1991 Ford F-150



April, 16, 2010 AT 10:59 PM

Electrical problem
1991 Ford F150 V8 Two Wheel Drive Automatic

I have 91 ford f 150 with dual tanks, the front fuel pump went out the rear tank was full at the time I noticed that after a day or two my rear tank was lower than norm, thne noticed fuel running out of front tank, could it be possible the front tank syphoned from rear

1 Answer


Dave H

April, 17, 2010 AT 4:15 AM

Fuel delivery systems differ in design depending upon model. System designs use one of the following configurations: Single tank with single pump.
Single tank with dual pump.
Dual tank with mechanical selector valve/reservoir.
All vehicles are equipped with a high pressure pump, but some vehicles use multiple pump systems. These systems have a primary, low pressure in-tank pump for supplying fuel to reservoir. The low pressure pump rests in a sump (depression) in fuel tank. A nylon screen protects low pressure pump inlet from contaminating particles, but allows passage of small amounts of water which may accumulate in fuel tank sump.
When dual tanks are used, each tank is equipped with a low pressure pump. Such a system has a total of 3 pumps: 2 low pressure and one high pressure.
The high pressure fuel pump is positioned inside fuel tank. A reservoir is built onto pump and sender assembly, instead of as part of tank.
In a 2-tank system, sender assembly handles switching of high pressure fuel through internal valves. Should one tank overfill during use (return line returns fuel to wrong tank), pump and sender unit in overfilling tank need replacement.
The high pressure fuel pump is capable of pumping over 33 gallons (125 liters) of fuel per hour at a working pressure of 39.2 psi (2.75 kg/cm2 ). This pump also has internal pressure relief and discharge check valves.
The 7.3L diesel engine uses a mechanical fuel pump to pump fuel from fuel tank to a combination fuel filter/fuel heater/water separator.
A driver-operated selector switch controls selector valve for switching fuel supply from one tank to other. The mechanical selector valve is contained within 6-port reservoir assembly. This valve switches fuel supply and return lines from one tank to other in response to fuel pressure from in-tank pumps acting on its actuating diaphragm.
The diaphragm switches tank connection when less than 2 psi of fuel pressure is acting on upper side of front tank and lower side of rear tank. Valve functioning depends upon proper operation of in-tank low pressure pumps. In all dual tank vehicles, excess fuel not used by engine is returned to same tank from which it was pumped.

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