GM Blazer/Jimmy/Typhoon/Bravada 1983-1993 Repair Guide

General Information


See Figures 1 and 2

Multi-port fuel injection first became available in the S/T series utility with the special edition Typhoon for the 1992 model year. The Typhoon was equipped with a 4.3L MFI-Turbo engine. MFI was likely chosen for its combination of efficiency and power. The 4.3L MFI-Turbo offered performance which could not be expected in the S/T series previously.

The MFI system functions with electronic engine control like most fuel injection systems. A throttle body is used to meter intake air, but unlike TBI systems, the fuel is delivered further downstream of the air flow. The defining feature of an MFI system is that a separate fuel injector is used for each cylinder. A fuel rail and injector assembly is mounted to the intake manifold. Fuel is delivered (through the fuel rail assembly) to each of the injectors.

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Fig. Fig. 1: Common fuel supply system schematic

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Fig. Fig. 2: Fuel injection circuit - 4.3L (VIN Z) engine

As with other fuel injection systems, all injection and ignition functions are regulated by the computer control module. The module 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 equipped) 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 injectors 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 through the fuel pump relay and oil pressure switch. The fuel pressure regulator (mounted to the end of the fuel rail) maintains a constant system pressure by metering flow to the fuel return line. A spring diaphragm is capable of restricting flow until sufficient pressure is achieved, then preventing over-pressurization by allowing increased flow in the return line, as necessary. When the fuel injectors are energized by the control module, the ball valve will open to allow pressurized fuel through the nozzle. Fuel is sprayed in a conical pattern towards the intake valve.

Other system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position (TP) sensor, Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, a Manifold Absolute Pressure (MAP) sensor and an oxygen sensor.

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 regulate idle speed, even when the engine is cold or when there is a high engine load at idle.

The computer module used on all MFI 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 and TP sensor in order to determine the best air/fuel ratio for starting. 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 opened all the way and engine rpm is less than 600, the computer module will pulse the fuel injector at an air/fuel ratio of 20:1 while the engine is turning over (in order to clear the engine of excess fuel). If throttle position is reduced below 80 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 3

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:

  1. Oxygen sensor output voltage is varied, indicating that the sensor has warmed up to operating temperature.
  3. The ECT shows an engine coolant temperature above a specified level.
  5. The engine has been operating for a programmed amount of time.

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Fig. Fig. 3: Closed loop operational schematic

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 required and stop the flow by temporarily shutting off the injectors.

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 alternator output and 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 module will enter the Field Service Mode. If the engine is running in Open Loop Mode, the CHECK ENGINE or SERVICE ENGINE SOON (as applicable) 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 Closed 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:

  1. New trouble codes cannot be stored in computer memory.
  3. 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.