Modular engine management system - MPi (Injectors): Description and operation

The ECU inputs and outputs are shown in the table.

Injectors

The four fuel injectors are fitted between the pressurised fuel rail and inlet manifold. Each injector comprises of a solenoid operated needle valve and a specially designed nozzle to ensure good fuel atomisation.

Engine coolant temperature sensor

The coolant temperature sensor is mounted in the thermostat housing and is immersed in the engine coolant. The sensor is a resistive device in which the resistance varies with temperature.

Throttle housing

On engines fitted with alloy inlet manifolds, the throttle housing is attached to the inlet manifold via a rubber sandwich plate whilst on engines fitted with plastic inlet manifolds, the throttle housing is sealed to the manifold by an 'O' ring. The throttle housing incorporates a throttle disc which is connected to the throttle pedal via the throttle lever and a cable.

There are two breather pipes connected to the throttle housing; one either side of the throttle disc. When the engine is running with the throttle disc open, both pipes are subject to manifold depression and draw crankcase fumes into the manifold. When the throttle disc is closed, only the pipe on the inlet manifold side of the disc is subject to manifold depression. This pipe incorporates a restrictor to prevent engine oil being drawn into the engine by the substantially greater manifold depression.

Also incorporated in the throttle housing are the throttle potentiometer and stepper motor.

Throttle potentiometer

The throttle potentiometer is attached to the throttle housing and is directly coupled to the throttle disc. It is non adjustable, closed throttle is detected by the potentiometer which initiates idle speed control via the stepper motor.

Three wires connect the throttle potentiometer to the ECU; a 5 volt supply to the potentiometer, an earth return to the ECU and an output voltage to the ECU which indicates the rate of throttle disc movement.

Stepper motor

The stepper motor is contained within the throttle housing and operates a cam and push rod via a reduction gear. The push rod is in direct contact with the throttle lever and moves the throttle disc to control idle and fast idle speed. The stepper motors maximum movement is 3.75 revolutions accomplished in steps of 7.5°. The reduction gear converts this into 180° of cam movement.

The throttle lever has a throttle position setting screw which rests on the stepper motor operating pin when the throttle pedal is released and is used to set the relationship between engine speed and stepper motor position.

In the side of the throttle housing is a throttle air bypass bleed screw to provide easier and more sensitive setting of the stepper motor position at idle.

The stepper motor position is checked using Microcheck and should be within the range of 20 to 40 steps when the engine is run in. If it is identified as being outside this range it can be adjusted to within range by turning the throttle air bypass bleed screw. It is important to follow the