Apparatus for diagnosing abnormality in fuel injection system and fuel injection control system having the apparatus

A fuel injection control system for controlling an internal combustion engine, said fuel injection control system comprising:

• an oxygen sensor for detecting a concentration of oxygen contained in an exhaust gas emitted from said internal combustion engine;
• first operation means, coupled to said oxygen sensor, for calculating a first air-fuel ratio correction value based on the concentration of oxygen so that an air-fuel ratio is equal to a target air-fuel ratio;
• second operation means, coupled to said first operation means, for generating a second air-fuel ratio correction value so that said first air-fuel ratio correction value is within a first predetermined range;
• air-fuel ratio correction means, coupled to said first and second operation means, for correcting a fuel injection period of a fuel injection valve of the internal combustion engine on the basis of said first and second air-fuel ratio correction values;
• first comparator means, coupled to said first operation means, for comparing said first air-fuel ratio correction value with a first upper limit value and a first lower limit value;
• setting means, coupled to said first comparator means, for forcibly setting the second air-fuel ratio correction value to a second upper limit value when it is determined by said first comparator means that said first air-fuel correction value has reached said first upper limit value and for forcibly setting the second air-fuel ratio correction value to a second lower limit value when it is determined by said first comparator means that said first air-fuel ratio correction value has reached said first lower limit value;
• second comparator means, coupled to said setting means, for determining whether or not said first air-fuel ratio correction value obtained after said second air-fuel ratio correction value is set by said setting means is within a second predetermined range;
• and
• decision making means, coupled to said second comparator means, for making a decision that a fault has occurred in the fuel injection system when said second comparator means determines that said first air-fuel ratio correction value is outside of said second predetermined range.