[River56]

What would I need to do to the fuel system on a 91 Ram 250 (Cummins) to run B100 biodiesel?

Thanks

 

[Drewyzr1]

there are no specific modifications needed. The biodiesel will clean out varnish deposits and the like from the fuel system so you need to change the fuel filters in the beginning after just a couple of tanks. After you see the filters staying clean you can go back to regular changes.

 

[illflem]

Since you have a '91 the fuel hoses and other fuel system rubber parts likely aren't compatible with BD.
After '94 most engine components are compatible.