Hard Starting 98 5.4 Expedition

I have a 98 4x4 5.4L Expedition that I've been having a starting problem with for a while. It doesn't affect the truck after it starts and it doesn't seem to be affected by temperature. It will turn over just fine but it takes a long time for the engine to fire. Sometimes it will start to fire up and then rumble till I turn the ignition off. When I turn it over the second time it usually fires right up. Usually it just turns over for a long period of time and then fires up. Sometimes it will puff out a cloud of blue smoke but that's rare. It's got 151k miles on it and other than this hasn't had many problems. I've heard some talk about the Idle Air Control Valve, Fuel Injectors, Fuel Air Intake Valve, and the Idle Control Sensor. The fuel filter has been replaced recently and the injectors have been cleaned but those things didn't seem to help much. Could the injectors just be old and need replacing or is it something as simple as an IACV? Any help is greatly appreciated as this has stumped both me and my usual mechanic. Thanks!

Also, I need to change the transfer case fluid on the front transfer case. I read that it takes ATF and I saw what looked like a drain plug on the back of it. Is it as simple as draining out the old and filling in the new through the top plug? My apologies for such a long post.

First, yes it takes atf in the transfer case and just fills through the top plug. Second, it sounds like maybe your fuel system is losing it's prime. Try putting a gauge on the fuel rail and seeing how long it takes to reach 40-50psi, it should be almost instantaneous when the key is turned on. Losing prime isn't real common, but common enough to be worth looking into. Good luck.

Got the transfer case fluid changed. Real easy job, thanks for the help. I've had the fuel pressure checked and it's ok according to my mechanic. The fuel filter has been changed recently also. The starting is still a problem and crazy as it sounds, it seems to be worse when it's warmer outside. Any more ideas on this problem would be greatly appreciated!!!