Abstract: A method of operating a fuel injected multi-cylinder internal combustion engine includes electronically storing a first value or reference value indicative of an engine speed response to a commanded fueling duration change during fueling the engine with a first fuel or reference fuel, such as a known type of diesel fuel. The method further includes operating the internal combustion engine in a fuel testing mode during fueling with a second fuel or test fuel, such as an unknown type of diesel fuel. The fuel testing mode includes determining a second value or test value indicative of an engine speed response to a commanded fueling duration change, comparing the test value with the reference value and outputting a fuel attribute signal which is based at least in part on comparing the reference value and the test value.