GM Full Size Vans 1987-1997 Repair Guide

General Information


The electronic fuel injection system is a fuel metering system with the amount of fuel delivered by the Throttle Body Injectors (TBI) determined by an electronic signal supplied by the Electronic Control Module (ECM). The ECM monitors various engine and vehicle conditions to calculate the fuel delivery time (pulse width) of the injectors. The fuel pulse may be modified by the ECM to account for special operating conditions, such as cranking, cold starting, altitude, acceleration and deceleration.

The ECM controls the exhaust emissions by modifying fuel delivery to achieve, as near as possible, an air/fuel ratio of 14.7:1. The injector "on" time is determined by various inputs to the ECM. By increasing the injector pulse, more fuel is delivered, enriching the air/fuel ratio. Decreasing the injector pulse, leans the air/fuel ratio.

The basic TBI unit is made up of two major casting assemblies: (1) a throttle body with a valve to control airflow and (2) a fuel body assembly with an integral pressure regulator and fuel injector to supply the required fuel. An electronically operated device to control the idle speed and a device to provide information regarding throttle valve position are included as part of the TBI unit.

The fuel injector is a solenoid-operated device controlled by the ECM. The incoming fuel is directed to the lower end of the injector assembly which has a fine screen filter surrounding the injector inlet. The ECM actuates the solenoid, which lifts a normally closed ball valve off a seat. The fuel under pressure is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve. The excess fuel passes through a pressure regulator before being returned to the vehicle fuel tank.

The pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and air cleaner pressure on the other. The function of the regulator is to maintain a constant pressure drop across the injector throughout the operating load and speed range of the engine.

The throttle body portion of the TBI may contain ports located at, above, or below the throttle valve. These ports generate the vacuum signals for the EGR valve, MAP sensor and the canister purge system.


Starting Mode

When the ignition switch is first turned ON , the fuel pump relay is energized by the module for 2 seconds in order to build system pressure. In the start mode, the computer module checks the ECT, TP sensor and crank signal in order to determine the best air/fuel ratio for starting. The modules on later model vehicles may also use the IAT or MAT (as equipped) and the MAP sensor. Ratios could range from 1.5:1 at approximately -33°F (-36°C), to 14.7:1 at 201°F (94°C).

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 600, the computer module will pulse the fuel injector at an air/fuel ratio of 16.5:1 while the engine is turning over in order to clear the engine of excess fuel. If throttle position is reduced below 65 percent, the module will return to the start mode.

Open Loop Mode

When the engine first starts and engine speed rises above 400 rpm, the computer module operates in the Open Loop mode until specific parameters are met. In Open Loop mode, the fuel requirements are calculated based on information from the MAP and ECT sensors. The oxygen sensor signal is ignored during initial engine operation because it needs time to warm up.

Closed Loop Mode

See Figure 1

When the correct parameters are met, the computer module will use O 2 sensor output and adjust the air/fuel mixture accordingly in order to maintain a narrow band of exhaust gas oxygen concentration. When the module 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 computer module will shift into this Closed Loop mode when:

Oxygen sensor output voltage is varied, indicating that the sensor has warmed up to operating temperature

The ECT shows an engine coolant temperature above a specified level.
The engine has been operating for a programmed amount of time.

Click image to see an enlarged view

Fig. Fig. 1: Computer module fuel control schematic for common closed loop engine operation (same for PCMs and VCMs)

Acceleration Mode

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

Deceleration Mode

As the throttle closes and the manifold pressure decreases, fuel flow is reduced by the module. If both conditions remain for a specific number of engine revolutions indicating a very fast deceleration, the module may decide fuel flow is not needed and stop the flow by temporarily shutting off the injectors.

Highway Fuel Mode (Semi-Closed Loop)

On some vehicles, the computer control module is programmed to enter a special highway mode to improve fuel economy. If the module senses the correct ECT, ignition control, canister purge activity and a constant engine speed, it will enter highway mode. During this operation, there will be very little adjustment of the long and short term fuel trims, also, the oxygen sensor values will usually read below 100 millivolts.

Decel En-Leanment Mode

On some vehicles, the computer control module is programmed to further reduce emissions by leaning the fuel spray on deceleration. The module does this when a high MAP vacuum (low voltage or pressure) is sensed, BUT it should be noted that the module may do this when the vehicle is not moving. This mode of operation may be misdiagnosed as a lean condition. When diagnosing the control system using a scan tool with the transmission in Park, the oxygen sensor signal low (usually below 100 mV), and both fuel trim numbers around 128 counts, lower the engine speed to 1000 rpm. If the sensor and long term trim numbers respond normally, it is possible that the system was fooled into decel en-leanment operation. If the oxygen sensor and long term numbers do not respond at the lower rpm, there are other problems with the vehicle.

Battery Low Mode

If the computer module 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 alternate output and, in some cases, ignition dwell time to allow for proper engine operation.

Field Service Mode

When the diagnostic terminal of the test connector is grounded with the engine running, the computer control module will enter the Field Service Mode. If the engine is running in Open Loop Mode, the CHECK ENGINE or SERVICE ENGINE SOON Malfunction Indicator Lamp (MIL) will flash quickly, about 2 1 / 2 times per second. When the engine is in Closed Loop Mode, the MIL will flash only about once per second. If the light stays OFF most of the time in Close Loop, the engine is running lean. If the light is ON most of the time, the engine is running rich.

While the engine continues to operate in Field Service Mode certain conditions will apply:

The distributor operates with a fixed spark advance (some early model vehicles).
New trouble codes cannot be stored in computer memory.
The closed loop timer is bypassed.

For more information concerning the computer control module, self-diagnosis systems and other electronic engine controls, please refer to Emission Controls of this guide.