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Discussion Starter · #1 ·
Ok, so my 73 has an issue with its fuel tanks. When I run the primary tank it syphons fuel from the aux tank and fills the primary tank. When the aux tank is empty, no more fuel will transfer to the motor causing me to be stranded. And today I came to tell conclusion that even when the aux tank is being used it's still transferring fuel to the primary tank. Suggestions on where to start? I can't seem to find a fuel line diagram to check it. I have a feeling this is a simple problem, but I'm not sure where to look.

Thanks in advance
 

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Discussion Starter · #4 ·
ok so the valve has been by passed. anyone have a fuel line diagram so i can get it corrected? or does the chilton have one in it?
 

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ok so the valve has been by passed. anyone have a fuel line diagram so i can get it corrected? or does the chilton have one in it?
found on the web from a `73 truck owner, maybe Charlie can confirm, what I`m understanding is that they are the same from `73 to `86.
 

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Coming in late on this. I would check your switch first it may have both pumps on at same time. I don't know how it could syphon over. If it is then the valve would be bad, and I still can't see how that would happen. If your gas caps are vented I don't see drawing a vacuum, from one tank to the other. Your gas is being pushed from the tanks. Rich has probably the best drawing out there.
 

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Discussion Starter · #7 ·
ok thanks, that will help a lot. i am assuming the valve is bad. it has been completely by passed. hoses are ran every where. looks like PO just did what he had to do to get it fixed. shitty considering that he was a mechanic at a local honda delealership. oh well. ill take some pictures before i fix it so you can see the mess im dealing with. if it warms up here this weekend ill climb under it and see if i can get it taken care of.

thanks again for the help.
 

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hope we did help I would check that switch too. HAHA Your not comparing a C10 to a Honda Your's shouldn't have many metric parts LOL
anim_25
 

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Discussion Starter · #11 ·
i took some pictures but they didnt turn out that great, i attached the only one that came out somewhat decent. anyways turns out the the tank selector valve was bad, it had a crack in the side of it and the PO decided to just completly bypass the system and run a fuel line directly to the aux tank. after looking at the fuel lines that were ran i realized that there was no way to syphon the fuel into the main tank. looks like i was always running the fuel off the aux tank. guess it helps when i actully take a moment to look at the lines. all the lines on the main tank was disconnected so yeah. anyways i replaced the selector valve and now im running the main tank and making sure everything works correctly.

thanks again for the assistance
 

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i took some pictures but they didnt turn out that great, i attached the only one that came out somewhat decent. anyways turns out the the tank selector valve was bad, it had a crack in the side of it and the PO decided to just completly bypass the system and run a fuel line directly to the aux tank. after looking at the fuel lines that were ran i realized that there was no way to syphon the fuel into the main tank. looks like i was always running the fuel off the aux tank. guess it helps when i actully take a moment to look at the lines. all the lines on the main tank was disconnected so yeah. anyways i replaced the selector valve and now im running the main tank and making sure everything works correctly.

thanks again for the assistance
Outstanding picture , plain to see you benefitted from
some professional help !! Good you got the problems solved !!!
Dual tank issues can be maddening !! I've sometimes wished I
had only one tank to worry about !!!
 

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Glad you got it figured out, it makes like easier....wish I had help like you do....mines all grown and doing his own thing...anim_63
 

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I have a 1987 gmc 1/2 ton and i cant get it to switch over to the other fuel tank the guy I bought it from said he replaced the switch and pump on that side any ideas?
 

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I have a 1987 gmc 1/2 ton and i cant get it to switch over to the other fuel tank the guy I bought it from said he replaced the switch and pump on that side any ideas?
Check all your grounds, they are bad for corrosion where they are located on top of the tank directly connected to the sending unit.....Welcome to the forum...! sign0016
 

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Is this the same issue?

I've got a brand new switch on my '83. Just to check it, I filled up one tank. It registered full. I switched to the other tank, and it still registered full.

Is this an issue, or does one tank just fill the other?

Thanks

John
 

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This is a question for Charlie....never had dual tanks, he`ll be along shortly...
 

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I need to know where your fuel pump is located??? in the tank or on the engine.. If on the engine the tanks will fill from the aux tank to the main. If in the tank they will fill each other when key is on. SOO I guess what your question is will filling on tank fill the other YES. It depends on the routing of the hoses at the valve. It dose take a bit of time for this to happen with pump on engine. The bad part is you think you have two full tanks and really just one bigger one. They will cross fuel in the return lines and the main feed lines. The only way to beat this is with a manual valve and the gauge switched seperatlly. ALSO there are different valves that work electrically sold as the same valve. Isn't it fun??

You can Google your question and see different way the valves are plumbed but that just confused me using low air pressure and a battery you can determine what you have.
 

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dual tanks

Then what's the point in having the switch if it really is just one big tank? (which would be fine by me!)

John
 
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