The engine oil cap is the removable cover located on top of the valve cover. It allows you to add new oil to the engine during maintenance. It features a rubber O-ring or gasket to create a tight seal, preventing oil from splashing out onto the hot engine and keeping dirt and debris from falling into the crankcase. A missing or loose oil cap can cause a vacuum leak in the PCV system, leading to a rough idle, “Check Engine” light, and a messy engine bay covered in oil film.