Combustion based gas admission valve evaluating during liquid fuel operation of dual fuel engine

For a dual fuel internal combustion engine (100) comprising a plurality of cylinders (9) and at least one gaseous fuel admission valve for each cylinder (9) of the plurality of cylinders (9), a method of evaluating operability of gaseous fuel admission valves (58) comprises operating (step 312) the dual fuel internal combustion engine (100) on liquid fuel; supplying (step 354) a purge gas to the gaseous fuel admission valves (58) at a pressure above an intake manifold air pressure thereby setting a differential gas pressure across the gaseous fuel admission valves (58); measuring (step 360), for each cylinder of the plurality of cylinders, a combustion process parameter such as cylinder pressure; and evaluating (step 362) operability of the gaseous fuel admission valves (58) based on the measured combustion process parameters. Leaking purge gas may result in cylinder-specifically deviating combustion process parameters, allowing to identify malfunctioning gaseous fuel admission valves.