Ford Repair: intermitten starting problem, ford expedition, check valve

Question
I'm working on a 2001 ford expedition e.d. that starts perfectly fine some times and sometimes it just cranks. just did fuel filter and spark plugs it went 2 weeks without doing it then started back up again.Any tips about what to look at next or what i should test to see if it is bad would greatly help. thanks

Answer
best guess is you have a check valve in the pump that is sticking. here's what to do to comfirm it. find the day it is just cranking. cycle key from the off to on position for 5 seconds. repeat again 3 times. then try to start. if it fires up then the pump is the problem.. real common.