When the ratio of the amount of vaporized fuel (purge amount) to be introduced into an intake passage of an engine from a
fuel tank through a purge passage
pipe, a purge control valve, etc. to the total amount of fuel (total fuel amount) to be supplied to the engine is large, an
abnormality determination
system acquires, as a parameter Pon, the air-fuel ratio imbalance index value that increases as the difference between the air-fuel ratios of the respective cylinders increases. When the purge amount is small relative to the total fuel amount, the determination
system acquires the air-fuel ratio imbalance index value as a parameter Poff. When the difference between the parameters Pon and Poff is less than a predetermined value and at least one of these parameters is greater than a predetermined threshold value, the determination
system determines that an inter-
cylinder air intake amount variation
abnormality is occurring.