EFI Fuel Line Identification

[DesertDog]

On the 4.0 fuel rail, which line is the supply and which is the return line? I had assummed the line on the front of the motor by the regulator was the supply but I am now thinking I may be mistaken and the line near the firewall is the supply. I'm doing a 4.0 swap into my Bronco II and I am troubleshooting fuel supply. (No pressure on the rail)

 

[MrShorty]

Fuel flow through the rail is: supply line --> fuel rail --> FPR --> return line. It's been a while since I've looked, so I don't remember where each line attaches to the rail. Doesn't the return line hook directly up to the FPR?
Getting them backwards would certainly explain 0 pressure to rail. FPR will act as a one way valve and prevent any gas flowing if hooked up backwards.

 

[DesertDog]

Fuel flow through the rail is: supply line --> fuel rail --> FPR --> return line. It's been a while since I've looked, so I don't remember where each line attaches to the rail. Doesn't the return line hook directly up to the FPR?

Thanks, I just needed to confirm this. I would have thought you would regulate the pressure to the rail first with the FPR before suppling the injectors. The other reason I hooked them up backwards is there are small and large barrel connections on the fuel lines and my '89 Bronco II barrels are the opposite of the Explorer 4.0. Go figure.

 

[Doug904]

The large line is always the feed because the reduced size of the return line helps maintain the pressure.

Later Doug

 

[410Fortune]

Dont even go there......
On one of our first 4.0L conversions we couldnt get the sucker running for a few days, triple checked everything 3 times. thats the day I learned the FPR is on the return side of things, not the input side of things

 

[MrShorty]

I imagine that it's a pretty common mistake. When most people think of a pressure regulator, they think of the regulators that come on propane, acetylene, or other compressed gas cylinders. These "line pressure" regulators take the high cylinder pressure and step it down to a usable pressure for the given application.
Our fuel pressure regulators, on the other hand, are "back pressure" regulators. These are designed to allow the upstream pressure to build up to a certain point (~40 psig in our case), then vent the excess pressure/flow (in this case back to the fuel tank).