Panel coker test and hot tube test were used to study the changes of the engine oils detergency on IIIG engine test.The IIIG engine test piston deposits of the second land was analyzed by FTIR and XPS,and the composition of the engine oils at different IIIG engine test hours was analyzed by FTIR and thin layer chromatography.The results show that the detergency of the engine oils is deteriorated after aging in the IIIG engine test.IIIG engine test piston deposits of the second land contain a significant hydrocarbon and oxygen compound,and a small amount of CaSO4,ZnSO4 and CaCO3.In the IIIG engine test,the saturated hydrocarbon content in the oils is decreased and the content of aromatic hydrocarbon and oxygen compound in the oils is increased.