Description & Operation
All TBI 2.5L models operate under a constant fuel pressure of 14.5 psi (1.02 kg/cm2 ). Fuel pressure regulator is mounted on throttle body assembly. Excess fuel pressure is returned to fuel tank. ECU has no control over fuel pressure relief valve. Fuel pump is immersible type with permanent magnet electric motor.
Fuel is drawn in through a filter sock and pushed through electric motor to pump outlet. A frame mounted in-line fuel filter is used. Fuel pump is attached to fuel gauge sending unit in fuel tank. Voltage to operate pump is controlled by Electronic Control Unit (ECU). A ballast resistor is used in the fuel pump control circuit.
Ballast resistor is by-passed during start mode. During running mode, ballast resistor reduces speed of pump by lowering pump voltage. This ensures normal pump speed in running mode. The one ohm ballast resistor is mounted on right side of plenum chamber.
Fuel pump control relay is located on front of right strut tower. Battery voltage is supplied to relay from ignition switch. Relay is energized when ECU provides a circuit to ground.
A multi-cell, roller type pump is used on all 4.0L MPFI models. Pump and fuel filter are located on a plate, forward of rear axle. Fuel pump control relay location for 4.0L models is on right inner front fender well. Battery voltage is supplied to relay from ignition switch. Relay is energized when ECU provides a circuit to ground.
Pump contains 2 check valves. One valve relieves internal pump pressure and regulates maximum pump output. Second valve, located near pump outlet, restricts fuel movement in either direction when pump is not in operation. System operates under a constant fuel pressure of 31 psi (2.17 kg/cm2 ).