Do i need all three fuel lines?

[GsxrMike]

Hey there...I am putting the wiring, fuel lines and brake lines back on today and I didn't know if I need all three fuel lines. Well I know I don't "need" them but should I run them. I am going with a holley fuel pump and a pressure regulator that doens't require a return line and as for the vent line, can I just put a vent cap on it or what?

 

[gmc4cw]

my truck would flood without the return line. it would stall every time I would rev it up and let off the gas.

 

[GsxrMike]

Well the pressure regulator will keep it at a constant 1 - 14psi......wherever I set it. I ran the lines earlier so I will go ahead and hook up the return line anyway.

 

[dyeager535]

You can vent cap the return, but it's a poor solution IMO. I believe on some trucks GM did it, but other than those rare vehicles, either the gas cap was vented (early) or the tank was run to the EVAP cannister.

Gas fumes being vented aren't a real good idea anyways, but my main concerns were what it can do to paint over time, and more important to me, easier to allow condensation to form inside the tank.

 

[myraley]

I dont mean to hi=jack, but almost all that stuff is missing from my truck. I have one line from the fuel tank to the pump and one line from the pump to the carb. Are you saying I should have a charcoal cannister, etc? I saw the three metal lines on the frame, but figured if I had a vented gas cap, I was ok. Wrong?

 

[gmc4cw]

you need a return line. otherwise the pump will flood the carb every time you let off the gas. I learned that the hard way. it was a long day of wheeling. if I attempted any obstacle at above idle it would die after I let off the gas.

 

[dyeager535]

Cannister and what not were based on year.

The return line prevented "boiling" gas by overheating it. Constantly circulating the fuel through the tank keeps it all cooler.

Cannisters are a good solution, but not necessary. Gas fumes eat paint over time, and a cannister helps prevent condensation in the fuel tank. But if the truck never had one, I don't think I'd worry about it.

 

[myraley]

Ok, I will connect a return. But on the bottom of the fuel pump, I have three lines out. A big square one, which is going to the carb, and then a medium one that is capped off, and then the smallest one, which is capped off. Which would be the return, and what is the other line? To the charcoal canister?

 

[dyeager535]

Really not sure on the third one. It's probably some sort of vent.

If you really wanted to, you could probably find a 70's application with a fuel return line fitting, and no vent line. Maybe a 1 ton truck.

Be careful with the "vented" caps...I know for a fact that very early on the caps were vented, I've got an original one. However, I went to Napa, and noticed that all the caps were listed as interchanging from '73+ or something along those lines. Sure enough, the caps were NOT vented correctly, they were vented at a very high pressure, just like the later trucks that used the EVAP cannister. "Old" vented caps should vent at something like 2PSI, later I believe were 16.

 

[myraley]

So the right cap for a 72 would be 16 psi or 2 psi? I have the hose that runs up next to the filler neck, and I believe that is vented, so I should use the 16 psi?

 

[Wasatch K5]

I think the concept of the three lines is this: One line is the fuel feed line which delivers fuel to the TBI. The second line is the return line which brings unused fuel back to the tank. The third line is a fuel tank vent line to the charcoal canister.

The return line is a great idea. Many engines, during hot summer days, have vapor lock issues. Vapor lock is caused by the fuel percolating, or boiling, in the carburetor and changing from a liquid to a gas (or vapor). By returning unused fuel to the tank it prevents the fuel from changing to a gas and prevents vapor lock.

I would recommend you hook up all the lines as they originally were installed.

 

[Steve GMC]

not to hijack but my fuel pump has no return line... its an aftermarket holley if i recall... what should i do with the return line thats there?

 

[myraley]

You could get a fuel pump like mine. It has four lines. One in, and three on the bottom! One to the carb , one I will now connect up as a return and one mystery line, probably designed by the previous owner, just to mess with me, like the electrical system and suspension were
Seriously though, where could a third outgoing line go?

 

[dyeager535]

So the right cap for a 72 would be 16 psi or 2 psi? I have the hose that runs up next to the filler neck, and I believe that is vented, so I should use the 16 psi?

EVAP cannister AFAIK is the only determining factor for a 16PSI cap. I believe later on GM standardized the caps and the sending units on the tank, so there could have been a 16PSI cap, but a vent line off the gas tank.

I don't know the older trucks well, so this may not be correct for yours, but it sounds to me like it is set up like the later trucks with that other hose, but that is to vent the tank while you fill it with gas.

I'm sure at one point I knew why there would be 4 total fittings on a fuel pump, but I can't think of it now. Fuel in, fuel return, and EVAP are the only ones I can think of.