The effect of temperature on the rate of lacquer deposit formation from neat sunflower oil on the needles of fuel injection nozzles was investigated. Boson fuel injection nozzles were tested on a fuel injection calibration stand. A pneumatic leak test was developed to monitor the needle clearance reduction due to deposit buildup.For fuels with physical and chemical properties similar to those of neat sunflower oil, excessive residue on the internal surfaces of the injection nozzles is likely to occur with the ultimate result of complete needle immobility. The rate of the lacquer buildup on the needle increases with temperature. Prior to final needle sticking, delay in start of injection, sluggish needle lift, increases in duration of injection, maximum, final residual, and maximum residual line pressures, and decrease in maximum needle lift can be observed.Based on the obtained results, the temperature of injection nozzles handling plant oil fuels should be kept as low as possible. The heating of plant oil fuels to reduce their viscosity should be restricted to the necessary minimum.