Please Help....Dual tank plumbing question

[PANCAKE]

recently insatlled a second tank in my 1983 GMC 3500 single cab long bed 4x4 with a 496 ci engine. The engine has a carb...not FI.

I installed a switching valve in the stock location under the passenger side cab and a selector switch in the dash. The valve and the switch both work fine.

The sending units on both tanks have two hose fittings on them. I attached the supply line of each sending unit to the swithcing valve and each fuel gauge wire from the sending unit to the switching valve.

My question is what do I do with the second hose from each sending unit?

There are plugs on the switching valve so I can't plug them in there.

Are these samller hoses from the sending unit a breather hose? Can I plug them and put a vented gas cap on?

Should I put some kind of breather cap/filter on them. Wiil gas overflow from the tank out of these lines?

On the orignal tank the samller line is pluged with a bolt. When I open the gas cap a bunch of air comes out. I dont think it is vented.

Any help would be great.

Thanks.

 

[4X4HIGH]

Sounds like the smaller lines would be for the charcoal cannister.

 

[GonicGM]

I have a 1983 K30 gas. It was a single tank. I added the second with all OEM parts. The correct OEM senders have three lines, 3/8, 5/16 middle, and 1/4". The correct plastic tank valve has 6 nipples, 2 forward, 4 rearward. The 2 forward lines are 3/8 and 1/4. The 3/8 is fuel pump feed. The 1/4 is fuel pump vapor return from the pump. The rearward connections are likewise 3/8 and 1/4" and are paired left and right of each other to connect to the respective tanks. The 5/16 line from the senders have nothing to do with the tank valve. If you have a HD emissions truck, over 8500GVW, these 5/16 lines simply have a short length of rubber hose and a nylon vent which is the same as a differential vent. They simply vent to the atmosphere. Small GVW trucks, under 8500 GVW, have a charcoal canister by the radiator support. A 5/16 line runs from the cannister to the rear fuel lines. If you have a single tank, it simply connects to the center 5/16 nipple on the sender. If you have dual tanks, there is a nylon "tee" to connect the two lines from the center sender 5/16 nipples and then to the cannister line. Those cannister lines have nothing to do with the switching valve.

 

[imiceman44]

This is more than anyone could expect as far as explanation.

I have a 1983 K30 gas. It was a single tank. I added the second with all OEM parts. The correct OEM senders have three lines, 3/8, 5/16 middle, and 1/4". The correct plastic tank valve has 6 nipples, 2 forward, 4 rearward. The 2 forward lines are 3/8 and 1/4. The 3/8 is fuel pump feed. The 1/4 is fuel pump vapor return from the pump. The rearward connections are likewise 3/8 and 1/4" and are paired left and right of each other to connect to the respective tanks. The 5/16 line from the senders have nothing to do with the tank valve. If you have a HD emissions truck, over 8500GVW, these 5/16 lines simply have a short length of rubber hose and a nylon vent which is the same as a differential vent. They simply vent to the atmosphere. Small GVW trucks, under 8500 GVW, have a charcoal canister by the radiator support. A 5/16 line runs from the cannister to the rear fuel lines. If you have a single tank, it simply connects to the center 5/16 nipple on the sender. If you have dual tanks, there is a nylon "tee" to connect the two lines from the center sender 5/16 nipples and then to the cannister line. Those cannister lines have nothing to do with the switching valve.

 

[PANCAKE]

Thanks GonicGM for the detailed reply.

Since I do not have a 3-outlet sending unit, and no charcoal canister I have decided to run the 5/16" vent line towards the rear of the truck and put a nipple on the end. I want to run it towards the back so there is no gas smell near the cab.

The switching valve that i have installed has caps on the small outlets. The tanks and fuel gauge switch over fine.

Thanks again for the input.

Hope to see you all at BB 2011!!!