Electronic controls in internal combustion engines require an in-cylinder combustion sensor to produce a feedback signal to the ECU (Engine Control Unit). Recent research indicated that the ion current sensor has many advantages over the pressure transducer, related mainly to lower cost. Modified glow plugs in diesel engines, and fuel injectors in both gasoline and diesel engines can be utilized as ion current sensors without the addition any part or drilling holes in the cylinder head needed for the pressure transducer. Multi sensing fuel injector (MSFI) system is a new technique which instruments the fuel injector with an electric circuit to perform multiple sensing tasks including functioning as an ion sensor in addition to its primary task of delivering the fuel into the cylinder. It is necessary to fundamentally understand MSFI system. In this study the author will firstly explore the influence of piston motion (as one side of variable capacitance) on the ion sensor signal through modeling and simulation, and then look into the origin of the MSFI signal of fuel injection; and finally the author will look at how to analyze MSFI signal to duplicate the injection command profile for on-board diagnostics (OBD). On the basis of the above analysis, a new methodology will be put forward to predict real fuel injection timing. This investigation is conducted on a high speed turbocharged diesel engine with a Piezo injector under low load operating conditions.