Our 1993 UD 1800 made by Nissan has a MAJOR intermittent fuel flow problem.
It seems to have a vacuum build up on the suction side of the fuel system. The truck runs great usually (other than a slight abundance of white smoke) until it seems to completely run out of fuel & stalls, usually on the freeway. The only remedy is to pull over and open the drain on the sedimenter water trap for a few seconds. A loud hissing noise is heard while A LOT of bubbles rush into the clear water trap bowl. Tighten it back up, the truck runs for anywhere from minutes to hours before stalling again. We removed the vent on top of the fuel tank, cleaned it in solvent, and blew it out with high pressure air and reinstalled. The truck has new fuel filters and air filter. The priming pump on the fuel pump doesn't build up pressure after the truck stalls, however, when the truck is running right the priming pump works properly. The truck has seen 2 mechanics, including a local UD dealer who only replaced all fuel lines with new rubber lines. Both have failed to diagnose the problem.

Any help or suggestions on what to look at would be greatly appreciated as this truck is the center of our business.

You've got trash or something in the tank restricting the fuel pick-up. Thats the only way to develope a vacum. You'll have to drop the tank(s) and remove the trash. It could be a bad case of microbial growth plugging the pick up. Biocide for the growth, clean and flush the pick up with solvent once out of the tank.
Doug

I agree, we had one with a burger wrapper in the bottom of the tank that would get caught on the pickup tube every once in a while.