Its hard to imagine that your Duramax didn’t come from the factory with a low pressure lift pump, but its true. It was determined that the CP4 injection pump did an adequate job on its own. Well, if “adequate” is good enough for you, then maybe it is. However, in stock form, and especially with the addition of even a mild programmer, performance gains are severely limited due to the lack of a lift pump. Cure this issue with the addition of a FASS lift pump!