86 F150 5.0L Dual Tank Problem

> Sounds like the tank select valve. Look at this:


I thought this valve was purely mechanical in nature... how can it be
affecting the gage readings?
 >> Stay informed about: 86 F150 5.0L Dual Tank Problem 

>
> 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?

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?
 >> Stay informed about: 86 F150 5.0L Dual Tank Problem 

"adupyours" <adjvickers.DeleteThis@yahoo.com> wrote in message
news:1176684495.183097.39660@o5g2000hsb.googlegroups.com...
>
> > Sounds like the tank select valve. Look at this:
>
>
> I thought this valve was purely mechanical in nature... how can it be
> affecting the gage readings?
>

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
 >> Stay informed about: 86 F150 5.0L Dual Tank Problem 

Big Al wrote:
> "adupyours" <adjvickers.RemoveThis@yahoo.com> wrote in message
> news:1176661627.939207.293200@l77g2000hsb.googlegroups.com...
>> 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!
>>
>
> Sounds like the tank select valve. Look at this:
>
> http://stangler.com/motorsports/articles/FuelTankValve/index.htm
>
> Al
>
>

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?
 >> Stay informed about: 86 F150 5.0L Dual Tank Problem 

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

"adupyours" <adjvickers RemoveThis @yahoo.com> wrote in message
news:1176661627.939207.293200@l77g2000hsb.googlegroups.com...
>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!
>
 >> Stay informed about: 86 F150 5.0L Dual Tank Problem