GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

General Information


Some 1994-95 fuel injected vehicles were equipped with a Throttle Body Injection (TBI) system. The system uses a TBI unit mounted centrally on the intake manifold where a carburetor would normally be found on older vehicles. The throttle body assembly is equipped with electronic fuel injectors in order to supply fuel to regulate the air/fuel mixture. All fuel injection and ignition functions are controlled by the computer control module, which is sometimes referred to as the ECM, PCM or VCM, depending on the application. 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 module will attempt to maintain the ideal 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 module will illuminate the CHECK ENGINE or SERVICE ENGINE SOON light (as applicable) and will continue to operate the engine, although it may need to operate in a backup or fail-safe mode.

Fuel is supplied to the injector(s) through an electric fuel pump assembly which is mounted in the vehicle's fuel tank. The module provides a signal to operate the fuel pump though the fuel pump relay and oil pressure switch.

Other system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position (TP) sensor, Manifold Air Temperature (MAT) or Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, a Manifold Absolute Pressure (MAP) sensor and an oxygen sensor. The fuel injectors are solenoid valves that the control module pulse on and off many times per second to promote proper fuel atomization. The pulse width determines how long an injector is ON each cycle and this regulates the amount of fuel supplied to the engine.

The system pressure regulator is normally part of the TBI unit fuel meter cover. The regulator is designed to keep fuel pressure constant at the injector regardless of engine rpm. This is accomplished by controlling the flow in the return line (a calibrated bypass).

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 computer control module can closely control idle speed even when the engine is cold or when there is a high engine load at idle.

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: A cross-sectional view of TBI unit operation-model 220 found on V6 engines

The module used on TBI vehicles has a learning capability which is used to provide corrections for a particular engine's condition. If the battery is disconnected to clear diagnostic codes, or for safety during a repair, the learning process must start all over again. A change may be noted in vehicle performance. In order to "teach'' the vehicle, make sure the vehicle is at normal operating temperature, then drive at part throttle, under moderate acceleration and idle conditions, until normal performance returns.


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 -33ºF (-36ºC), to 14.7:1 at 201ºF (94ºC).

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 2

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. 2: 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 Routine Maintenance of this guide.