Mercury - Quicksilver Quickleen Engine and Fuel System Cleaner Fuel Treatment Liquid Formula 8M0047921. Overview. If you've ever noticed an engine that just doesn't feel as smooth as it used to, chances are the fuel system is asking for attention. This Quicksilver Quickleen cleaner is one of those products you keep on hand because it quietly keeps everything running the way it should. Pour it in, run the engine, and let it do the work in the background. It's simple peace of mind, especially when