Generator Set Unit that operates in the PLTD, during operation there is a knocking sound at cylinder position# 14, after checking and removing the injector there is diesel in the combustion chamber and indicated glowing in the exhaust manifold. The purpose of this study is to determine the root causes of knocking and diesel fuel in the combustion chamber. This research method uses oil sample testing to be tested in the laboratory. The results of the analysis of oil contaminated with diesel fuel are caused by diesel oil flowing into the pan oil and after checking the cylinder head crack (groove-shaped) is found on the side of the injector seal holder, when the unit is not operating the diesel flows into the combustion chamber and into the oil reservoir. The groove in the cylinder head is formed, caused by a hard injector seal caused by fuel temperature and high water temperature. The solution of this research is that the pressure on the fuel return from the engine should not exceed 27 kPa, while the maximum fuel temperature is 66 C (150 F), and what needs to be done is an improvement of diesel cooling and routine radiator maintenance.