Truck is a 90 with a 5.0. Vacuum line attached, I’m getting about 23 psi. Pull vacuum line and it jumps to around 38 psi. Bad fpr right? Making sure my thought is right or not..

There is no fuel in vacuum line.

 

I would a agree its likely bad. 23 is too low with the vacuum line connected.

 

I think it's fuel pump related. How many tanks does the truck have?

 

Factory was 2, but it only has one. I had to replace the entire system from tank to injectors when I bought it. Couldn’t afford the second tank.

 

The FPR is working, not leaking and doing its job.It's job is to keep the pressure from going too high. You can go one step farther. With gauge attached and vacuum line off the regulator, goose the throttle and hold it. If the pressure drops then quickly bounces back up, that's good. If it drops a lot and stays low or very slowly increases, that indicates a weak fuel pump.My guess is your pump is weak or the filter is obstructed. Pressure should be 30-45 psi.

 

My thought was if the fpr is designed to keep a certain pressure and I can get that pressure with vacuum line removed, then it would be the problem. I will see what it does like you said and report back.

 

This is this morning at 33* weather with vacuum line ON… guess I will have to see what the pressure is when it’s warm outside this afternoon to see if I’m chasing an electrical issue or it was a fluke ALL day…

 

Vacuum line OFF

 

Well, first real drive this morning and pressure has stayed up like in video. Turn truck off and back to “dead battery” attempts to start.

 

The fuel pressure regulator is after the injectors, on the return fuel line. It has an internal spring and diaphragm. The fuel pump builds pressure against the spring until around 45 psi, then the return line opens, and allows fuel to return to the tank.
The vacuum line assists in opening the valve. You have the most vacuum at idle, so that's when the most fuel is returned to the tank, and the lowest fuel pressure at the injectors.
It sounds like you have a fuel pump or wiring/ground issue.
How are your battery terminals and ground points?
Check out the main ground cable from the battery negative terminal to the engine block and frame. It can corrode internally and need replacement.