I have a 2001 blazer and have been having starting problems. I checked the fuel pressure and it read around 50psi and when I shut off the key the fuel pressure dropped to zero. I decided it needed a new fuel pump. I put in the new pump and it didn't run. I lowered the tank again and hooked up the old pump that was running before I pulled it. I didn't installed it in the tank just hooked the wires to it and it would not run when I turned the key. I checked the fuel pump wires for power and got power to the gray wire and the ground wire using a circuit tester. I also swaped out a the fuel pump relay with no change.
I have a few questions, when I reinstalled the tank I didn't tighten it completely to the frame thinking if it didn't work it would easier to pull would this be a problem. Also, will the fuel pump run out side of the tank when hooked up to the 4 prong fuel pump wire harness. What other steps do you suggest.

 

All it takes is power and ground for the pump to run. If you have both, then the pump should run.