The effect of increased clearance between the needle and its guides in a fuel injection nozzle on the rate of lacquer deposit formation from neat sunflower oil was investigated. Bosch fuel injection nozzles were tested on a fuel v injection calibration stand. The needle clearance reduction due to deposit buildup was monitored with a pneumatic leak test. Two test series of 100 hours duration each were performed at a temperature of 350°C. Each series consisted of ten 10-hour segments with a complete system shutdown after each segment. For the first test series the system was allowed to cool down before each shutdown. During the second test series the system was stopped while still hot.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. Although increased, clearance does not decrease the rate of initial lacquer formation, it appears that for nozzles with greater clearance the effect of lacquer deposits is less detrimental to the nozzle performance.For the hot shutdown tests, the needles displayed significant lacquer buildup. Some needles showed various degrees of wear and rubs.The test results indicate that the clearance used for the standard diesel fuel injection nozzles is not optimum for fuels like vegetable oils. Secondly, a long cooldown period for engines running on plant oil fuels should be used before each shutdown.