I have a 01 cummins 3500 and the lift pump is failing I was wondering If i were to just tap into the fuel line and add a aftermarket one what do I do with the factory one?

 

In tank pump or block mounted? If you have the cash to do so, a FASS or AirDog is the best way to go.

 

The factory pump is on the block

 

After dealing with this BS for a while and through several iterations of mechanical and electrical pumps I settled on what has been the best solution for me so far.

I pulled the whole stock assembly off and pitched it. Eliminated the banjo bolts at the same time and installed an airdog. I already had a fuel pressure gauge but if you do not, get one.

The short answer to your question as I understand it... Leave it where it is if you're being lazy or pull the assembly off and throw it in the trash. IF you take it off, make sure you replace any bolts that are also holding the head on.