GM Caprice 1990-1993 Repair Guide

Description of System


The TBI unit equipped on the 4.3L, 5.0L (VIN E) and 5.7L engines is made up of 2 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(s) 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's 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.

The Throttle Position Sensor (TPS) is a variable resistor used to convert the degree of throttle plate opening to an electrical signal to the ECM. The ECM uses this signal as a reference point of throttle valve position. In addition, an Idle Air Control (IAC) assembly, mounted in the throttle body s used to control idle speeds. A cone-shaped valve in the IAC assembly is located in an air passage in the throttle body that leads from the point beneath the air cleaner to below the throttle valve. The ECM monitors idle speeds and, depending on engine load, moves the IAC cone in the air passage to increase or decrease air bypassing the throttle valve to the intake manifold for control of idle speeds.

Cranking Mode

During engine crank, for each distributor reference pulse the ECM will deliver an injector pulse (synchronized). The crank air/fuel ratio will be used if the throttle position is less than 80% open. Crank air fuel is determined by the ECM and ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 201°F (94°C).

The lower the coolant temperature, the longer the pulse width (injector on-time) or richer the air/fuel ratio. The higher the coolant temperature, the less pulse width (injector on-time) or the leaner the air/fuel ratio.

Clear Flood Mode

If for some reason the engine should become flooded, provisions have been made to clear this condition. To clear the flood, the driver must depress the accelerator pedal enough to open to wide-open throttle position. The ECM then issues injector pulses at a rate that would be equal to an air/fuel ratio of 20:1. The ECM maintains this injector rate as long as the throttle remains wide open and the engine rpm is below 600. If the throttle position becomes less than 80%, the ECM then would immediately start issuing crank pulses to the injector calculated by the ECM based on the coolant temperature.

Run Mode

There are 2 different run modes. When the engine rpm is above 400, the system goes into open loop operation. In open loop operation, the ECM will ignore the signal from the oxygen (0 2 ) sensor and calculate the injector on-time based upon inputs from the coolant and manifold absolute pressure sensors.

During open loop operation, the ECM analyzes the following items to determine when the system is ready to go to the closed loop mode:

  1. The oxygen sensor varying voltage output. (This is dependent on temperature).
  3. The coolant sensor must be above specified temperature.
  5. A specific amount of time must elapse after starting the engine. These values are stored in the PROM.

When these conditions have been met, the system goes into closed loop operation In closed loop operation, the ECM will modify the pulse width (injector on-time) based upon the signal from the oxygen sensor. The ECM will decrease the on-time if the air/fuel ratio is too rich, and will increase the on-time if the air/fuel ratio is too lean.

The pulse width, thus the amount of enrichment, is determined by manifold pressure change, throttle angle change, and coolant temperature. The higher the manifold pressure and the wider the throttle opening, the wider the pulse width. The acceleration enrichment pulses are delivered nonsynchronized.Any reduction in throttle angle will cancel the enrichment pulses. This way, quick movements of the accelerator will not over-enrich the mixture.

Acceleration Enrichment Mode

When the engine is required to accelerate, the opening of the throttle valve(s) causes a rapid increase in Manifold Absolute Pressure (MAP). This rapid increase in the manifold pressure causes fuel to condense on the manifold walls. The ECM senses this increase in throttle angle and MAP, and supplies additional fuel for a short period of time. This prevents the engine from stumbling due to too lean a mixture.

Deceleration Leanout Mode

Upon deceleration, a leaner fuel mixture is required to reduce emission of hydrocarbons (HC) and carbon monoxide (CO). To adjust the injection on-time, the ECM uses the decrease in manifold pressure and the decrease in throttle position to calculate a decrease in pulse width. To maintain an idle fuel ratio of 14.7:1, fuel output is momentarily reduced. This is done because of the fuel remaining in the intake manifold during deceleration.

Deceleration Fuel Cut-Off Mode

The purpose of deceleration fuel cut-off is to remove fuel from the engine during extreme deceleration conditions. Deceleration fuel cut-off is based on values of manifold pressure, throttle position, and engine rpm stored in the calibration PROM. Deceleration fuel cut-off overrides the deceleration enleanment mode.

Battery Voltage Correction Mode

The purpose of battery voltage correction is to compensate for variations in battery voltage to fuel pump and injector response. The ECM modifies the pulse width by a correction factor in the PROM. When battery voltage decreases, pulse width increases.

Battery voltage correction takes place in all operating modes. When battery voltage is low, the spark delivered by the distributor may be low. To correct this low battery voltage problem, the ECM can do any or all of the following:

  1. Increase injector pulse width (increase fuel)
  3. Increase idle rpm
  5. Increase ignition dwell time

Fuel Cut-Off Mode

When the ignition is OFF , the ECM will not energize the injector. Fuel will also be cut off if the ECM does not receive a reference pulse from the distributor. To prevent dieseling, fuel delivery is completely stopped as soon as the engine is stopped. The ECM will not allow any fuel supply until it receives distributor reference pulses which prevents flooding.

Backup Mode

When in this mode, the ECM is operating on the fuel backup logic calibrated by the CalPak. The CalPak is used to control the fuel delivery if the ECM fails. This mode verifies that the backup feature is working properly. The parameters that can be read on a scan tool in this mode are not much use for service.

Highway Mode

When driven at highway speeds the system may enter highway or semi-closed loop mode. This improves fuel economy by leaning out fuel mixture slightly. The ECM must see correct engine temperature, ignition timing, canister activity and a constant vehicle speed before if will enter this mode. The system will switch back to closed loop periodically to check all system functions.

A scan tool scan determine highway mode by looking at the integrator/block learn values and oxygen sensor voltage. Integrator and block learn will show very little change and the oxygen sensor voltage is be less than 100 millivolts.

ALCL/ALDL Connector

The Assembly Line Communication Link (ALCL) or Assembly Line Diagnostic Link (ALDL) is a diagnostic connector located in the passenger compartment. It has terminals which are used in the assembly plant to check that the engine is operating properly before it leaves the plant. This connector is a very useful tool in diagnosing EFI engines. Important information from the ECM is available at this terminal and can be read with one of the many popular scanner tools.