Assuming a "boucle" is a CLOSED loop.

Engine fuel control systems generally include CLOSED LOOP CONTROL of the air/fuel ratio of the mixture supplied to the engine. In these systems, an oxygen sensor monitors the engine exhaust gases and provides an indication of the rich or lean status of the air/fuel mixture relative to the stoichiometric ratio. Integral and proportional correction terms are generated from the oxygen sensor output and summed to generate a closed loop correction term used in computing the fuel amount injected into the engine to maintain the stoichiometric ratio. In addition to this closed loop correction term, many systems also employ a long term calibration adjustment used in computing the fuel amount injected into the engine. This long term correction term, sometimes referred to as a block learn term, provides for an OPEN LOOP TRIM on the fuel calculation to adjust for such things as system aging, system-to-system variations or changes in vehicle operating conditions. It is typical for a separate block learn term to be generated for each of several engine operating points each of which is defined, for example, by a specific combination of engine speed and load. The learn term for each operating point is derived from the integral term portion of the closed loop correction term and is adjusted over time so that corrections made by the integral term are minimized. In other words, the integral term correction is transferred to the learn term.
http://www.freepatentsonline.com/5094214.html

Read more at : http://forums.mightymustangs.net/index.php?showtopic=18859


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Note added at 58 mins (2008-03-17 11:37:23 GMT)
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When the Engine is COLD or at W.O.T [wide-open throttle], the ECU is running in "OPEN LOOP MODE"(No O2 sensor feedback). During Open Loop Mode the ECU maps are programmed to intentionally run somewhat richer for a higher & smoother idle when cold (which operates like a Choke) or for more power at W.O.T.
http://www.ozaccord.com/forums/showthread.php?t=5

Closed loop operation occurs when the engine is idling, or running at near-constant throttle position at near-constant engine speed. During idle, the ECU actually has two goals: one, to deliver the correct fuel, and two, to maintain the idle speed at 750 RPM. The fuel is handled by controlling the injectors and using the oxygen sensor for feedback. The idle speed is controlled by the idle speed control motor (ISC), using the crankshaft angle sensor as feedback.
Relatively LARGE CHANGES IN THROTTLE POSITION at a given RPM will cause the ECU to change to OPEN-LOOP OPERATION. Once the throttle position and RPM are back "in sync" with each other, the ECU will generally switch back to closed-loop operation. This can occur if the throttle position moves again, or if the engine speed begins to level out at the new throttle position. However, there are some special cases that may change this behavior.
http://members.shaw.ca/costall/ECUprimer/ECU_ch5.html
En fonctionnement moteur, deux situations sont possibles.

a) Le calculateur ne tient pas compte de l’information
sonde.
Le système est dit « Débouclé » (non opérationnel).
Le système restera débouclé tant que les conditions de fonctionnement moteur seront incompatibles avec la régulation de richesse (dosage non adapté) et/ou tant que la sonde n’aura pas atteint sa température normale de fonctionnement.
. Temporisation de démarrage (mélange riche).
. Fonctionnement à FROID.
. PF et VARIATIONS DE CHARGES RAPIDES (dosage de puissance).
. Coupure d’injection en décélération.
. Mode dégradé (sonde défectueuse).
http://mecaniquons.fr/index.php?mod=articles&ac=commentaires...