Just picked up a project truck. 85 F250 with the 6.9 IDI. The guy I bought it from self professes to know NOTHING about diesels or trucks in general. He said he drove the truck for months with zero issues and then one day it puttered out and stalled and he couldnt get it to re-start so its sat for 4 months and he finally decided to sell it to me cheap. Im 99% its a dumb fuel issue but where might be the first place to look?. He did say it wasnt out of fuel because that was the first question I asked. Thanks guys

 

Welcome to the site..

Id start by checking that the lift pump (fuel pump) is supplying spec fuel volume, no air, from the tank on up to the filter and injection pump.
Its fairly common for the injection pumps to go south as well introducing a low volume air filled mix into the system and to the injectors making for a hard/no start.
Its easy to check fuel flow and for circulating air using some clear tubing off the filter housing to IP, and the returns..
Plenty of vids on the net and YouTube on troubleshooting the systems.. Or ask here...