Injection pumps play an integral role in the operation of diesel-powered vehicles. The part allows diesel cars and trucks to run by delivering an exact amount of fuel, under very high pressure, to each injector at a specified time interval. As the fuel is delivered from the injection pump, each injector disburses the fuel into the motor's cylinders, where it combusts, and keeps the motor running. During routine maintenance, such as fuel filter replacements, you may need to prime the injection pump to remove air from the fuel system.