This the code I use to enable the fuel injector interrupt.
Code:
EICRA = (EICRA & ~(1 << ISC01)) | (1 << ISC00); // set injector sense pin control
EIFR = (1 < INTF0); // clear fuel injector sense flag
EIMSK = (1 << INT0); // enable fuel injector sense interrupt
Does yours look the same?