Saturn Coupes/Sedans/Wagons 1991-1998 Repair Guide

General Information


See Figures 1 and 2

The 1995-98 SOHC and all DOHC engines are equipped with Multi-port Fuel Injection (MFI), which utilizes one injector for each cylinder. All fuel injection and ignition functions are controlled by the Powertrain Control Module (PCM). It accepts inputs from various sensors and switches, calculates the optimum air/fuel mixture and operates the various output devices to provide peak performance within specific emissions limits. The PCM will attempt to maintain an air/fuel ratio of 14.6:1 in order to optimize catalytic converter operation. If a system failure occurs that is not serious enough to stop the engine, the PCM will illuminate the SERVICE ENGINE SOON light and operate the engine in a backup or fail-safe mode. In the backup mode, the PCM delivers fuel according to inputs from the Manifold Absolute Pressure (MAP) sensor and the Coolant Temperature Sensor (CTS). Other operating modes are in the PCM program and are described later.

Fuel is supplied to the engine from a pump mounted in the fuel tank. The fuel pump module includes the gauge/sending unit, which can be replaced separately. Otherwise, the module must be replaced as an assembly. The pump is operated through a relay mounted in the Instrument Panel Junction Block (IPJB), which is located under the center of the instrument panel on the passenger' side of the vehicle. A check valve in the tank unit maintains pressure in the system for a period of time after the engine is stopped to aid hot starting. The fuel tank must be removed to access the pump module.

Other system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position Sensor (TPS), Air Temperature Sensor (ATS), Coolant Temperature Sensor (CTS), a power steering pressure switch and an oxygen sensor. The fuel injectors are solenoid valves that the PCM pulses on and off many times per second, in order to promote good fuel atomization. The pulse width determines how long the injector is on each cycle, and this regulates the amount of fuel supplied to the engine. Fuel injectors are operated simultaneously, not sequentially.

On 1991-97 models, the system pressure regulator is mounted on the end of the fuel rail that feeds the injectors. For 1998, all models have a new Returnless fuel system, which makes the pressure regulator an integral component of the fuel filter, which is mounted below the vehicle near the fuel tank. Intake manifold pressure is supplied to the regulator diaphragm, making system pressure partly dependent on engine load. The idle air control valve is a 2-coil stepper motor that controls the amount of air allowed to bypass the throttle plate. With this valve, the PCM can closely control idle speed even when the engine is cold or when there is a high engine load at idle.

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Fig. Fig. 1: All four fuel injectors are pulsed once per revolution, each time the No. 1 cylinder is 60 degrees before TDC-1995-98 SOHC and all DOHC engines

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Fig. Fig. 2: Fuel injection control schematic-1995-98 SOHC and all DOHC engines


See Figure 3

Starting Mode

When the ignition switch is first turned ON , the fuel pump relay is energized by the PCM for 2 seconds to build system pressure. When the crankshaft position signal tells the PCM that the engine is turning over or cranking, the pump will run continuously. In the start mode, the PCM checks the TPS and CTS to determine the best air/fuel ratio for starting. Ratios could range from 0.8:1 at -40°F (-40°C), to 14.6:1 at 220°F (104°C) engine coolant temperature.

Clear Flood Mode

If the engine becomes flooded, it can be cleared by opening the accelerator to the full throttle position. When the throttle is open all the way and engine rpm is less than 400, the PCM will close the fuel injectors while the engine is cranking, in order to clear the engine of excess fuel. If throttle position is reduced below about 75 percent, the PCM will return to the start mode.

Open Loop Mode

When the engine first starts and engine speed rises above 400 rpm, the PCM operates in the Open Loop mode until specific parameters are met. Fuel requirements are calculated based on information from the MAP sensor and CTS.

Closed Loop Mode

When the correct parameters are met, the PCM will use O 2 sensor output and adjust the air/fuel mixture, in order to maintain a narrow band of exhaust gas oxygen concentration. When the PCM is correcting and adjusting fuel mixture based on the oxygen sensor signal, along with the other sensors, this is known as feedback air/fuel ratio control.

The PCM will shift into Closed Loop mode when:

Oxygen sensor output voltage varies, indicating that the sensor has warmed to operating temperature; that is, a minimum of 600°F (318°C)
Coolant temperature is above 68°F (20°C)
The PCM has received an rpm signal greater than 400 for more than 1 minute

To operate in closed loop mode, 1992 and later vehicles also require a change in throttle position.

Acceleration Mode

If the throttle position changes and manifold pressure is quickly increased, the PCM will provide extra fuel for smooth acceleration.

Deceleration Mode

As the throttle closes and the manifold pressure decreases, fuel flow is reduced by the PCM. If both conditions remain for a specific number of engine revolutions, the PCM decides fuel flow is not needed and stops the flow by shutting off the injectors.

Fuel Cut-Off Mode

When the PCM is receiving a Vehicle Speed Sensor (VSS) signal and rpm goes above 6750, the injectors are shut off to prevent engine overspeed. The PCM will also shut off the injectors if the VSS signal is 0 and engine speed reaches 4000 rpm.

Battery Low Mode

If the PCM detects a low battery, it will increase injector pulse width to compensate for the low voltage and provide proper fuel delivery. It will also increase idle speed to increase alternator output.

Field Service Mode (1991-95 Models)

When terminals A and B of the ALDL are jumpered with the engine running, the PCM will enter the Field Service Mode. If the engine is running in Open Loop Mode, the SERVICE ENGINE SOON light will flash quickly, about 2 1 / 2 times per second. When the engine is in Closed Loop Mode, the light will flash only about once per second. If the light stays OFF most of the time in Closed Loop, the engine is running lean. If the light is ON most of the time, the engine is running rich.

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Fig. Fig. 3: Data flow schematic for closed loop fuel injection control-MFI system