Fuel return flow acceptable PSI?

[Garbage_Mechan]

I have been fighting poor performance, random idle miss and poor fuel mileage on my 91 for a long while. Systematically testing and ruling out possible fuel issues. Recently upgraded electric fuel pumps and now have free flow of 1 gallon per 80 seconds. Or apx 45 gph.
Maintaining a max of 6.5 to min of 5 psi measured after the filter.
My question is with all this newfound fuel flow is it possible there is excess pressure in the return to tank line that is affecting the injection pump. I measured it with fuel pumps running but engine off ( folks sleeping). Teeing into the return line gave me about 3/4 psi. How much back pressure in the return to tank flow is OK?

Thanks!

Garbage Truck Mechanic

 

[Thewespaul]

You want as close to no pressure as possible, I believe the spec is less than 1 psi is acceptable, but return pressure can be caused by several things, try removing the tank cap and see if the reading changes, it’s possible it’s just a ventilation issue, or it could be an issue with the size of the return lines.

 

[Garbage_Mechan]

Wes thanks for getting back.... The fuel cap was off since I had to keep pouring “test fuel” back in. I have replaced the “showerheads”, replaced O-rings in quick couplers on tanks ( but not on tank selector) and all rubber hoses. It has nylon lines most of system.
I measured the return flow rcoming out of the bleed port on top of the fuel filter. With the engine off there is a quart a minute apx 15 gph flowing back to tank through the loop of lines through the injector return caps. Didn’t measure the return from the top of the pump. Thinking of running a new parallel return hose back toward the tank selector.

 

[Thewespaul]

Try plugging the return off the filter and see if the drop in return volume improves performance

 

[Garbage_Mechan]

The only problem with that is that my fuel pressure will go to high if I do that because I have an adjustable pressure relief installed in that line. Without it psi gets up to 8.5.

 

[typ4]

The pump case port does not like any pressure. Fought a 6.2 for a while, tiny flapper of hose in the bleed line, truck would go like hell loaded, light throttle with more bleed fuel it would shut off, usually just as you were trying to leave a stop.

 

[Clb]

Try plugging the return off the filter and see if the drop in return volume improves performance

@Thewespaul
I'm confused here...How could that work?
Would that not bump up the psi the i.p. is seeing?
On a side note (thinking out loud) could putting on a 1 way check valve on thr i.p.'s return cure the high return psi issue?