wwoody

I have a 12 valve 98 D250 - I let fuel gel after thawing/bleeding/new filters, I drove truck - It idled perfect but 45 mph max speed (no power whatsoever). I replaced injector pump, mechanical fuel pump, complete sending unit in tank also, new diaphragm on injector pump. Nothing changed running whatsoever - still 45 mph, no smoke, seems to be starving for fuel.

kawi600

Are you pulling in air?
Maybe install a temporary clear see-through filter so you can check for air. Or you can install a fuel pressure gauge which should read 25-30psi at idle, 30-35psi at 2000rpm no load.
You might even try pulling the AFC as a test. Could be the governor arm hanging? sounds a lot like a gov arm riding under the plate.

dozer12216

Air leak into fuel line sounds right. No smoke, no power. Look very carefully at lines. someltimes you can apply light pressure to disconnected lines and find leak in reverse.