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There is a separate circuit and relay that primes the fuel system when you start the vehicle. If you sit in a silent location and turn the key to the ON position (without cranking), you should hear the fuel pump run for about 5-6 seconds. This is to bring the fuel pressure up in order to let the vehicle start. Then the normal pump circuit takes over. Sounds like your priming circuit is not working. I would try the relay first.
If your talking about a 7.3 diesel then you have injector problems..when they alot of miles on them,they start to lose their oil prime when sitting overnight..Then it causes a hard start or no start..unless you plug it in,and then it will run rough until it get warm..only way to realy diagnos which injectors is to look at scan tool and do some different tests
The check valve is a small brass fitting where the return line attaches to the filter head. I believe it takes a 7/16" wrench. There is a 'duck bill' inside that prevents fuel from leaking back to the tank causing air in the system. It should allow flow in only on direction. Some times you can see light through it if it's bad enough. Also, the lines between the injectors can dry out and start to leak air. Air will leak in before fuel leaks out, and you won't see it. There are plastic, cup-shaped connectors for these hoses that press onto the top of each injector. They are sealed by two o-rings on each injector body where it meets the connector. These o-rings can dry out and also leak air and fuel. Your Ford dealer or diesel service professional should have the proper o-rings. Any other o-rings might not seal, as they won't be the exact size. You already changed the fuel solenoid when you replaced the pump. It is inside. There has to be battery cranking voltage to it while cranking, and battery voltage with key in 'on' position. The wire attaches to the top of the pump. This is not likely to be the problem, since it restarts every 2 hours.
it seems that because it sit's for a while that perhaps it's losing it's prime. if ythis happens generally you will have to reprime the fuel system. in a diesel system you also have glow plugs to help ignite the fuel. check to make sure your glow plugs work, but i would start with whether you have proper prime in the fuel system.. please rate this
just bump the ignition without starting it let sit for 20 seconds oruntill you hear lift pump shut off then shut off ignition and repeat do it 2 or 3 times should start then cummins is a self priming system has been for years even in semi trucks