Dual tanks dilemma

Muleskinner · Jun 11, 2022

Just purchased 1980 GMC Sierra classic 4x4 with dual tanks. Seller filled drivers side tank, passenger side was empty. Missing gas cap on drivers side. Half mile from Sellers house truck died; no gas. Seller paid 15 mile towing bill to my house. Not getting current to fuel selector valve in L or R positions (don't know if I am supposed to). Jumped valve from battery: could hear it actuate. Put little gas pass. side and nuther gas cap. This morning wouldn't start, switch on pass. side, this afternoon Put switch to drivers side started right away. I am so confused. 68,000 Original miles. Not driven long time. Help?

SirRobyn0 · Jun 13, 2022

Ok, so I think your going to be in for a bit of work. It would be nice if you could could answer a few questions first.
#1. What is the general condition of the truck, is this a really nice rig, well worn driver, or looks like it has been sitting?
#2. What style of tank switch does it have? Is it a one wire or does it have a flat plug with 7 or so wires running into to it?

Basically you have three possibilities.

A fuel pump with an intermittent problem.
A bad tank switch, or dash switch, (need info from you on the style of switch to troubleshoot)
Rusty fuel tanks or tanks that have sat with bad gas in them and now the socks in the tanks are clogged with varnish or rust or both.

That's about it. You can try blowing though the lines backwards to see if you can get air into the tank to see if the socks are plugged. But honestly on a 40 year old truck it would pay in the long run to pull the tanks and see what kind of shape they are in. New tanks are only about $100 each and sending units with new socks around $60. If you don't have clean fuel storage you'll never be able to make it reliable. If you have access to a borescope you could run the camera into the tank to get an idea of condition without having to pull it.

One wire tank valves are prone to failure. A new valve is less than $100. You could pull the old one off to test it, or just change it out and see what happens.

Hopefully this helps.

Ron Sebastian · Jun 13, 2022

Muleskinner said:
Just purchased 1980 GMC Sierra classic 4x4 with dual tanks. Seller filled drivers side tank, passenger side was empty. Missing gas cap on drivers side. Half mile from Sellers house truck died; no gas. Seller paid 15 mile towing bill to my house. Not getting current to fuel selector valve in L or R positions (don't know if I am supposed to). Jumped valve from battery: could hear it actuate. Put little gas pass. side and nuther gas cap. This morning wouldn't start, switch on pass. side, this afternoon Put switch to drivers side started right away. I am so confused. 68,000 Original miles. Not driven long time. Help?

Before you turn the key, it the tank switch. If it doesn't start or sputters, turn the key off, then hit the tank switch again to the other side. If it runs you have isolated the issue. Assuming gas in both tanks.

dvdswan · Jun 13, 2022

Are you getting gas to the carb?

75gmck25 · Jun 13, 2022

It sounds like you have the old 3 or 6 port solenoid with only one power wire.

With no power a properly plumbed 1980 should default to the main tank, which is on the passenger side in 1980. The first simple fix is to put about 5 gallons of gas in the passenger side tank and put the switch into the main position. Then disconnect the fuel line at the carburetor and run the hose into a bucket. Turn the engine over with the starter for about 30 seconds and see if you get fuel into your bucket.

If that does not work, bypass the solenoid by connecting the tank’s 3/8” line directly to the 3/8” metal line running up the frame rail. Do the fuel flow test again.

Muleskinner · Jun 13, 2022

Thanks for the replies. Here's the upshot. I put gas in both tanks replaced the one missing gas cap. I ran a hot wire from battery to fuel selector valve (valve only has one wire). Wire actuated the valve (I could hear it click). I juiced it several times and left it energized. She started right up. Dash switch was in R-side position. Didn't skip a beat when switched to L-side. I connected switch wire back to valve and started fine on either tank. That was evening next morning wouldn't start again. That evening, started right up. This morning had to crank long time to start. Riddle me this Batman.
P.S. 350 w/mechanical fuel pump. Someone removed carburetor filter and spliced in metal canister filter and severely kinked rubber line to carburetor inlet. I am hoping the restricted flow didn't damage the fue pump diaphragm.

SirRobyn0 · Jun 14, 2022

Muleskinner said:
Thanks for the replies. Here's the upshot. I put gas in both tanks replaced the one missing gas cap. I ran a hot wire from battery to fuel selector valve (valve only has one wire). Wire actuated the valve (I could hear it click). I juiced it several times and left it energized. She started right up. Dash switch was in R-side position. Didn't skip a beat when switched to L-side. I connected switch wire back to valve and started fine on either tank. That was evening next morning wouldn't start again. That evening, started right up. This morning had to crank long time to start. Riddle me this Batman.
P.S. 350 w/mechanical fuel pump. Someone removed carburetor filter and spliced in metal canister filter and severely kinked rubber line to carburetor inlet. I am hoping the restricted flow didn't damage the fue pump diaphragm.

The restricted flow isn't going to hurt the pump. If after the pump would be no different than if the the float closes off flow.

Muleskinner · Jun 14, 2022

Would the restriction affect the amount of gas getting to carb.? It is a four barrel. I fixed that though and put it back to original. I am really having trouble sending pictures.

SirRobyn0 · Jun 14, 2022

Muleskinner said:
Would the restriction affect the amount of gas getting to carb.? It is a four barrel. I fixed that though and put it back to original. I am really having trouble sending pictures.

Don't worry about the pictures. Every bit of info we can get helps us help you, so pictures would be nice to have but don't worry if you can't. Yes, of course a kick could restrict flow to the carb. Particularly if it's before the pump, the pump could suck that bit shut, you loose fuel, then it sits for a while and fires up again. Less likely if it's after the fuel pump because there is pressure in the line, but it's also possible engine heat could allow it to kink more, shutting of fuel flow.

Muleskinner · Jun 14, 2022

Thank you. I did find that the flared fitting at the carb. Housing was leaking. With a little steel wool it's now sealed. That may have caused a pressure leak? 7:00 AM started truck. First hit fired and nothing, cranked for long time until I pumped it, then fired up. Oops, I am used to fuel injection. I think it's going to be okay. I will be doing a lot of tinkering and tuning anyway. I cant tell you how good it is though to have this forum. thank you very much.

Muleskinner · Jun 14, 2022