Diagnostic system and diagnostic method for internal combustion engine

A diagnostic system for carrying out a diagnosis of a difference in air-fuel ratio among a plurality of cylinders in an internal combustion engine that includes the plurality of cylinders, in-cylinder injectors that respectively inject fuel inside the corresponding cylinders and port injectors that respectively inject fuel outside the corresponding cylinders, the diagnostic system comprising:

• an electronic control unit configured to receive signals from an air-fuel ratio sensor and make an in-cylinder injector abnormality diagnosis of an air-fuel ratio due to the in-cylinder injectors based on signals from the air-fuel ratio sensor and then to make a port injector abnormality diagnosis of the air-fuel ratio due to the port injectors based on signals from the air fuel ratio sensor,
• the electronic control unit being configured to execute a program stored in a memory to control the engine to inject fuel from the in-cylinder injectors and the port injectors, wherein the electronic control unit is configured to execute the program to:
• inject fuel from only the in-cylinder injectors and make the in-cylinder injector abnormality diagnosis of the air-fuel ratio due to the in-cylinder injectors in an operating situation in which fuel is injected from only the in-cylinder injectors by determining whether there is an abnormality of the air-fuel ratio due to the in-cylinder injectors based on signals from the air-fuel ratio sensor, and
• inject fuel from both the in-cylinder injectors and the port injectors after making the in-cylinder injector abnormality diagnosis and then make the port injector abnormality diagnosis of the air-fuel ratio due to the port injectors, irrespective of whether an abnormality of the air-fuel ratio due to the in-cylinder injectors is detected, by increasing a ratio of an injection amount of the port injectors and determining whether there is an abnormality of the air-fuel ratio due to the port injectors based on signals from the air-fuel ratio sensor,
• the electronic control unit being configured to make a combined injector abnormality diagnosis of the air-fuel ratio when fuel is injected from both the in-cylinder injectors and the port injectors after making the in-cylinder injector abnormality diagnosis,
• wherein when the electronic control unit then has diagnosed that there is a combined injector abnormality when fuel is injected from both the in-cylinder injectors and the port injectors, the electronic control unit is configured to make the port injector abnormality diagnosis of the air-fuel ratio due to the port injectors by further increasing the ratio of the injection amount of the port injectors, and
• wherein the electronic control unit is configured to make the port injector abnormality diagnosis of the air-fuel ratio due to the port injectors by increasing the ratio of the injection amount of the port injectors to 100% when the electronic control unit determines that there is an in-cylinder injector abnormality of the air-fuel ratio due to the in-cylinder injectors.