Fuel Problem 1998 Nissan Sentra

I had the fuel pump and sending unit replaced on my 1998 Sentra. It ran great, but then stopped. I believe it isn't getting fuel, because I can put starter fluid in the air filter box and it runs great for a few seconds.

I'm trying to determine if the fuel pump relay is working. I checked the fuse and it's fine. I can't find the relay anywhere. Does anyone know where this is located on this car? I found a diagram online that shows it close to the left headlight, but it's not there.

Also, when I turn the ignition to On I hear a noise coming from a round black component on top of the manifold. I tried to find this online and it's called a DP. (The link for the diagram is below.) I don't know what this abbrevication stands for. It makes a strange buzzing noise, doesn't seem to be like a normal relay-type sound, sounds more like something's rattling. The car will run with starter fluid, so whatever this is isn't keeping the car from running, but could it have something to do with it starting? Here's the link that shows the DP:

http://www.freepdfmanual.com/wp-content/uploads/2009/01/4runner-engine-diagram.png

Thank you so much.

Why not go back as far as you can to the fuel pump and check for power on the black with yellow wire that feeds power to the fuel pump. If you check for power at the pump and you find power you just eliminated the fuse, relay, wiring as potential causes. I would check for power with the pump plugged in. there should be power any time the engine is cranking or running. If there is a problem with corroded wires, burnt contacts in the relay it may not show up with the pump unplugged.

Your link looks like it's for a 2009 Toyota 4runner; why not look for a diagram for a 98 Sentra?

http://www.nicoclub.com/FSM/Sentra/1998_Sentra_GA/

Sounds good. Regarding testing it at the pump, the sending unit sits on top of the pump. It has 2 connectors, one with 4 wires and one with 2. Which one do I want to test? Also, which connector combinations do I want to test? I don't want to short anything out during testing.

The black wire with the yellow tracer is the wire that supplies power to the pump