My F150 starts sometimes and sometimes not. When it does start it seems to run fine. When it doesn't, here are the symptoms:

It begins to turn over and then seems to lock up. I thought it might be a starter armature short so I replaced the starter. It worked for a few days and then I had the problem again. I immediately pull the starter to check the teeth on the flywheel before anything moved and they were fine.

Now what!

 

When this happens, engine won't turn over, try this. Turn on headlights and beep horn, at the same time engage starter. Horn and lights should remain on while engine cranks. If they don't the battery is to weak to start the engine. Possible corrosion at battery cable so alternator isn't charging battery, alternator is defective, or other dirty electrical connections or grounds. Or battery is shot, have it tested.

Another senario is an injector is leaking and this engine hydro locks. Not good.

 

Bad ignition switch?

 

Battery is good. All electrical checks good.

Lets suppose it is a leaking injector and the engine hydro locks; What do you mean, " not good"?

 

First of all, would think if its an injector leaking, it would happen quicker than 15 minutes. Hydro lock, its correct name is hydraulic lock. Liguids do not compress. If a liquid enters the cylinder and is displacing more volume than is available with piston at top dead center (TDC) it will not turn any further. This has happened to a couple of members, not that common. Still feel its battery, did you perform the test mentioned? Please perform the test, its free and will narrow down the posibilities. Have you seen the posts about instruments pegging out on start ups. First sign of battery failure. I don't understand why, thats how these trucks are.

 

Ignition switch

 

I had a similar issue years ago. I cut the positive battery cable at the molded battery connector and found that the copper wire internal to the cable was totally corroded. I replaced the wire and all was fine.