When a diesel engine is running at steady state, the diesel combustion process still has some level of variation from cycle to cycle, even if engine load and all control inputs are fixed. This variation is a disturbance for the speed governor, and it could lead to less than optimal engine performance in terms of fuel economy, exhaust gas emission and noise emission.The most effective way to reduce this steady state combustion variation is by applying fuel path feedback control. The control action can be performed at a fixed frequency, or at a defined cycle event time. Intra-cycle control has the highest capacity to suppress the combustion deviation, as it measures the current cycle combustion performance and compensates for it within the same cycle using a very fast control response. Correct knowledge and a model of the disturbance sources and combustion variation patterns are essential in the design process of this intra-cycle control strategy.This paper presents a detailed study of identifying disturbance sources to the combustion process within fuel injection system. The study of influence of other factors on combustion variability such as: residual fraction mass, mixture compositions and distribution, coolant flow speed, air path fluctuation is not the content of this paper. Statistical correlation analysis, principal component analysis and factor analysis methods were used in singling out specific aspects of the disturbance, some of which can be attributed to known sources of variability in the fuel path. The preliminary analysis points to two dominant sources of disturbances: variations in the common fuel rail pressure and variations in the injector current.