My neighbour has an '86 with a 6.9 ,and I noticed that there was a metal screen in the end of the tail pipe. What is the metal screen for?

 

I usually see these to prevent a backfire from igniting brush, like on a tractor or lawnmower, but i don't know here. Do diesels even backfire like that?

 

It does look like a spark arestor, but it would take one hell of a back fire to get out of the tailpipe.

 

On gas engines, unburned fuel can build up in the exhaust on deceleration, and it ignites, and bang.....

it's usually only when timing's way off, but maybe diesels do this too....they ignite at a lower temp, and I bet the manifold is still pretty hot.

 

Spark arrestor
I have seen carbon build up blow out my exhaust as glowing sparks while running it hard for the first time in a long while.
I have stacks on mine so it is up where I can see them easily in my west coast mirrors.