new to diesel please help
 

I have a 1996 GMC with a 6.5.The first crank of the day it will usually fire right up after that you have to try to turn it over three or four times to crank.When it does this it has black smoke at first and quickly turns white.It does not have any trouble turning over just the actual firing up and running part is tricky.If it was gas engine I could find problem but this is diesel and I have no clue and I do not wish to start buying parts until I find the correct one.Thanks in advance for any help.

It could be injectors or injection pump. Does it have any strange knocks? Rev the engine to 2000 rpm hold it there for a short time and slowly back off the throttle. It should return to idle smoothly. If it doesn't you probably have a pump going out.

idle is excellent and as far as knocks nothing that i can tell is out of the ordinary.runs out good going down the road or setting in the yard.it holds roughly 40 lbs of oil pressure.gere is one thing i have noticed today it only does when motor is warm.first crank of the day it fires right up and if it sets for a while it fires up no problem.is this timing?if so does a diesel adjust the same as gas power or is there a procedure?does injector pumps make an unusual rattle?

The knocks would be a fuel knock. Have you tried revving the engine like I said yet? Do it while the truck is hot. My guess would be the pump is laying down and as it gets hotter things start acting up for it.

tried reving engine and worked fine.did it cold and hot worked fine both ways.is reving the engine the only way to check the pump?

No but the only other way I know is to take the pump off. You said the it has a unusual rattle? If so you may have injectors laying down. The only way to know for sure is take them out and have them tested.

if you let it run anymore with bad injectors it could start spraying the incorrect pattern and burn a hole in your piston