**Background**

I've had a couple of situations with diesels. One in particular where I swapped a Ford F350 Powerstroke 7.8 transmission. When I was done. The truck wouldn't start. It was a painful troubleshooting situation for me because of my diesel ignorance. It turned out that the fuel lines needed to be bled.

It made no real sense to me but it was the solution. The fact that air was in the system at all didn't make sense to me.

**My Question**

Why do diesel fuel lines have to be bled?

**Here's a generic diesel fuel system image**

[![enter image description here](https://i.stack.imgur.com/dP36g.gif)](https://i.stack.imgur.com/dP36g.gif)