A pre-vaporized fuel system for use in fueling an internal combustion engine is provided with a dynamic infrared sensor. The infrared sensor senses the hydrocarbon content of the vaporized fuel which information is used by the engine control module to control engine fueling to minimize emissions. The infrared source is operated to maintain the pre-vaporized fuel below its lower explosive limit and the thermopile detector electronics is synchronized with the light pulse frequency to develop fast signal responses suitable for fueling control.