Abstract: A tool for tuning an engine having a fuel injector unit with a follower and a body with a flange portion protruding outward from the body, includes a support structure having a follower contact surface, a plunger member slidably connected to the support structure, for bearing against the flange portion, a light source connected to a power source, a circuit for delivering electric current from the power source to the light source, and a switch which closes the circuit when the plunger member is moved to a predetermined position, and opens the circuit when the plunger member moves out of the predetermined position. The support structure preferably includes a tubular housing having two ends and which receives the plunger member in one end. A spring is preferably provided for biasing the plunger member in an extended position relative to the support structure. The follower contact surface preferably includes an outward projecting arm member, including a plate having a recess for receiving the top of the follower.