Help with 89 D250 hard start
 

All of a sudden I'm getting hard start, for last two years even in coldest weather it starts up on first crank (after waiting for wait to start light to go out). It is 20 degrees C at night and 32 C during the day so wait to start does not light. When I crank the wait to start light does light and I searched and this is normal to show bulb is working when its above 15 degrees apparently. To get it to start I have to hold pedal half to three quarters way down, once the truck warms up, if I restart immediately it is ok but if I wait for 5 mins then I have to do the throttle thing again. When it starts I get a puff of black smoke then it runs ok. When running performs normally, ie good power and no smoke. Fuel pressure is around 14 psi so that seems ok.... Any thoughts on where to start investigation?

I'd check for air leaks in the fuel line between the tank and lift pump.

exactly. my truck did the same thing when my return line clamp was loose so it was allowing fuel todrain out overnight. check for leaks or air intake in the fuel lines

I couldn't find any fuel leaks, ie fuel leaking out of any lines. I filled tank with diesel and it made starting a bit easier but still not right. I then changed the fuel filter and bled at the injectors this morning and got most but not all the air out so it does seem as though it could be sucking air somewhere. It idled smoother after that. I then left it for 2 hours and it just started perfectly. Guess I'll see how it does for a few days.... I have been reading about people pressurizing fuel tank with a shop vac to find leaks, is that a good way to look for a leak that is occurring under suction?

my truck had a cracked fuel pick up tube in the tank as long as it was 3/4 or better full no prob but if got to low it would run fine but if it got turned off and try to restart that is when i had the same starting prob

Small leaks upstream of the lift pump won't show because it a vacuum-draw system, just suck air. Carefully pressurize the tank (5psi MAX) and look for wet spots.