In a traditional fuel injection gas engine, our best fuel additive is Berryman “B-60 High Mileage Fuel System Rejuvenator” (part #7516).  If you have a direct-injection gas engine or “GDI,” then you’ll need a fuel additive (such as 7516) as well as a kit that goes in through the air-intake or manifold vacuum induction system.  For that application, we recommend Berryman “Intake Valve & Combustion Chamber Cleaner” (part #2611) in the red box.  For a more complete GDI solution, we also have a 4-step kit that includes mass airflow sensor cleaner, air-intake cleaner, 2611, and 7516.  That product is called “Professional 4-Step Air & Fuel System Maintenance Kit” (part #2640).