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Old 06-05-2021, 10:26 AM   #114
hatzie
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Location: Wentworth, NH
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Re: 1973-1991 Dual fuel tank systems theory of operation

Quote:
Originally Posted by 69Tom View Post
So my truck has the cable system. It's a 74. Interestingly enough, I'm looking at the 73-74 parts book and part 329854 is a rocker-type selector. I thought that wasn't available until 75, but maybe I'm wrong.

In any event, I'd like to learn a bit more about the system on my truck, as I'll be shortening the bed and replacing the 20 gal tanks with 16 gal tanks. I want to may sure my system works correctly, as I'm going to need both thanks with a 454. To what site is being referred above? Does anyone have the link or more info?

Thanks.
There are a bunch of inconsistencies around the dual tank systems in the 73-75 parts and wiring manuals.
http://forum.73-87chevytrucks.com/sm...0970#msg270970
^^ This is the thread I was referencing. Not a lot of information there but it's the most I've seen anywhere..

My understanding, never having owned one myself, is the 73-75 cable system is a cable operated valve under the truck with some kind of switch built into the dash assembly to select which sender was connected to the fuel gauge.
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^^This is the picture VileZamboni posted in the thread on Chris's board.

If someone has a better picture of the dash switch/bezel and the valve showing the hose connections and the switch on the bezel it would be an appreciated addition to post it here.

1973-1975 are not likely to have return lines so you likely only have three fuel lines at the original valve.

As long as you mark the valves' hoses and hook them up to the correct tanks' hardline you shouldn't have any issues. Painters tape and a sharpie...

The 16 & 20 gallon saddle tanks use the exact same senders. If yours are working I'd keep them.

Most 3/4 & 1 tons of that vintage likely used a transmission/axle vent on the end of a chunk of SAE J30R7 fuel line as the vapor system. Adding a three hose 1970's Corvette or F-body vapor can is an easy way to make sure the garage will not smell like raw fuel. I'm not a pushy obnoxious environmentalist. If the smell of raw fuel in the garage doesn't bother you just carry on without an evap can.
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RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 06-05-2021 at 05:47 PM.
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