1991 f250 starts and runs fine turn it off wait 30 minutes it starts runs about 30 secounds rough then shuts off wont start wait several hrs and it will start and run great

 

does it fire up ok when the motor is cold, then not start when hot?

if so, i am gona say your injection pump is shot. the alumium pump expands when hot, throwing the tolorances out the door and causing the hard start/no run issue.

 

it doesny real matter yesterday ran for about 4 hrs shut off for maybe 10 min.started fine waites about an hour started fine about 1/2 hr later started ran rough and shut off about 2 hrs later started fine

 

possibly air intrusion into fuel system. do u have ANY fuel leaks? return lines on top of motor all dry? what condition are they in?

 

yes return lines dry seems like air but cant figure where its coming from
also ty for your help

 

do u have dual tanks?

 

yes i do have dual tanks

 

has the selector valve ever ben touched? sometimes they go bad and can get stuck between tanks and sucks air. but that is a long shot. i have no clue, that last one is a major guess.

 

thanks for your help i will keep searching anymore ideas pls lrt me know TY

 

Check the hard fuel line running under the motor along the cross member, they can rub through sometimes. The fuel pump also has a weep hole that will leak if your diaphragm tears.