The main engine load sensor is the MAP sensor. A vacuum hose connects the MAP sensor (located within the ECU) and the inlet manifold. Manifold vacuum acts upon the MAP sensor diaphragm, and the ECU converts the pressure into an electrical signal. MAP is calculated from the formula: Atmospheric Pressure less Manifold Pressure = Manifold Absolute Pressure.
Using the speed/density method, the ECU calculates the AFR from the MAP signal and the speed of the engine (CAS). This method relies on the theory that the engine will draw in a fixed volume of air per revolution. The.inlet manifold on the MPi models is a 'dry' manifold. Since fuel does not enter the manifold - due to injection being made onto the back of the inlet valve, there is no risk of fuel being drawn into the MAP sensor to contaminate the diaphragm, and a fuel trap is not used.
The inlet manifold on the SPi models is a 'wet' manifold. Fuel is injected into the inlet manifold, and there is a risk of fuel being drawn into the MAP sensor to contaminate the diaphragm. This is prevented by running the vacuum hose upward to the air filter, through a fuel trap and then to the ECU (which contains the MAP sensor).

The CTS is incorporated in the cooling system, and contains a variable resistance that operates on the NTC principle. When the engine is cold, the resistance is quite high. Once the engine is started and begins to warm-up, the coolant becomes hotter, and this causes a change in the CTS resistance. As the CTS becomes hotter, the resistance of the CTS reduces (NTC principle), and this returns a variable voltage signal to the ECU based upon the coolant temperature. The open-circuit supply to the sensor is at a 5v reference level, and this voltage reduces to a value that depends upon the CTS resistance. Normal operating temperature is usually from 80°C to 100°C. The ECU uses the CTS signal as a main correction factor when calculating ignition timing and injection duration. The coolant temp sensor is the black sensor located in the inlet manifold. It can be found underneath on the right side of the manifold as you stand at the front of the car. The coolant temperature sensor works on the same principle as the air temperature sensor, whereby as the temperature falls the resistance rises or vica versa.
A faulty CTS will have a dramatic effect on the fuelling of the engine as the ECU may never leave the cold start and warm up programs which operate in open loop control where the signal from the oxygen sensor is almost ignored. Again this sensor will usually read out low and a permanently rich condition will result.
This sensor is connected directly to the ECU and then the ECU provides a signal to the Temperature Gauge on the dash. The gauge should read not quite midway, just a little below the white line when the engine is warm. It's always worthwhile checking the plug connectors as these can suffer contact corrosion. This sensor can only be accurately checked with the diagnostic and a calibrated thermometer.
If this sensor is faulty and causes the ECU to over fuel for long periods, the Oxygen Sensor will usually fail too.

The ATS is mounted in the air inlet casing (MPi) or air filter casing (SPi), and measures the air temperature before it enters the inlet manifold. Because the density of air varies in inverse proportion to the temperature, the ATS signal allows more accurate assessment of the volume of air entering the engine. The open-circuit supply to the sensor is at a 5.0v reference level, and the earth path is through the sensor return. The ATS operates on the NTC principle. A variable voltage signal is returned to the ECU based upon the air temperature. This signal is approximately 2.0 to 3.0 volts at an ambient temperature of 20°C, and reduces to about 1.5v as the temperature rises to around 40°C. Although the air filter casing used on SPi models contains a thermal valve system, the air temperature is calculated solely by reference to the ATS. The air temperature sensor is the green plug and sensor mounted in the underside of either of the types of air filter, Standard or K&N.
This sensor is used by the ECU to determine the density of the air entering the engine. The sensor does not have to totally fail to cause issues with engine running. The sensor is a NTC (Negative Temperature Coefficient) thermistor, resistance rises as the temperature falls. The sensor element is constructed of two dissimilar metal wires fused together (the bead) at the tip of the sensor, this generates a resistance that changes with temperature. This type of sensor is prone to contamination with moisture which causes corrosion at this fused point, generating a higher resistance. It will most likely give a low reading compared to the actual air entering the engine and the ECU will determine the air is denser (colder) than it is and add more fuel to compensate. The Rover part number is NNK10001
This only contributes to 20% of the fueling equation with the main reasons for rich running being the Throttle Position Sensor and the Coolant Temperature Sensor. When this sensor fails, causing rich running conditions, it usually causes the Oxygen Sensor to fail as the carbon particles clog the pours of the sensor. Symptoms of rich running are black sooty spark plugs( make sure the correct plugs are used first NGK BPR6ES), rich misfire, erratic idle and elevated CO and Hydrocarbon emissions.

The MEMS injection system fitted to catalyst vehicles implements a closed-loop control system, so that exhaust emissions may be reduced. Closed-loop systems are fitted with an oxygen sensor (OS) (Lambda Sensor) which monitors the exhaust gas for its oxygen content. A low oxygen level in the exhaust signifies a rich mixture. A high oxygen level in the exhaust signifies a weak mixture. The lambda voltage will be seen to oscillate between a high reading and a low reading in normal operation. The OS only produces a signal when the exhaust gas, has reached a minimum temperature of approximately 300°C. In order that the OS will reach optimum operating temperature as quickly as possible after the engine has started, the OS contains a heating element. The OS heater supply is made from the OS relay terminal number 87 (on the right hand side of the engine bay on an Spi). This ensures that the heater will only operate whilst the engine is running. Under full-load conditions, the heater supply is cut-off by the ECU by inhibiting the earth path of the OS relay.

The primary signal to initiate both ignition and fuelling emanates from a Crankshaft Angle Sensor (CAS), also referred to as the Crankshaft Position Sensor which is mounted next to the flywheel. The CAS consists of an inductive magnet that radiates a magnetic field. The flywheel incorporates a reluctor disk containing 34 steel pins set at 10° intervals. As the flywheel spins, and the pins are rotated in the magnetic field, an AC voltage signal is generated to indicate speed of rotation. The two missing pins (set at 180° intervals) are a reference to TDC, and indicate crankshaft position by varying the signal as the flywheel spins. One missing pin indicates TDC for cylinders 1 and 4, and the other missing pin indicates TDC for cylinders 2 and 3. The peak-to-peak voltage of the speed signal can vary from 5 volts at idle to over 100 volts at 6000 rpm. The ECU contains an analogue-to-digital converter to transform the AC pulse into a digital signal.

The throttle potentiometer sensor (TPS) is provided to inform the ECU of rate of acceleration. The TPS is a potentiometer with three wires. A 5.0 volt reference voltage is supplied to a resistance track, with the other end connected to earth. The third wire is connected to an arm which wipes along the resistance track, and so varies the resistance and voltage of the signal returned to the ECU. From the voltage returned, the ECU is able to calculate just how quickly the throttle is opened. From model year 1993 onwards, the TPS also informs the ECU of idle position with a voltage of approximately 0.6 volts.
A fault will be indicated with the TPS circuit if the voltage reading is below 0.6 volts.

The air valve stepper motor is an actuator that the ECU uses to automatically control idle speed during normal idle and during engine warm-up. When the throttle is closed, the throttle valve is locked in a position where very little air passes by. The throttle position then, will have no effect upon the idle speed. A by-pass port to the throttle plate is located in the inlet manifold. A valve is positioned in the port. As the valve moves, the volume of air passing through the port will vary, and this directly affects the idle speed. The idle speed then, depends upon the position of the stepper air valve in the by-pass port. This method of idle control is fitted to some models (principally those with the plastic inlet manifold) from the middle of 1994.