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 that 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 injection time correction factor ALPHACL_MEAN that makes it possible to regulate richness of exhaust gases leaving the engine, ALPHACL_MEAN being given by the relation:
• effective injection time=B+ALPHACL_MEAN*GAIN*A*Mair,
• in which:
• B is an offset value;
• 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 the measured or estimated mass of air admitted into a cylinder of the engine;
• (b) comparing ALPHACL_MEAN with predetermined minimum and maximum threshold values THRESHOLD_MIN and THRESHOLD_MAX;
• (c) diagnosing a defective state when ALPHACL_MEAN is outside a window included between THRESHOLD_MIN and THRESHOLD_MAX.