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Fuel Injection Technical Information
ELECTRONIC CONTROL UNIT (ECU) - The function of the ECU is to "tweak" or "fine tune" the engine operation to obtain the most complete and efficient combustion process. The ECU microprocessor receives input signals from various sensors from the engine and generates specific outputs to maintain optimum engine performance. The engine operating modes controlled by the ECU is the following: |
- Cold and hot start
- Acceleration enrichment
- Battery voltage compensation
- Deceleration cut/off or enleanment
- Run mode (open loop or closed loop)
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MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP) - The MAP sensor is a three-wire sensor located on or attached to the intake manifold. The function of this sensor is to measure the changes in the intake manifold air pressure and generates an electric signal that is proportional to the change of pressure. |
This signal is fed into the ECU and is used to:
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Adjust the fuel delivery
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Spark ignition calculations
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Barometric pressure readings upon starting the engine
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| MASS AIR FLOW SENSOR (MAF) - The mass air flow sensor is positioned in the air intake duct or manifold and measures the mass of incoming air. From this acquired data the ECU calculates the required fuel for the specific air mass flow rate. The MAF works on the hot wire or hot film concept. The hot wire/film is maintained |
| at a constant calibrated temperature. The passing air cools down the hot wire/film and the added energy required to maintain the calibrated temperature is directly proportional to the mass of air passing by the hot wire. |
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| The MAF also compensates for humidity as humid air, denser or cooler, absorbs more heat from the sensor, requiring more current to maintain the calibration temperature. |
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THROTTLE POSITION SENSOR (TPS) - The TPS is a three-wire sensor that is mounted on the throttle body assembly and is actuated by the throttle shaft. The TPS is |
| basically a variable resistor (potentiometer) that sends a voltage signal to the ECU that is proportional to the throttle shaft rotation.When the throttle shaft is open the sensor emits a high voltage signal and when the throttle shaft is closed it emits a low voltage signal. The voltage signal from the TPS changes between 0.45 V at idle to 4.5 to 5.0V at wide open throttle. |
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| OPEN LOOP - Open loop defines the engine operation where the fueling level is calculated by the ECU with only the input signals from the throttle position sensor (TPS), from the coolant and/ or air charge temperature, and from the manifold absolute pressure (MAP) or the mass air flow sensor (MAF). |
| CLOSED LOOP - Closed loop defines the engine operation where the fueling level is calculated and corrected by the ECU based on the voltage signal from the O2 sensor. When the O2 sensor emits a voltage signal above 0.45V due to a rich mixture in the exhaust manifold, the ECU reduces the fueling level by reducing the pulse width of the injector. The O2 sensor voltage is the feedback that modifies the fuel control program that is based on other signals. |
| IDLE AIR CONTROL VALVE (IACV OR IAC) - The IAC is located in the throttle body of the TBI, MPFI and CMFI systems. The valve consists of a stepper motor that adjusts the position of its pintle to vary the bypass air during idle and off idle conditions. |
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During the closed throttle condition (idle), the ECU constantly compares actual engine speed with the programmed desired engine speeds. Discrepancy between these two values result in activation of the stepper motor increasing or decreasing the bypass air around the throttle plate(s) until desired engine speed is achieved. The following input signals or conditions determine the position of the valve:
- Throttle position sensor
- Engine load (MAP/MAF, A/C compressor, power steering pressure switch, gear selection)
- Battery Voltage
- Engine coolant temperature
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THROTTLE AIR BYPASS VALVE - The throttle air bypass valve is located on the throttle body of engine fuel management systems. This solenoid valve allows additional bypass air when the engine is subjected to certain load conditions or cold starts.
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AIR CHARGE TEMPERATURE SENSOR - The air charge sensor is located in the engine air intake to sense the air induced into the engine manifold. The sensor consists of a thermistor,which generates a voltage signal, that is proportional to the air temperature.This voltage signal is used by the ECU to calculate the air density and using these results to adjust the fueling levels for a particular engine load. Other functions of the air temperature signal are:
- Adjust fueling during cold start
- Activation of the EGR valve
- Modify spark advance
- Regulate acceleration enrichment
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