There are many things that can trigger the P0087 code. One is restricted fuel filter, fuel lines or gell in the lines. Also early trucks had a problem with the rubber fuel hoses at the driver side valve cover, and below the drivers feet area. The lines get kinked, or collapse. Usually more so when it it hot out, and pulling heavy load.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.