I found the problem with my 99 Blazer fuel gauge. Of course I replaced the pump first and broke the vent valve. Before you dive into the pump replacement take a look at the sending unit wiring. The harness runs between the frame and the tank. Just past the fuel filter the two sending wires branch off towards the right side of the vehicle. Just follow the fuel lines. So the harness rubbed on the front axle driveshaft. I attached a picture for help. Also I have read many claims about a ground at the rear of the vehicle. I didnt have any ground on my 99 for the fuel pump. All four wires run forward. The two fuel pump wires go to the fuse block on the left side of the engine compartment and the two sending unit wires run to the ECM on the right side of the passenger compartment.