A cracked fuel filter cap can lead to air in the fuel system. That can result in hard starting or potential fuel injector issues. A cracked fuel filter cap can also cause a fuel leak, which is definitely a bad idea on a hot engine. Never worry about that again with a Sinister Diesel Fuel Filter Cap. Constructed from a single solid aluminum billet, the Sinister Diesel Fuel Filter Cap is far stronger than the OEM plastic cap.