An optimal fuel injection has everything to do with the relationship between the amount of oxygen and fuel. If there is a shortage of oxygen, not all the fuel burns, resulting in a loss of power, but if there is an excess of oxygen, there is not enough fuel and results in a loss of power. The optimum combustion ratio for a complete combustion is between 14.7 : 1 or 1 gram of petrol to 14.7 grams of air. Unfortunately, this ratio is not suitable for every situation. The most power is achieved when you have at a ratio of 12.5 : 1 and the best economy when the ratio is 15 : 1. The ECU calculates the ratio based on the sensor values which are used to determine the load on the engine and the environment. The MEMS ECU has a hard-wired map that it uses to determine the correct ratio depending upon the load on the engine.
The Manifold Absolute Pressure Sensor (MAP sensor) provides information about the pressure in the intake manifold. This information is used to calculate the air density and the air mass ratio. A leak in the vacuum pipes will adversely affect performance as the ECU will be unable to calculate the correct air to fuel ratio. Until the ECU has determined a fault condition this can result in very erratic engine behaviour.
Coolant temperature sensor measures the temperature of the water circulating the engine. The Mini SPi should be fitted with an 88C thermostat to ensure correct operating temperature is reached. Below 80C the ECU will determine that the engine is still warming and may over-fuel and remain in open loop where the Lambda (O2) readings are not used to adjust the air fuel ratio. This sensor is also used to determine if the electric fan should be activated and the engine temperature on the instrument panel.
The Lambda Sensor or oxygen sensor (O2 sensor) measures the amount of oxygen in the exhaust gases. If there is too much oxygen then the ECU will add more fuel to enrich the mixture. If there is too little oxygen in the exhaust then the ECU will reduce the fuel and decrease the making the mixture more lean. The ECU tries to balance the air : fuel ratio using the ECU mapping table. When operating in closed loop the ECU will use the dynamic values read from the O2 sensor and the engine RPM sensor to adjust the air : fuel ratio.
The Crankshaft Position Sensor is used to ‘synchronise’ the engine at start-up, so that the ECU knows when to inject the fuel. The crankshaft position sensor is often used as a source to measure the engine speed. A fault with this sensor has a direct effect on the overall operation of the fuel injection system and will create very erratic behaviour including stalling and misfires.
The Throttle Position Sensor is used to determine the position of the accelerator pedal. As the accelerator pedal is pressed the amount of air and fuel is increased. On the Mini Spi the TPS is used to determine the throttle position with reference to Idle Air Control (IAC) sensor.
The Idle Air Control Sensor (IAC) is used to adjust the engine idle speed when starting from cold. The ECU increments and decrements the Stepper Motor to adjust the throttle. The ECU has no direct feedback as to the position of the stepper motor. The ECU attempts to reset the stepper motor when the ignition is switched on by applying a voltage for a short period. The IAC position can give an indication of a faulty stepper motor.