This paper deals with the influence of biodiesel on some tribology characteristics of a bus diesel engine with a mechanically controlled fuel injection system. The tests have been performed on a fully equipped engine test bed, on a fuel injection test bed and on a discharge coefficient testing device. The tested fuel was neat biodiesel produced from rapeseed. Attention was focused on the biodiesel influence on the pump plunger surface roughness, on the carbon deposits in the combustion chamber, on the injector and in the injector nozzle hole. The pump plunger surface was analyzed by experimentally determined roughness parameters and by a microscope. The carbon deposits at fuel injector and in the combustion chamber were examined using endoscopic inspection. The deposits in the injector nozzle were investigated indirectly by measuring the nozzle discharge coefficient. Numerical simulation has been performed in order to estimate the influence of the discharge coefficient variation on the computed injection characteristics. The obtained results indicate that biodiesel usage may even improve the pump plunger lubrication conditions. Furthermore, the carbon deposits in the combustion chambers did not vary significantly in quantity but they were noticeably redistributed. Finally, it was found out that the variation of the nozzle discharge coefficient has to be taken into account only if high accuracy of numerical simulation is desired. (C) 2008 Elsevier Ltd. All rights reserved.