i ChangeD the DriversValve CoverGasket On My 98 6.5 And Chaged The Fuel Filter. I Got Tge Truck Restarted And Let It Idle A Few.Minutes And Drove It For 20 Minutes, Now I Have Drove It Back And Forth To Work A Few Times(15Minutes But It StillHard Starts Doesn'tMatter Cold Or Hot. It Takes About 8 SecondsBefore It FIres Up

 

does it seem to be turning over slower than normal? if so check the cable connections, and batteries, if on is week may cause this, or could be starter showing signs, of wear.

 

Is check engine light on?
Any codes?

Glow plug relay get disturbed?
Glow plugs plugged in and working?
CPS harness get disturbed?
Is the lift pump working?

 

I had that problem and it turned out that my starter was weak and wasn't turning the engine over fast enough,I replaced the starter and it fixed my problem