I'm working on a buddy's truck that he bought from someone on here, and it has low fuel pressure. Truck is a 2000 rcsb 5 spd with an lq9. Still running the return fuel system. The truck only has about 40 psi at idle and goes up to around 48 wot. I have verified this with the autometer gauge in the truck as well as a mechanical gauge I have. We put a new pump in the truck when he bought it, then warrantied it for another new one just to try it. Tried swapping pressure regulators with 2 other known-working trucks, still exact same pressure each time. I also have checked voltage at the plug, getting 13.5 there. Any ideas welcome!!
Thanks

 

Faulty pressure guages.

 

First post, have that all covered with multiple new parts, used parts, etc...exact same everytime. Also tried two mechanical gauges, still same. This is a buddy's truck, not sure if he ever figured it out.