Help me diagnose F250 starting problem

I've been having some problems starting my F250 on occasion ever since the start of the winter.

It cranks just fine, repeatedly, no problems with the battery at all.

However if the truck hasn't been driven for several days I usually have to crank it several times before the engine gets started- sometimes there is some turn over on a failed crank but usually it just cranks and fails to turn over on a failed start.

I've been able to start it every time though it takes several attempts if the truck hasn't been driven in a long time. Have no other problems at all with the vehicle.

Any ideas of what the problem might be here?

Is it fuel injected? If it is, try turning the key on and off about 3 times and see if it starts easier. Leave the key on about 2 or 3 seconds each time. If it starts easier, the check valve in the fuel pump may be failing allowing the fuel to drain out of the lines.

Yes I concur with the last poster, something in your fuel system is not working right, allowing the fuel to drain back into the tank when the truck sits for a while. There may be other things that can cause that too but I'm not certain on that. (ie possibly regulator?)

When you go to start the truck, turn the key to run for a few seconds to allow the pump to pressurize the system. Can you hear it run?
Then start the truck. Otherwise more work for your starter as I'm sure you are aware.

Thanks for the advice. Yes it is fuel injected- and it's actually a natural gas system- the engine and system are almost the same as regular gas so I suspect you guys are right about it being something with the fuel injection system. I'll try turning the key a few times like you guys recommend, and yes it is much easier to start if I give it a preliminary crank.

If the pump runs you can try one of these.
http://www.thetoolwarehouse.net/p-6444-sur-r-ckv7.aspx
Make sure the line size is correct before ordering.
Always use a tubing cutter as they have copper crush washers.