Think of the barrel and plunger as a super-precise, high-pressure syringe. Each cylinder in the engine has one set within the fuel pump. Their job is a two-step process: Drawing Fuel: The plunger moves downward, sucking a precise amount of diesel fuel into the barrel chamber. Pressurizing and Metering: As the plunger is driven upward by the pump camshaft, it traps the fuel. The...