The Hitachi Fuel Injector Kit - 135921 is a comprehensive package of products designed to improve the performance and efficiency of a car's fuel injection system. It includes various components such as fuel injectors, O-rings, seals, and gaskets that are essential for proper fuel delivery to the engine. The kit also includes a fuel pressure regulator, which helps regulate the amount of fuel entering the system. Additionally, it contains a fuel filter, which helps to remove any impurities from the fuel before it reaches the engine. Lastly, the kit includes a fuel injector cleaner, which helps to remove any built-up deposits from the fuel injectors, ensuring they continue to function properly. This kit is suitable for various makes and models of cars and is an essential maintenance tool for ensuring the smooth operation of a car's fuel system.