A system and method comprises operating an engine during a first cycle to drive a piston in a cylinder without energizing a fuel injector of the cylinder; acquiring first pressure data of the cylinder for a predetermined crank angle window during the first cycle; energizing the fuel injector for an energizing time during a second cycle; acquiring second pressure data of the cylinder for the predetermined crank angle window during the second cycle; calculating a pressure ratio difference average (PRDA) from the first pressure data and the second pressure data; and modifying the operation of the fuel injector based on the PRDA value.