An unmodified, direct-injected diesel engine was operated on diesel fuel and three blends of diesel fuel and sunflower oil. Heating of the fuels was used to change their viscosities.At normal fuel temperatures, specific fuel consumption and smoke emission increased for any power as sunflower oil content increased. Overall efficiency and exhaust temperature showed virtually no changes with fuel composition. Increasing fuel temperature caused a shift of best overall efficiency from high to low speeds, the magnitude of the shift depending on the plant oil concentration of the fuel. Thus fuel heating as a means of viscosity control may result in an efficiency penalty in the normal operating range of an engine.Typical plant oil induced engine contaminations such as wet stacking, excessive carbon accumulations, nozzle orifice blocking, and lubrication oil gelling were experienced.