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Hyundai Coupes/Sedans 1986-1993 Repair Guide

General Description


For a complete description of the fuel injection system and its operation, as well as diagnosis and testing, refer to Emission Controls of this guide.

The fuel injection system used on Hyundai vehicles, is classified as a Multi-point Fuel Injection (MPI) system. The basic function of the system is to control the air/fuel ratio, based on input signals from various engine sensors. The air/fuel ratio is controlled by varying the injector driving time. The system is controlled by an Electronic Control Unit (ECU), which monitors the engine conditions, then calculates the injection timing and air/fuel ratio according to the signals from the sensors. The ECU consists of an 8-bit microprocessor, Random Access Memory (RAM), Read Only Memory (ROM) and input and output signal interface system.

The MPI system consists of 2 operating modes:

Open Loop-air/fuel ratio is controlled by information programmed into the ECU by the manufacturer.
Closed Loop-air/fuel ratio is varied by the ECU based on information supplied by the oxygen sensor.

An electric fuel pump supplies sufficient fuel to the injection system and the pressure regulator maintains a constant pressure to the injectors. These injectors inject a metered quantity of fuel into the intake manifold in accordance with signals from the Electronic Control Unit (ECU) or engine computer. After pressure regulation excess fuel is returned to the fuel tank.

The injectors have 2 modes (Injector Drive Timing) of operation:

Non-synchronous Injection (Simultaneous Injection)
Synchronous Injection (Sequential Injection)

Non-synchronous injection is activated during engine starting (cranking). There are 2 fuel injections, for each engine rpm, to all 4 cylinders. Also, during acceleration, fuel proportionate to the magnitude of acceleration, is injected to 2 selected cylinders during the intake and exhaust strokes.

Synchronous Injection is activated after the engine has started. The injectors are activated at the exhaust stroke of each cylinder in a sequential manner, according to crankshaft angle sensor signal. There is 1 injection per cylinder for every 2 crankshaft revolution, according to firing order.


Air Flow Sensor (AFS)

The AFS measures the intake air volume. The ECU uses this intake air volume signal to decide the basic fuel injection duration. The AFS on all models except the Turbo Scoupe is a hot film type, the Turbo Scoupe uses a hot wire type. The results that are obtained by the 2 types of sensor, however, are the same.

Atmosphere Pressure Sensor

The atmosphere pressure sensor signal is used by the ECU to compute the altitude of the vehicle and so the ECU can correct the ignition timing and air/fuel ratio. The atmosphere pressure sensor is contain in the AFS.

Intake Air Temperature Sensor

The air temperature sensor is a resistor based sensor for detecting the intake air temperature. The ECU provides fuel injection control based on this information. The air temperature sensor is located on the AFS.

Engine Coolant Temperature Sensor

The coolant temperature sensor is located in the coolant passage of the intake manifold. The ECU uses this signal to determine the base fuel enrichment for cold and warm engine operation.

Throttle Position Sensor (TPS)

The TPS is a rotating type variable resistor that rotates with the throttle body shaft to sense the throttle valve opening. Based on TPS voltage signals, the ECU computes the throttle valve opening and accordingly corrects fuel for engine acceleration.

Idle Switch

The idle switch is a contact type switch. The switch is installed at the tip of the ISC. This switch provides the ECU with idle or off idle signal.

Motor Position Sensor (MPS)

The MPS is a variable resistor type sensor and is installed in the ISC servo. The MPS senses the ISC servo plunger position and sends the signal to the ECU. The ECU controls the valve opening, and consequently the idle speed by using the MPS signal, idle signal, engine coolant temperature signal, load signals and vehicle speed sensor.

Cylinder TDC and Crankshaft Angle Sensor

The No. 1 TDC sensor and the crankshaft angle sensor are composed of a disc and unit assembly in the distributor. The No. 1 cylinder TDC is detected by the signal obtained through the single inner slit of the disc. The ECU, based upon this signal, determines the fuel injection cylinder. The crankshaft angle sensor signal that comes from the 4 slits at the outer circumference of the disc, serves to detect the position of the crankshaft. The ECU, based on this signal, determines the fuel injection timing, and also calculates the a