Will it do any damage if I only use the front tank (1987 F-250, 6.9 Diesel)? I keep the front full and don't let it go under 1/2 tank.

 

Without being experienced with any particulars in regard to diesel fuel myself, I would have to say that no, you won't do any damage.
You could experience some small long-term effects, such as the tank selector switch and valve taking a set and not wanting to switch later down the road if you need it. I don't think your '87 was using FDM's (fuel delivery modules) a that time, was it? Just a valve on the frame maybe?

Of course, if there is any fuel at all left in the old unused lines and tank, that could pose a deterioration issue down the road as well. Especially if you live where weather changes are a factor.
Or will you be completely removing the old tank, or leaving it bone dry empty?

What's the reason for this move, out of curiosity? Needing some parts for the rear system and just don't want to mess with it now?
Should be ok, but wait for some diesel experts to chime in.

Paul

 

I'm not an expert but the rear pump may get stiff from non use or the in tank filter may get crusty if there is no diesel in the tank in a worst case situation.
I don't know. I think it would be fine. If it were my truck, assuming the tank is empty, I would put in 3-4 gallons and some fuel treatment and call it good.