All vehicles covered herein are utilize an electronic multi-port fuel injection system. The heart of this system is a micro-processor called the Powertrain Control Module (PCM) or Electronic Control Module (ECM). The PCM/ECM receives data from a number of sensors and other electronic components (switches, relay, etc.). Based on information received and information programmed in the PCM/ECM's memory, it generates output signals to control various relay, solenoids and other actuators. The PCM/ECM in this system has calibration modules located inside the assembly that contain calibration specifications for optimizing emissions, fuel economy and driveability. The calibration module is called a PROM.
The following are the electronic engine controls used by MPV, Navajo and B Series Pick-ups:
The MAF sensor (a potentiometer) senses the position of the airflow in the engine's air induction system and generates a voltage signal that varies with the amount of air drawn into the engine. The IAT sensor (a sensor in the area of the MAF sensor) measures the temperature of the incoming air and transmits a corresponding electrical signal. Another temperature sensor (the ECT sensor) inserted in the engine coolant tells if the engine is cold or warmed up. The TP sensor, a switch that senses throttle plate position, produces electrical signals that tell the PCM when the throttle is closed or wide open. A special probe (the HO2S) in the exhaust manifold measures the amount of oxygen in the exhaust gas, which is in indication of combustion efficiency, and sends a signal to the PCM. The sixth signal, camshaft position information, is transmitted by the CMP sensor, installed in place of the distributor (engines with distributorless ignition), or integral with the distributor.
The microcomputer circuit processes the input signals and produces output control signals to the fuel injectors to regulate fuel discharged to the injectors. It also adjusts ignition spark timing to provide the best balance between driveability and economy, and controls the IAC valve to maintain the proper idle speed.
Because of the complicated nature of this system, special tools and procedures are necessary for testing and troubleshooting.
The Multi-port Fuel Injection (MFI) system includes a high pressure, inline electric fuel pump mounted in the fuel tank, a fuel supply manifold, a throttle body (meters the incoming air charge for the correct mixture with the fuel), a pressure regulator, fuel filters and both solid and flexible fuel lines. The fuel supply manifold includes 4 or 6 electronically-controlled fuel injectors, each mounted directly above an intake port in the lower intake manifold. Each injector fires 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 in parallel to the fuel injectors; it is positioned on the far end of the fuel rail. Any excess fuel supplied by the fuel pump passes through the regulator and is returned to the fuel tank via a return line.
The pressure regulator reduces fuel pressure to approximately 39-40 psi under normal operating conditions. At idle or high manifold vacuum condition, fuel pressure is further reduced to approximately 30 psi.
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 Powertrain Control Module (PCM), although electrical power is routed through the fuel pump relay and an inertia switch (Navajo/B Series Pick-up). The fuel pump relay is normally located in the power distribution box, under the hood, and the inertia switch is located on the toe-board, to the right of the transmission hump, in the passenger-side footwell. The inline fuel pump is usually mounted in the fuel tank. Tank-mounted pumps can be either high- or low-pressure, depending on the model.
The inertia switch, if equipped, 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.
On Navajo and B Series Pick-up models, check that the inertia switch is reset before diagnosing power supply problems to the fuel pump.
The fuel injectors used with the MFI system are electro-mechanical (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.
Exercise care when handling fuel injectors during service. Be careful not to lose the pintle cap and always 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 supply manifold (fuel rail) assembly. The complete assembly includes a single, pre-formed tube with four or six connectors, the 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 the fuel injectors and pressure regulator attached. Fuel injector electrical connectors are plastic and have locking tabs that must be released when disconnecting the wiring harness.