Apparatus for injecting dye in a fuel flow line is disclosed. The apparatus includes a fluid powered motor adapted for placement in a fuel flow line, a dye reservoir and a fluid injector connected between the reservoir and the flow line and driven by the fluid powered motor. The fluid powered motor, dye reservoir, and fluid injector are all housed within a container having a thermostatically controlled heater disposed in it. The arrangement insures that the temperature of dye applied to the fuel flow line is maintained above a certain level during cold weather, thereby preventing the viscosity of the dye from rising above a certain level and preventing clogging and fouling of the dye delivery system which may be caused by high viscosity of the dye.