Valve flushes
— additives designed to service, or rather, clean engine valves.
Contamination impairs the efficiency of valves and can eventually cause the valves to simply not close properly. To eliminate these undesirable phenomena, special cleaning nozzles are used. Note that such compositions are rarely intended "only for valves", many of them also act on piston rings, nozzles and other elements of the fuel system. And some are generally claimed as general cleaning additives that affect the entire fuel system. However, if cleaning the valves is the most critical point, you should choose an additive from this category.