Hyundai Coupes/Sedans 1986-1993 Repair Guide

General Information


See Figures 1, 2 and 3

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.

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Fig. Fig. 1: Multi-Point Fuel Injection (MPI) system-typical

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Fig. Fig. 2: Non-synchronous injection (simultaneous injection)

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Fig. Fig. 3: Synchronous injection (sequential injection)


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 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

See Figure 4

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.

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Fig. Fig. 4: Idle Speed Control (ISC) system

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 comes from the 4 slits at the outer circumference of the disc serve to detect the position of the crankshaft. The ECU, based on this signal, determines the fuel injection timing, and also calculates the amount of intake air, the ignition timing, etc. for each stroke of engine.

Phase Sensor

The Phase Sensor is built into the distributor. It senses the TDC point of the No. 1 cylinder in its compression stroke. These signals are fed to the ECU to be used in determining fuel injection sequence.

Crankshaft Position Sensor

The crankshaft position sensor consists of a magnetic coil which is installed in the transaxle housing. The sensor gets its signal from a sensor ring mounted to the flywheel. The voltage signal is fed to the ECU for detecting engine rpm and crankshaft position.

Oxygen Sensor

The oxygen sensor, by detecting the oxygen content in the exhaust gas, maintains the stoichiometric air/fuel ratio. In operation, the ECU receives the signals from the sensor and changes the duration during which fuel is injected. The oxygen sensor is located in the exhaust manifold.

Vehicle Speed Sensor

The vehicle speed sensor uses a reed type switch. The speed sensor is built into the speedometer and converts the speedometer gear revolution into pulse signals, which are sent to the ECU.

Neutral (Inhibitor) Switch

The ECU, based on this signal, senses when the automatic transaxle is in NEUTRAL or PARK and operates the ISC servo to keep the idle speed correct.

Idle Speed Control (ISC) Servo

The ISC servo consists of a motor, worm gear, worm wheel and plunger. The MPS is used to detect plunger position and an idle switch to detect closed throttle position.

As the motor rotates, according to signals from the ECU, the plunger extends or retracts depending on direction of the motor rotation. This actuates the throttle valve via the ISC lever. The ECU controls the idle speed by changing the throttle opening through this function.

Knock Sensor

The Knock Sensor is attached to the cylinder block and senses engine knock. The sensor functions by reading the knocking vibration from the engine and converts this into a voltage signal delivered as output to the ECU. The ECU uses this signal to adjust the ignition timing.

Waste Gate Control Solenoid Valve

The waste gate control solenoid valve controls the turbo boost pressure. This valve is integrated with the rest of the engine sensors to ensure proper engine operating conditions under varying boost conditions. It also helps to reduce turbo lag and engine knock.


Fuel Pump

The fuel pump is a compact impeller design and is installed inside the fuel tank. The pump assembly consists of the impeller (driven by the motor), the pump casing (which forms the pumping chamber), and cover of the pump. This pump is called a wet-type pump, because the inside is also filled with fuel. Never operate this type of pump when removed from the vehicle or explosion will result, due to fuel fumes, electric sparks and fresh air, which is not available in the fuel tank.

Fuel Injectors

The injectors are solenoid valves. When the solenoid coil is energized, the plunger is retracted. The needle valve that is attached to the plunger is pulled to the full open position.