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Carbonaceous deposits can reduce the durability and performance of diesel engines. Chemical cleaning methods are widely used to remove the deposits in remanufacturing industry but the wasted liquid is harmful to the environment. In this paper the supercritical CO2cleaning technology, an environmentally friendly approach, has been used to remove these contaminants. The surfaces of diesel engine valve before and after cleaned were examined by field emission scanning electron microscope (FE-SEM) and Fourier transform infrared spectroscope (FT-IR). The testing results showed that most organic compounds were dissolved and the stubborn contaminants were changed to be removed easily after cleaning by supercritical CO2fluid. The new cleaning method can reduce carbonaceous deposits significantly without changing the surface properties of the diesel engine valve.