86 F150 5.0L Dual Tank Problem

I have an 1986 F150 with the dual fuel tank option. When I switch it to the front tank the fuel gage reads full and the engine runs normally. When I flip the switch to the rear tank, the fuel gage reads
empty (even though it is full of fuel) and the engine runs normally but it is drawing fuel from the front tank. I know how is is supposed to function, but I cant figure out why it continues to use fuel from the front tank even though the switch is set to the rear tank, and why the gage reads an empty rear tank when it is full.
Any help will be greatly appreciated!
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Sounds like the tank select valve. Look at this:
http://stangler.com/motorsports/articles/FuelTankValve/index.htm
Al
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Big Al wrote:

i never did understand the purpose of the dual tanks. their combined size is about the same as a single regular tank. my 95 ford had dual tanks.
anyone care to explain their purpose?
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They put them in for long distance traveling. The front one is 16 gallons and I think the rear one is the same or bigger. As you adequately put it, they only add up to one big tank. Maybe they did this for spare part sales, who knows.
I pulled the selector switch out of the dash and checked it and it checks good. It switches positive between the two pumps and also switches which sending unit goes the dash gage... I used a jumper wire to jump the rear tank with voltage, and the jumper got hot. I put the jumper in the front tank wire and it remained cool. I checked the wiring from the relay, switch, impact switch, through the engine compartment connector to the fuel tanks.
Maybe I have a bad fuel pump? If so, is it common to see both the pump and sending unit fail at the same time? or is it related to the mechanical 6 port selection valve as Big Al suggested?
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I thought this valve was purely mechanical in nature... how can it be affecting the gage readings?
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As far as I remember you are correct. On my truck the front tank would overflow when running off the rear tank. The return from the fuel pressure regulator was always going to the front tank, no matter what tank was selected. The fuel gauge read correctly, but the readings didn't make sense.
Al
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Sounds like a concern with the fuel tank module.... .
The 86 had three fuel pumps. A single, low pressure pump in each tank and a third high pressure pump on the left frame rail. . It is the pressure delivered by the in tank pump that "decides" which way the changeover valve directs fuel.... If one in tank pump fails, the valve will not change tanks when commanded and the high pressure pump will still draw from the "good" tank even though the low pressure pump in that tank isn't turned on....
If the pump in the tank you are running on fails, the truck will continue to run until that tank is empty. You will be able to change to the other tank but not be able to change back...
HTH

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