A definition and how and when a gm engine goes into closed loop. CLOSED LOOP – Closed Loop is a term in which the ECM uses feedback from the O2 sensor to make corrections to the air fuel ratio. Another name for this term is short term fuel trim. The ECM makes immediate but temporary corrections to the fuel delivery to maintain the AFR at 14.7. The only ratio that can be maintained in closed loop is 14.7. This is due to the nature of the type oxygen sensor used on most passenger cars. There are other types of O2 sensors called wide band sensors, but they are expensive. They can be used to monitor the AFR at other than 14.7 AFR. The short term fuel trim value is called the Integrator in most early scantools. The value of the integrator varies above and below 128 with 128 being no correction. For ex., if the integrator is 140, the ECM is adding fuel because the O2 sesnor is reading a lean mixture. If the Integrator is 115, the ECM is removing fuel because the O2 sensor is reading a rich mixture. Anytime the system is not in closed loop, the Integrator will immediately return to a value of 128 and stay there. There is only one Integrator and its value is solely dependent on the O2 sensor. When the engine is started, the ECM will keep the Integrator at 128 until the ECM determines that the O2 sensor is working correctly and that the engine temperature and time delays before entering closed loop constraints have been satisfied. The problem with the Integrator is that its value cannot be used to determine how far away from 14.7 AFR you are. The value of the Integrator does however increase or decrease the longer you are above or below the 14.7 AFR. The time aspect is probably where the Integrator gets its name. If you have studied calculus, you'll know what I mean. Otherwise, its beyond the scope of this document to explain. The Integrator and BLOCK LEARN work together to re-tune the system to match any engine's charactersitics, up to a practical limit.
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