Three Canadian tar sands derived experimental diesel fuels with cetane numbers (CN) of 36, 31 and 26 and a reference fuel with a 47 CN were tested in a Deutz F1L511D, single cylinder, 4 stroke, naturally aspirated, diesel engine. Tests were performed with intake and cooling air temperatures of 0°C and −12°C over the entire engine operating range, Performance and combustion behaviour with the 36 CN fuel was marginally acceptable at 0°C and unacceptable at −12°C. High maximum combustion pressures and rates of combustion pressure rise were encountered at the low engine speed, high load condition with the experimental fuels. Poor combustion behaviour was also experienced with the two low CN fuels at light loads. Operation of this engine configuration with the 31 and 26 CN fuels is not recommended at 0°C and lower intake air temperatures.