Finally got a decent fuel pump (to replace the stock pump) Edelbrock 13 PSI. Installed it along with a pressure regulator. Factory pump has 3 lines: an inlet an outlet, and a vapor return line. I installed everything. With the Vapor return line NOT connected to anything, I get great, consistant fuel pressure. But about 5 minutes of idling, fuel starts flowing out of the vapor return line. (why? what's pressurizing the tank? If I crack the gas cap, there is no audible pressure released, but the fuel flow stops) So, I hook up the return line to the other outlet of the fuel pressure regulator, and now my pressure guage is bouncing between 2 and 5 PSI (not consistant pressure) I realize that by hooking up the reg to the return line that my pressure will be cut in half, approx. What did anyone do to fix this? Do I just turn the regulator up till I get consistant pressure?(which will be about 10 PSI divided by 2 which is not sufficient pressure) I fear that if I plug the return line, a vacuum will form in the tank. WTF? Anyone have any ideas? Vapor canister gone. Metal line plugged.

I think that you probably need to plug it and get a vented gas cap. Thats what I did. My tank still holds pressure so I assume that the gas cap only keeps air from coming out.

Ben

If you leave the return line unplugged it's possible that pressure buildup inside the gas tank can start to push fuel out through that line. I don't recall ever having that problem myself, but I don't see why it wouldn't be possible.

Even vented gas caps have a "blow-off" pressure of about 1 PSI. So they don't vent fuel vapors to the atmosphere but rather force them into the charcoal canister through the vent line (i.e. for emissions reasons). That's plenty of pressure to start forcing gasoline through an open line, if it's got a relatively unrestricted path to exit.

Vented gas caps will let air INTO the tank with no restriction (so you don't start to draw a vacuum in there as fuel level goes down) but they will restrict air/vapors trying to get OUT of the tank, as I described above.

Thanks, Damon and Ben. It all makes sense. I'll block it off, because the car runs better w/ constant pressure.

Let's clarify something: The stock pump has three lines - inlet (supply), outlet (to the carb) and return. NOT vapor return, fuel return.

The "typical" mechanical pump regulates pressure simply by not pumping above 6 psi. On factory stock pumps with that 3rd line, pressure is regulated by a by-pass valve that returns excess fuel to the tank (same way an oil pump regulates pressure). The net effect is that the fuel at the pump stays cooler, because it isn't staying in the inlet line as long. This means the fuel going to the carb will be cooler as well.

The best way to rig up a higher-than-6psi mechanical pump & regulator is to use a reg with a return port, so that it sends the proper 6 psi to the carb and the excess is returned to the tank.

Kinda like the factory did, don't cha know. So nice of them to build in a return line for us, and then we go and plug it off...

By the way, the vapor/vent line goes to the evap canister, aka charcoal canister.

My Holley regulator has 2 outlets. If I hook the return line up to it, my fuel pressure goes to sh*t. So I need a different regulator?

Two outlets are intended to feed dual bowls. It's not the same as a true 3-port with a return.

Got it. Boy, the sh*t they got these days would scare you. Found the correct regulator. (Mallory 3-port bypass) Will be here in time for my FREE steel-braided line installation. (it's not what you know,...........)
Thanks, 57

GM sells a fitting that will go in the Holley regulator with a .042" restriction that will allow a return line to be used. Just ordered my fitting yesterday.

I know a racer who has a needle valve in the line so he can adjust how much goes back (he drives it on the street, too).

I installed a solonoid on my fuel pump return line that is controled by a micro sw. When I go to WOT the carb (Q-Jet) sec. linkage touches the sw. and the sol. (N. Open) closes. Works great and is automatic.

Auggie