Two com. gas oils were modified by addn. of a corn. fatty acid Me ester (20 or 30%), conditioned for 16 weeks at 20 degrees C and 60 degrees C and tested in an engine (load 50-100 Nm, rotation 1000-3800 rpm) to det. the torque changes and smoke content in the exhaust gas. The poor biodiesel stability in storage constituted a potential threat to engine performance and operational reliability.