Evaluating air-tightness of a gaseous fuel manifold during purge gas flushing

For a dual fuel internal combustion engine (100) with a plurality of cylinders (9) and at least one gaseous fuel admission valve (58) for each cylinder (9) of the plurality of cylinders (9) fluidly delimiting a gaseous fuel manifold (54) with respect to the respective cylinder (9), a method of evaluating air-tightness of the gaseous fuel manifold (54) comprises flushing (step 314) and/or pressurizing (step 316) the gaseous fuel manifold (54) with a purge gas at a pressure above an intake manifold air pressure thereby setting a differential gas pressure across gaseous fuel admission valves (58); measuring (step 320) a pressure associated with the flushing (step 314) and/or the pressurizing (step 316), thereby generating pressure data; and evaluating (step 322) the air-tightness of a gaseous fuel manifold (54) based on the measured pressure data.