Main Components

To better understand the overall operation of this fuel injection system, first become familiar with the functions of the individual components.

Engine Control Module
The engine control module (ECM) is located between the incoming information from the various sensors and the fuel injectors. Depending on the current load and temperature conditions, it enables the engine to receive a precisely determined quantity of fuel. To achieve this, the ECM varies the opening duration of the solenoid-operated fuel injectors. Since the fuel system pressure remains practically constant, the quantity of injected fuel can only be regulated by changing the injection duration. Where does the ECM obtain the information on which it bases this injection duration? This is provided by the following sensors:

2.8L MPI engine only: Air flow meter; it provides information on the quantity of intake air.

2.6L MPFI engine only: Exhaust manifold temperature sensor; in combination with the intake manifold pressure sensor (in the ECM), it reports the quantity/mass of intake air.

Engine coolant temperature sensor; it reports the engine temperature.

Throttle position sensor (potentiometer); it provides information on engine load.

Engine speed sensor; it sends the engine speed signal for the ignition/MPI/MPFI system.

Crankshaft position sensor; it reports the crankshaft position. In this way the ECM determines which cylinder is next for ignition or injection.

The start signal is received from terminal 50 (ignition switch and starter).

Oxygen sensors report whether the air-fuel mixture is correct.

Other signals come from knock sensors, the transmission, the tachometer, and even the air conditioning system.

Fuel Injectors
One fuel injector is located in the intake manifold for each engine cylinder. They supply each cylinder with the required quantity of fuel at the required time and at the same time ensure fine atomization of the gasoline.

The injectors are actuated by a solenoid. The needle valve lifts approximately 0.1 mm off its seat, allowing fuel to flow. It is noteworthy that each individual fuel injector is activated exactly in time with injection. This means the injected fuel has no time to condense on the walls of the intake manifold (no fuel loss).

Fuel Distributor
It supplies fuel evenly to all fuel injectors. In addition, the fuel distributor acts as a fuel accumulator, thereby preventing pressure fluctuations. The U-shaped pipe design, which delivers fuel to all six injectors, is very interesting.

Fuel Pressure Regulator
It is located at the rear right of the fuel distributor and, as its name indicates, maintains a constant pressure in the fuel distributor. This is achieved by a greater or lesser flow of fuel back to the fuel tank through the return line. If more fuel flows through the return line, the pressure drops; if less, the pressure rises.

The vacuum hose connection to the fuel pressure regulator also provides information on engine load. Under full load, the regulator further increases the pressure. As a result, more fuel is injected, which the engine needs to achieve maximum power.

Fuel Pump and Relays
For more detailed information on the electric fuel pump, fuel pump relay, and other MPI/MPFI relays, refer to the Fuel Tank and Fuel Pump chapter.

Air Flow Meter
MPI only

A heated wire is located in the intake air flow path. The air flow varies depending on the quantity of intake air, which causes the wire to be cooled more or less. The change in temperature changes the electrical resistance of the wire, which is measured by the ECM.

Intake Manifold Pressure Sensor
MPFI only

The intake manifold pressure sensor is located inside the MPFI ECM. It is connected to the intake manifold by a thin hose. Intake manifold pressure is the main information used by the ECM to calculate engine load. It affects injection duration and ignition timing.

Intake Air Temperature Sensor
MPFI only

The intake air temperature sensor is screwed into the intake passage of the third cylinder (rear right). In addition to the intake manifold pressure sensor, it serves as a source of information for the ECM to calculate engine load. At high intake air temperature (which corresponds to low air density), it is necessary, for example, to shorten the injection duration and retard the ignition timing slightly.

Throttle Body
Where the intake airflow enters the intake manifold, two throttle valves are located in one housing. The smaller valve is connected by a cable to the accelerator pedal. It meters the intake airflow into the engine up to the half-load position. When the accelerator pedal is depressed further, the linkage opens the second, larger valve, until both valves are fully open at full load.

Throttle Position Sensor
The throttle position sensor is driven by the throttle valve shaft. The sensor detects the current throttle valve position and sends this information to the control unit in the form of electrical resistance. The control unit needs this load information, for example, to regulate idle speed, select ignition characteristics and calculate injection duration.

Idle Air Control Valve
As the name implies, this valve constantly maintains a constant engine speed at idle, regardless of whether the engine is cold or warm, or whether high-power consumers such as air conditioning are switched on.

The valve itself is the actuator. The control center is the MPI or MPFI injection system control unit. It compares the current engine speed with the specified value and thus provides finely coordinated opening and closing of the control valve to balance the engine speed. In doing so, the cross-section of the additional air passage bypassing the throttle valves is varied. When the passage is open, more air enters, so the air flow meter or the intake manifold pressure sensor 'interprets' the increased air quantity as an open throttle valve. This causes the injection system to increase the fuel quantity to the required amount.

It should also be noted that different injection systems use different idle air control valves: in the MPI system, a continuously regulating valve; in the MPFI system, a valve driven by a so-called stepper motor. The latter regulates the opening of the passages in small, precisely adjusted steps.