Abstract: A device for controlling injection of treatment fluids in plunger lift systems includes a lubricator cap coupleable to a lubricator. The lubricator cap defines a fluid inlet coupleable to a fluid injection system. A sensor coupled to the lubricator cap detects arrival of a plunger within the lubricator a controller communicatively coupled to the sensor transmits an indicator corresponding to arrival of plunger to a fluid injection control system. In response, the fluid injection system injects fluid into the fluid inlet to recharge the plunger. In certain implementations, the sensor detects arrival of the plunger at the lubricator by detecting movement of a lubricator spring or a component of a lubricator spring, such as a spring follower.