The lift pump (located on the inside of the frame, drivers side) could be the culprit. It could also be the Oil Pressure Switch too. If the truck is not starting, unplug the electrical connector on the lift pump. Use a test light (make sure you have a clean ground to the frame) and test the connection coming to the lift pump. In order to test this connection, someone needs to either turn the key over or the truck needs to be running......thats right, a 6.5 will run at times without the lift pump working, usually runs poorly, but may continue to run. If your are getting power to the lift pump, this confirms that the OPS is working properly. If you are not getting power to the lift pump, then it is very likely that the OPS has failed. The OPS is located at the rear of the engine, next to the firewall, and behind the fuel filter (manager). IF you are not getting power to the lift pump, change the OPS.

If you are getting power down to the lift pump, you can try a couple of things.

1. If the truck will start, once it is running, open the tee valve (located next to thermostat housing). If fuel comes out of the tee valve, the lift pump works. If not, and you have confirmed you are getting power to the lift pump from the OPS, then the lift pump is shot.

2. If the truck will not start, use a 12v power supply and connect it to the lift pump. Listen for the pump to kick in. You could also crack the outlet side of the lift pump to see if fuel is coming out once you have the 12v power supply connected to the lift pump.