Method for diagnosing the condition of an engine fuel supply system

A method for diagnosing a state of a system for supplying fuel to a fuel-injected, controlled-ignition internal combustion engine, of a type including an electronic control device which makes use of an oxygen probe for closed-loop regulation of a value of an air-fuel ratio admitted into combustion chambers of the engine, and according to which a signal delivered by the oxygen probe is analyzed, the method comprising:

• a) deducing from the signal, a change in effective injection time of exhaust gas leaving the engine, given by relation:
• effective injection time=B+ALPHACL_MOYEN*GAIN*A*Mair,
• in which:
• B is an OFFSET value;
• ALPHACL_MOYEN is an injection time correction factor that makes it possible to regulate richness of the exhaust gases leaving the engine;
• GAIN is a coefficient making it possible to take account of drift in hydraulic characteristics of the fuel supply system;
• A is a factor that takes into account various phenomena notably linked to canister draining, wetting of walls;
• Mair is measured or estimated mass of air admitted into a cylinder of the engine;
• b) calculating
• CRITERION=∫(CRITERION1+CRITERION2+CRITERION3)
• in which
• CRITERION1=difference between a value of ALPHACL_MOYEN for which no correction to the injection time as a function of time is necessary to achieve a richness objective 1 in the exhaust, and a value ALPHACL_MOYEN applied to the injection time, to achieve the richness objective 1 at the exhaust,
• CRITERION2=difference between an instantaneous OFFSET value corresponding to use of a “theoretical” fuel supply system, which is a system that is not dispersed and not aged and whose average characteristic coincides with a value for which no modification of the injection time is applied, and an instantaneous OFFSET value applied to the injection time, for a given vehicle (specific to each vehicle produced),
• CRITERION3=difference between the instantaneous GAIN value corresponding to use of a “theoretical” fuel supply system, which is a system that is not dispersed and not aged and whose average characteristic coincides with a value for which no modification of the injection time is applied, and the instantaneous GAIN value applied to the injection time, for a given vehicle (specific to each vehicle produced);
• c) comparing CRITERION with predetermined minimum and maximum threshold values THRESHOLD_MIN and THRESHOLD_MAX;
• and
• d) diagnosing a defective state when CRITERION is outside a window included between THRESHOLD_MIN and THRESHOLD_MAX.