Ford Pick-ups and Broncos 1987-1996 Repair Guide

General Information



See Figure 1

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Fig. Fig. 1: A common fuel injection pressure gauge

Both the Multi-port Fuel Injection (MFI) and Sequential Fuel Injection (SFI) systems are types of "multi-point'' fuel injection. While Ford Motor Co. has changed terminology for many of its fuel system components, in many cases the technology has remained the same. For purposes of uniformity, the latest names of components will generally be used in this inormation.

The Multi-port Fuel Injection (MFI) and Sequential Fuel Injection (SFI) sub-systems include a high pressure inline electric fuel pump, a low-pressure tank-mounted fuel pump, fuel charging manifold, pressure regulator, fuel filter, and both solid and flexible fuel lines. The fuel charging manifold includes 6 or 8 electronically controlled fuel injectors, each mounted directly above an intake port in the lower intake manifold. On the 6-cylinder MFI system, all injectors are energized simultaneously and spray once every crankshaft revolution, delivering a predetermined quantity of fuel into the intake air stream. On the V8 MFI engines, the injectors are energized in 2 banks of 4, once each crankshaft revolution. On the SFI engines, each fuel injector is energized once every other crankshaft revolution in sequence with the engine firing order.

The fuel pressure regulator maintains a constant pressure drop across the injector nozzles. The regulator is referenced to intake manifold vacuum and is connected parallel to the fuel injectors and positioned on the far end of the fuel rail. Any excess fuel supplied by the pump passes through the regulator and is returned to the fuel tank via a return line.

The pressure regulator reduces fuel pressure to 39-40 psi (269-276 kPa) under normal operating conditions. At idle or high manifold vacuum condition, fuel pressure is reduced to approximately 30 psi (207 kPa).

The fuel pressure regulator is a diaphragm operated relief valve in which the inside of the diaphragm senses fuel pressure and the other side senses manifold vacuum. Normal fuel pressure is established by a spring preload applied to the diaphragm. Control of the fuel system is maintained through the EEC power relay and the EEC-IV or EEC-V control unit, although electrical power is routed through the fuel pump relay and an inertia switch. The fuel pump relay is normally located on a bracket somewhere above the Powertrain Control Module (PCM) and the inertia switch is located in the cab. The in-line fuel pump is usually mounted on a bracket at the fuel tank, or on a frame rail. Tank-mounted pumps can be either high or low-pressure, depending on the model.

The inertia switch opens the power circuit to the fuel pump in the event of a collision. Once tripped, the switch must be reset manually by pushing the reset button on the assembly. Check that the inertia switch is reset before diagnosing power supply problems to the fuel pump circuit.

Fuel Pump

The fuel delivery system uses either a high or low-pressure in-line or in-tank electric fuel pump, with some models equipped with both. It is a recirculating system that delivers fuel to a pressure regulating valve in the throttle body and returns excess fuel from the throttle body regulator back to the fuel tank. The electrical system uses two types of control relays, one controlled by a vacuum switch and the other controlled by the powertrain control module (PCM) to provide power to the fuel pump under various operating conditions.

Fuel supply lines on vehicles equipped with a high pressure fuel system will remain pressurized for long periods of time after engine shutdown. The fuel pressure must be relieved before servicing the fuel system.

An inertia switch is used as a safety device in the fuel system. The inertia switch is located in the cab, generally under the dashboard on the right side. It is designed to open the fuel pump power circuit in the event of a collision. The switch is reset by pushing each of 2 buttons on the switch simultaneously (some models use switches with only one reset button). The inertia switch should not be reset until the fuel system has been inspected for damage or leaks.

When the ignition switch is ON , it turns the EEC power relay ON . The EEC power relay provides power to the powertrain control module (PCM) and the control side of the fuel pump relay. Power for the fuel pump(s) is supplied through a fuse link or high current fuse attached to the starter solenoid (battery side). From the fuse link or high current fuse, current flows through the fuel pump relay and inertia switch to the fuel pump(s). The fuel pump relay is controlled by the PCM.

When the ignition switch is turned ON , the fuel pump(s) will operate. If the ignition switch is not turned to the START position the PCM will shut the fuel pump(s) OFF after 1 second. The PCM will operate the fuel pump(s) operate the fuel pump(s) when the ignition switch is turn to START position to provide fuel while cranking.

After the engine starts, the PCM will continue to operate the fuel pump(s) unless the engine stops, drops below 120 rpm or the inertia switch is tripped.

Fuel Charging Assembly

The fuel charging assembly controls air/fuel ratio. It consists of a butterfly valve throttle body. It has bore(s) without venturis. The throttle shaft and valves control engine air flow based on driver demand. The throttle body attaches to the intake manifold mounting pad.

A throttle position sensor is attached to the throttle shaft. It includes a potentiometer that electrically senses throttle opening. Some vehicles incorporate a throttle kicker solenoid fastens opposite the throttle position sensor. During air conditioning operation, the solenoid extends to slightly increase engine idle speed.

Fuel Pressure Regulator

The fuel pressure regulator controls critical injector fuel pressure. The regulator receives fuel from the electric fuel pump and then adjusts the fuel pressure for uniform pressure differential between the intake plenum and the fuel injector pressure. The regulator sets fuel pressure at 13-17 psi (90-120 kPa).

Fuel Manifold

The fuel manifold (or fuel rail) distributes fuel to each injector. The end of the fuel rail contains a relief valve for testing fuel pressure during engine operation and relieving fuel system pressure before work is performed on the system.

Fuel Pressure Testing

Fuel pressure must be relieved before attempting to disconnect any fuel lines.

The diagnostic pressure valve (Schrader type) is located on the fuel rail on multi-port systems. This valve provides a convenient point to monitor fuel pressure, release the system pressure prior to maintenance, and to bleed out air which may become trapped in the system during filter replacement. A pressure gauge with an adapter is required to perform pressure tests.

If the pressure tap is not installed or an adapter is not available, use a T-fitting to install the pressure gauge between the fuel filter line and the throttle body fuel inlet or fuel rail.

Testing fuel pressure requires the use of a special pressure gauge (T80L-9974-A or equivalent) that attaches to the diagnostic pressure tap fitting. Depressurize the fuel system before disconnecting any lines.

Fuel Injectors

The fuel injectors are electromechanical (solenoid) type designed to meter and atomize fuel delivered to the intake ports of the engine. The injectors are mounted in the lower intake manifold and positioned so that their spray nozzles direct the fuel charge in front of the intake valves. The injector body consists of a solenoid actuated pintle and needle valve assembly. The control unit sends an electrical impulse that activates the solenoid, causing the pintle to move inward off the seat and allow the fuel to flow. The amount of fuel delivered is controlled by the length of time the injector is energized (pulse width), since the fuel flow orifice is fixed and the fuel pressure drop across the injector tip is constant. Correct atomization is achieved by contouring the pintle at the point where the fuel enters the pintle chamber.

The computer, based on voltage inputs from the crankshaft position sensor, operates each injector solenoid 2 times per engine revolution. When the injector metering valve unseats, fuel is sprayed in a fine mist into the intake manifold. The computer varies fuel enrichment based on voltage inputs from the exhaust gas oxygen sensor, barometric pressure sensor, manifold absolute pressure sensor, etc., by calculating how long to hold the injectors open. The longer the injectors remain open, the richer the mixture. This injector "on'' time is called pulse duration.

Exercise care when handling fuel injectors during service. Be careful not to lose the pintle cap and replace O-rings to assure a tight seal. Never apply direct battery voltage to test a fuel injector.

The injectors receive high pressure fuel from the fuel manifold (fuel rail) assembly. The complete assembly includes a single, preformed tube with 4, 6, or 8 injector connectors, mounting flange for the pressure regulator, mounting attachments to locate the manifold and provide the fuel injector retainers and a Schrader® quick-disconnect fitting used to perform fuel pressure tests.

The fuel manifold is normally removed with fuel injectors and pressure regulator attached. Fuel injector electrical connectors are plastic and have locking tabs that must be released when disconnecting when disconnecting the wiring harness.

Throttle Air Bypass Valve

The throttle air bypass valve is an electro-mechanical (solenoid) device whose operation is controlled by the EEC-IV or EEC-V control unit. A variable air metering valve controls both cold and warm idle air flow in response to commands from the control unit. The valve operates by bypassing a regulated amount of air around the throttle plate; the higher the voltage signal from the control unit, the more air is bypassed through the valve. In this manner, additional air can be added to the fuel mixture without moving the throttle plate. At curb idle, the valve provides smooth idle for various engine coolant temperatures, compensates for air conditioning load and compensates for transmission load and no-load conditions. The valve also provides fast idle for start-up, replacing the fast idle cam, throttle kicker and anti-dieseling solenoid common to previous models.

There are no curb idle or fast idle adjustments. As in curb idle operation, the fast idle speed is proportional to engine coolant temperature. Fast idle kick-down will occur when the throttle is kicked. A time-out feature in the PCM will also automatically kick-down fast idle to curb idle after a time period of approximately 15-25 seconds; after coolant has reached approximately 71°C (160°F). The signal duty cycle from the PCM to the valve will be at 100% (maximum current) during the crank to provide maximum air flow to allow no touch starting at any time (engine cold or hot).