Most ECUs seem to operate in two modes. In one mode, the ECU is monitoring as many engine sensors as possible, trying to get the best fuel economy. This mode is normally referred to as "closed loop" because it is a closed feedback system. In the other mode, the ECU is generally trying to achieve the greatest level of performance possible. This mode is normally called "open loop" because it does not continuously monitor sensors and compute fuel correction: simpler routines are used. It's difficult to change the exact behavior of the ECU under closed loop conditions because it requires an understanding and possibly a re-write of that portion of the code running the ECU. Open loop conditions are generally stored in the form of a table where the ECU "looks up" fuel and timing values using a relatively simple algorithm. In most cases, the open loop tables also serve as values to use as a base for fuel correction routines