Injection article for more information on these sensors.This is known as open-loop fuel management.

Oxygen Sensor operation
Oxygen sensors are made of metals which, when heated, produce an electrical voltage in the absence of oxygen. Understanding the combustion process is important in understanding how an O₂ sensor works. As fuel is burned in the engine, it consumes a certain amount of the oxygen in the air that enters the engine through the intake. The ideal ratio of air to gasoline to burn cleanly is 14.7:1, meaning there is 14.7 times as much air as fuel in the engine. A rich mixture, meaning more fuel and less air, will result in less oxygen being left unburned in the exhaust, and therefore a higher voltage produced by the O₂ sensor. A lean mixture will result in more oxygen being left unburned in the exhaust, and a lower voltage produced by the O₂ sensor.
The normal reference for an O₂ sensor is 450mV (.45 volts). The vehicle’s engine control module or powertrain control module interprets voltage above 450mV as indicating a rich air-fuel mixture, while voltages below 450mV indicate a lean air-fuel mixture. The control module responds by adjusting the fuel injector pulse-width, or the length of time in milliseconds (ms) that the injectors open on each intake stroke, thereby adjusting the volume of fuel supplied to the engine.

Fuel Trim
The control module is programmed with a fuel strategy which determines how long the fuel injector pulse-width should be at any given time under any conditions reported by a variety of sensors, excluding O₂ sensors. Sensors monitor conditions such as intake air temperature, throttle position, manifold pressure, mass air flow, and coolant temperature among others. The control module runs these data through its fuel strategy program and determines fuel injector pulse width. During open-loop operation, this is essentially the end of the story. During closed loop operation, the control module uses feedback from the O₂ sensors to adjust fuel injector pulse width up or down from the value determined by its fuel strategy program. Fuel trim values are an expression of this adjustment. Positive fuel trim values indicate the control module has extended the fuel injector pulse-width beyond what the fuel strategy program indicates. Negative values indicate the control module has reduced the pulse-width. Technicians can monitor fuel trim values to determine the presence of problems such as clogged or leaking fuel injectors, intake vacuum leaks, weak fuel pumps, clogged fuel filters, or other fuel management problems.