Cylinder liner wear of a heavy-duty diesel engine was measured by means of radionuclide technology to identify the combined effects of diesel fuel sulfur and coolant temperature under engine operating conditions.Higher cylinder liner wear results from increasing load and decreasing engine speed. At low cooling temperatures, i.e. during engine cold start and warm-up, any reduction of the sulfur content leads to substantially reduced wear. At normal operational coolant temperatures, however, the effect of sulfur is extremely small. At higher than normal operating temperatures, diesel fuels with very low sulfur content, e.g. 0.05%, lead to increased wear, when compared with those of usual sulfur levels of 0.3 to 0.5%.