GM Storm/Spectrum 1985-1993 Repair Guide

General Information


See Figure 1

The non-carbureted engines covered in this guide are equipped Multi-port Fuel Injection (MFI) that uses 1 injector for each cylinder. All fuel injection functions are controlled by the Engine Control Module (ECM). 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 ECM will attempt to maintain the air/fuel mixture of 14.7:1 in order to optimize catalytic converter operation. If a system failure occurs that is not serious enough to stop the engine, the ECM will illuminate the CHECK ENGINE light and operate the engine in a backup or fail-safe mode.

Fuel is supplied to the engine from a pump mounted in the fuel tank. A separate fuel gauge sending unit is also mounted in the tank. Either component can be replaced separately.

Other related system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position Sensor (TPS), Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, Manifold Absolute Pressure (MAP) sensor, a power steering pressure switch (in some cases) and an oxygen sensor. The fuel injectors are solenoid valves that the ECM 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.

The system pressure regulator is mounted on the end of the fuel rail that feeds the injectors. Intake manifold pressure is supplied to the regulator diaphragm, making system pressure partly dependent on engine load. The pressure regulator compensates for engine load by increasing fuel pressure as vacuum drops.

The idle air control valve is a stepper motor that controls the amount of air allowed to bypass the throttle plate. With this valve the ECM can closely control idle speed even when the engine is cold or when there is a high engine load at idle.

The throttle position sensor is mounted on the side of the throttle body and monitors rotation of the throttle shaft. The sensor is a potentiometer (variable resistor) that creates a voltage drop in the reference signal to correspond with throttle position. The sensor is an auto zeroing type that cannot be adjusted.

Click image to see an enlarged view

Fig. Fig. 1: Multi-Port Fuel Injection (MFI) control schematic


See Figure 2

Starting Mode

When the ignition switch is first turned ON , the fuel pump relay is energized by the ECM for 2 seconds to build system pressure. When the ECM determines that the engine is turning over or cranking, the pump will run continuously. In the start mode, the ECM checks the MAP sensor, TPS and ECT sensor to determine the best air/fuel ratio for starting. Ratios could range from 1.5:1 at -33°F (-36°C), to 14.7:1 at 201°F (94°C) of 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 ECM will close the fuel injectors while the engine is turning over in order to clear the engine of excess fuel. If throttle position is reduced below about 80 percent, the ECM will return to the start mode.

Open Loop Mode

When the engine first starts and engine speed rises above 400 rpm, the ECM operates in the Open Loop mode until specific parameters are met. In Open Loop operation the ECM ignores oxygen sensor signals and fuel requirements are calculated based on information from the MAP sensor, TPS and ECT sensor.

Closed Loop Mode

When the correct parameters are met, the ECM 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 ECM 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.

If Trouble Code 13 is logged in ECM memory, the ECM will continue to operate in Open Loop mode.

The ECM will shift into Closed Loop mode when:

- Oxygen sensor output voltage varies, indicating that the sensor has warmed up to operating temperature.
- Coolant temperature has risen above a minimum.
- A specific amount of time has passed since engine start-up.

Acceleration Mode

If the throttle position and manifold pressure are quickly increased, the ECM will provide extra fuel for smooth acceleration.

Deceleration Mode

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

Battery Low Mode

If the ECM 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 and increase ignition dwell time for better spark control.

Fuel Cut-Off Mode

When the ECM is receiving an engine rpm signal above 6800, the injectors are shut off to prevent engine overspeed. When rpm drops below 6800, the injectors will again be pulsed in normal mode.

Click image to see an enlarged view

Fig. Fig. 2: Fuel system Closed Loop operation schematic


  1. Remove the fuel cap to relieve tank pressure.
  3. Remove the fuel pump relay from the underhood relay center.
  5. Start the engine and run until fuel in the lines is consumed and the engine stalls.
  7. Crank the engine for an additional 30 seconds to assure all pressure is released from the lines. It is easier on the starter if the 30 seconds is broken into shorter 10 second bursts, with a pause between each.
  9. Disconnect the negative battery cable.