1974 C10 Dual Tanks, No Fuel Return Line?

[Cadillac Kid]

Hello Everyone.

I was hoping for a Suburban, but I got a 1974 c10 longed.
I'm just about to get the thing running after sitting for 10 years and I'm replacing the rubber fuel hoses. I am also replacing the fuel pump. While I was crawling under the truck I noticed only the Vapor return line, and the fuel feed line from the switching valve. Is there no fuel return line for dual tanks on these trucks? If It is there, where is it located? The previous owner may have disconnected it. This truck has factory A/C so I would assume it has a fuel return line somewhere.

Thank you for you help.

-Gavin

[Dead Parrot]

My 75 has factory dual tanks, AC and no fuel return. It also came with the Heavy Duty option which exempted it from most emission requirements. No cats, pumps, vapor canister, and can use leaded gas.

Only fuel lines are the supply to the pump and the short line that runs across the frame from the driver's side tank to the switch valve. Tanks are vented to the air via a short hose capped with a small plastic vent cap. Similar to the vent on the rear differential. Switch valve is a two in one out type.

Emission levels varied a lot during the 70's so it is very possible that what you have on your truck is like it left the factory. It is also possible that a PO modified something. A lot of folks remove emission stuff when they do a rebuild.

[Cadillac Kid]

Thank you for getting back to me.

But oh no.. I was afraid someone was going to tell me that. So basically your saying that this truck is just going to be vapor locking constantly? I'm really not looking forward to pluming a fuel return system. Has anyone experienced vapor lock with the dual tanks and no return line? I live in CA and we have corn for gasoline. I've only ever driven old cars and everyone I owned that had no return line vapor locked once it got above 90 degrees.

Thanks

-Gavin
Posted via Mobile Device

[El Dorado Jim]

Originally Posted by Cadillac Kid

Thank you for getting back to me.

But oh no.. I was afraid someone was going to tell me that. So basically your saying that this truck is just going to be vapor locking constantly? I'm really not looking forward to pluming a fuel return system. Has anyone experienced vapor lock with the dual tanks and no return line? I live in CA and we have corn for gasoline. I've only ever driven old cars and everyone I owned that had no return line vapor locked once it got above 90 degrees.

Thanks

-Gavin
Posted via Mobile Device

yes, I have had a terrible vapor locking problem with the new fuels, you will have to use the vents as return ports,I had to do this, put a fuel pump with a return port, then drill tiny holes in your gas caps to vent them, but to do this you need to change your fuel selector valve to the newer style that is used on the 81-83 trucks, the one you have on your truck will not take the pressure from the return line,you can get the new style motorized valve new at autozone, you will need the 5 space electrical connector plug from a truck (wrecking yard) cant buy this new,make sure you leave enough wire to splice on to, then you need to change the switch on the dash to the (depress fully) one,they have these at autozone also, you will need to file the rectangle hole in the dash, new switch is just a little bigger, and you need to do some rewiring,I can post the link of how to wire it, when I find it .....alot of work , but nice once it is done....

[El Dorado Jim]

http://products.pollakaftermarket.co...S_41-revdd.pdf print these and you can figure it out....if you need help, let me know, the pics show a toggle switch, forget that, you will use the switch that replaces the one in your dash....

[Dead Parrot]

Our summer months usually include multiple week long runs over 100. Never had a vapor lock problem. Have AC and headers and run a 195 thermostat. I use the corn based fuel as it is about 30 cents cheaper then the real stuff and my truck doesn't seem to care.

I wouldn't worry about it unless and until you start having a vapor lock problem.

[Cadillac Kid]

El Dorado Jim and Dead Parrot, thank you so much for your input!
It is much appreciated.

The reason I asked about having a fuel line and having A/C is because Ive only ever driven old Cadillacs, and the way the GM set those up back in the day was if you had A/C, you had a fuel return line. If you had a non A/C car, you didn't have a fuel return line. This is my first chevrolet so please excuse the learning curve. I though chevrolet would have had it together by 1974, but they did have leaded fuel back in the day, so..


El Dorado Jim, thank you for posting all of you info about converting the dual tanks to accept retune lines. I think I found that harness/connecter that you were tanking about. STANDARD MOTOR PRODUCTS Part number: FV5C
I priced all of the parts out and it came to roughly $140 for everything. New pump, switching valve, steel fuel line, fuel switch, and connector. I think I'm going to get the truck running and drive it for awhile to see if it is something I am going to keep or sell before I jump into the retrofit.

Thanks for posting the link with the wiring diagram. I did have one question about wiring the dash switch and the switching valve. Starting with the stock configuration, do I need to add any new wires to the circuit or is it just a matter of rewiring the existing harness to work with the new parts?

I'm loving the information on this forum already.

Thank you.

-Gavin