truck runs fine after it sits for awhile it wont start unless i take the fuel cap off. then it starts just fine. what do you think ???

1997 3/4 diesel truck drw

thanks much

 

First, crawl underneath and reach for the fuel lines in the top of the tank. Make sure that the vent is clear and not covered with mud or something. I expect you will find a vent blockage to be the culprit. Do you have a guage for the fuel pressure? If not, suggest you make up a temporary guage to see what you have when it is working correctly. Good luck,<br>Ed

 

Also check the rubber fuel lines above the starter. There should be NO moisture there.