fuel return line in-cab tank 72 gmc

[60ratrod]

ok so i got a tpi motor from an 88 camaro cause going efi at this point would be nice and easier since FAST came out with it's ez efi retrofit harness. now this retrofit system requires a return fuel system and was wondering if my 72 gmc with it's in cab tank would have the return line from the factory. i haven't popped the hood since nick and i fiddled with it to get it back to my house so i have no idea if it has one or not. and if it didn't come with a return line then how hard would it be to put in a return line? i know the fuel pump is stock. and where would i look to see if it does have a return line and how many nossles would be on the sending unit?

[alderan33]

My 72 has a factory return line. You can look behind the seat on the sending unit to see if there are one or two lines. If there is one, then no return line. If two, the larger one is the fuel line and the smaller one is the return.

[MikeB]

Originally Posted by alderan33

My 72 has a factory return line.

I think that's a vapor line that runs to the charcoal canister. It replaced a vented gas cap used on earlier trucks, like my 69.

[RustyPile]

Originally Posted by MikeB

I think that's a vapor line that runs to the charcoal canister. It replaced a vented gas cap used on earlier trucks, like my 69.

The vapor line is not connected to the fuel sending unit. The line going to the vapor canister comes off the tank in the upper passenger side corner of the tank. The line travels up inside the "B" pillar, makes a U-turn and comes back down and through the floor, then forward along the frame to the canister.

[PbFut]

If you have a 72 c10 Chevy or GMC you will have 3 lines to the tank. Fuel supply, return and Vapor. Supply and return at the sender unit. Vapor in the upper passenger side corner. A C20 will not have the vapor port but will have supply and return at the sender. Remember, you will need a FI fuel pump to attain the needed 50 to 60 lbs. A carb pump puts our single digit pressure. In 72 the c10 had a sealed cap, c20 a vented cap.

[MikeB]

Originally Posted by PbFut

If you have a 72 c10 Chevy or GMC you will have 3 lines to the tank. Fuel supply, return and Vapor.

What is the purpose of the return line?

[alderan33]

Does anyone have photos or a diagram showing where all the lies are supposed to go from the top of the vapor canister? Some of mine are hanging loose and I don't know where they go.

[tdangle]

Originally Posted by MikeB

What is the purpose of the return line?

My understanding is a return line on a system with a mechanical pump is to keep the fuel a little cooler. In an electric pump system not only to keep cooler but it is the bypass to regulate fuel pressure. Dead heading an electric pump is very hard on the pump.

[Richard]

Attached a pic of the fuel sending unit on my stock 72 tank that I removed. Only one line. Another pic of the line that goes to Canister. Purchased truck in 98 which made it 26 years old at the time so plenty of time to change things up. Truck is a K10 and was built in Fremont California, sold in Nevada to a California resident, documentation indicates I am the second owner. The Canister and lines were still intact when purchased. County where purchased has no recurring smog requirements and all that equipment was long gone.

There are two sending units sold as replacements for a 72. One with the single line and another two lines indicated as "with automatic transmission" My truck is an automatic and was purchased as seen. Only one line from sender. So, with the information I have on hand. Why would an automatic trans truck have a return line and others do not? Do not think added line there is a return. Any proof of a fuel pump for the 72 year that has provisions for the return? Even if there was one I bet it is vent.
Could that line in sending unit be used as return? Yes.

[Richard]

Originally Posted by alderan33

Does anyone have photos or a diagram showing where all the lies are supposed to go from the top of the vapor canister? Some of mine are hanging loose and I don't know where they go.

If you look closely the connections are labeled on the canister.