Sounds like you have a bad low pressure pump in the rear tank?
FUEL DELIVERY SYSTEM
Fuel delivery systems differ in their design, depending upon vehicle. Systems are classified by type. Five types are available:
Type 1: Single tank with single pump.
Type 2: Single tank with dual pump.
Type 3: Dual tank with electric selector valve.
Type 4: Dual tank with mechanical selector valve/reservoir.
Type 5: Dual tank with pump sender in-tank reservoir.
LOW PRESSURE FUEL PUMP
All vehicles are equipped with a high pressure pump, but some are 2 pump systems having a primary, or low pressure in-tank pump for supplying fuel to reservoir (types 2, 3, and 4 systems). The low pressure pump rests in a sump, or 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.
HIGH PRESSURE FUEL PUMP
The type 5 system uses a high pressure fuel pump positioned inside fuel tank. This pump is similar to type 1 pump. Type 5 systems have a reservoir built onto pump and sender assembly instead of as a part of tank.
In a 2-tank system, sender assembly handles switching of high pressure fuel through internal valves. In a type 5 system with 2 tanks, should one tank overfill during use (return line returns fuel to wrong tank), it will be necessary to change pump and sender unit in tank that overfills.
The fuel pump is capable of pumping in excess of 33 gallons (125 liters) of fuel per hour at a working pressure of 39.2 psi (2.75 kg/cm2 ). These pumps also have internal pressure relief and discharge check valves.
RESERVOIRS & FILTERS
Fuel reservoirs are used to prevent fuel flow interruptions during extreme vehicle maneuvers with low tank fill levels. In-line reservoirs are used on type 2, 3 and 4 systems and are frame mounted between low and high pressure pumps. When high pressure pump is located in-tank (type 1 and 5 systems), reservoir is either molded or welded into tank or into fuel pump and sender plastic housing.
There are 2 types of in-line reservoirs: single function and dual function. Both contain a fine mesh in-filter and the dual function contains a mechanical selector valve.
A driver-operated selector switch controls selector valve for switching fuel supply from one tank to other. Two types of valves are used. These are electrical type (type 3) or mechanical type (type 4).
The electric type, when energized by selector switch, shuts off fuel supply and return lines from one tank and opens lines to other tank. Simultaneously, in-tank pump and fuel level sender are turned off for one tank and energized for other.
The mechanical selector valve is contained within 6-port reservoir assembly and is found on type 4 high pressure fuel pump system only. 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 under 2 psi of fuel pressure are acting on upper side of front tank and lower side for 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.
Thursday, January 1st, 2009 AT 6:55 AM