Jumping the fuel pump relay so there is power back to the fuel pump, and if it doesn't run/hum, then you can tap on the tank . this makes the brushes in the pump make contact. If the pump now runs, you know the pump is bad, and is the cause of the no start. This is how we have checked for a pump that will not work. It is a very good and very quick test to perform. It may sound out of the ordinary, and you might question the validity of it. But it does work. This same principle has been performed on starters that do not turn over but just click, for years.
It takes 2 minutes tops to determine if the pump is not making connection. It doesn't mean the pump is good. It is just a quick test to prove the pump is the failed component.
It is easier to do this and drive the vehicle in the stall then it is to push it in the stall.
You stated it is weird that it could set for a few weeks and then crank up fine.
This is due to temperature and humidity changes. Poor connections don't care what makes them move to get connection. Heat and cold temperature fluctuations or a tap on the tank.
You have to do what you can as fast as you can when your tying to diagnose an intermittent concern.
I have seen fuel pumps run good. drive the vehicle in my stall for something else. Go to start it to pull out, and it won't start, test fuel pressure 0psi, get the customer to come and look at it, and hit the key and it starts up. Customer doesn't replace pump, then gets stuck in a parking lot , and is mad at us for not repairing his vehicle. while it was in our shop.
I have personally experienced this condition every week, at least once.
The best time to check the vehicle is when it won't start.
The ECM B fuse is good,This powers the fuel pump. In front of the fuel pump relay you will see a open terminal slot. This is the fuel pump test terminal( it goes directly to the fuel pump.) Put 12 volts to this terminal and the pump should run. If not then remove the fuel pump connector . check for 12 volts at the gray wire. If you have voltage, Now test between the fuel pump connector terminals C and B ( gray and black) with a test lamp( 12 v) if the lamp lights, then replace the pump. If it doesn't light then check the ground splice pack 423 and ground connection G402. They are on the left rear frame rail
Check for corroded connections/ poor connections. This could be a very intermittent condition that could be affected by heat, and humidity that could change in 2 weeks. The diagram I sent you gives you all the connection points for the fuel pump.
To test the relay hook a test lamp in the fuel pump test terminal and to ground. Turn the key on , it should light up, for 2 seconds then go out. If it does, the ECM is working
If you are having difficulty testing this circuit or don't have the test equipment. Have someone crank the engine over and tap on the tank, It works about 98% of the time to quickly test for a bad pump. If the pump is really bad it usually wont work. But for ones that ran one time and just won't start now. Give it a try, It is quick and easy.
Thank you Hal