I have a stock fuel pump with 170,000 miles on it. I also have 255lph external fuel pump inline for my n2o system. Lets just say my main fuel pump dies, will my extternal be able to continue to pump fuel or will the main fuel pump close up and prevent it from pumping additional fuel from the fuel tank?.

 

The inline pump won't do the job alone, you will need a functioning in-tank pump as well.

 

Understood, but why, does the pump close up? or does it require a bigger pump to suck it out the tank and push it forward?

 

Yes, it works off the pull it out and push it forward principle.