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Old 06-04-2021, 06:13 PM   #113
69Tom
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Join Date: Jul 2015
Location: Phoenix, AZ
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Re: 1973-1991 Dual fuel tank systems theory of operation

Quote:
Originally Posted by hatzie View Post
There were three fuel valves used between 1973 & 1991.
None of the systems are rocket science to repair once you understand what makes em tick.

On all three systems...
-The fuel is routed from one of the tanks to a common OUT port. Only one tank at a time is valved open to the output port.
-If the truck is equipped with a fuel return line the common return port from the engine is switched from tank-to-tank as well.
-On systems with a return line... The return line should be routed to the tank that fuel is being drawn from. If you mix up the return lines you can overfill the tank not being used. Hosing down the guy in the next lane with raw fuel may not garner a friendly reaction.
-Both LH tank lines into the same side and both RH fuel lines into the opposite side.
-the COMMON side of the valve To/From-The-Engine only has 1 or 2 lines. Feed and, if equipped, Return.
NOTE: It's worth noting the 1981-1991 systems likely all have return lines. I haven't run into one without.

**1973 & 1974 & some 1975
Cable System... had a cable operated valve with a SPDT switch behind the dash knee panel that activated via the cable to run the fuel gauge. If you google it you'll find a thread on Chris's board about the valves with pictures of this setup. This is LONG obsolete. Folks replaced them with the 75/76-80 system when the parts ran out but there are a few survivors. NOTE: I'm not sure whether this system used the NL2 RPO or not... The 1975 wiring diagrams list the cable system and then NL2 in separate panes.
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.
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