GM Full-Size Trucks 1980-1987 Repair Guide

General Information

Print

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, and 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. Pulses are sent to the injector in two different modes: synchronized and non synchronized.

SYNCHRONIZED MODE



In synchronized mode operation, the injector is pulsed once for each distributor reference pulse.

NON-SYNCHRONIZED MODE



In non synchronized mode operation, the injector is pulsed once every 12.5 milliseconds or 6.25 milliseconds depending on calibration. This pulse time is totally independent of distributor reference pulses.

Non-synchronized mode results only under the following conditions.

  1. The fuel pulse width is too small to be delivered accurately by the injector (approximately 1.5 milliseconds).
  2.  
  3. During the delivery of prime pulses (prime pulses charge the intake manifold with fuel during or just prior to engine starting).
  4.  
  5. During acceleration enrichment.
  6.  
  7. During deceleration leanout.
  8.  

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(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 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 is 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.

COMPONENTS AND OPERATION



The Throttle Body Injection (TBI) system provides a means of fuel distribution for controlling exhaust emissions within legislated limits by precisely controlling the air/fuel mixture and under all operating conditions for, as near as possible, complete combustion.

This is accomplished by using an Electronic Control Module (ECM), a small "on-board" microcomputer, that receives electrical inputs from various sensors about engine operating conditions. An oxygen sensor in the main exhaust stream functions to provide "feedback" information to the ECM as to the oxygen content, lean or rich, in the exhaust. The ECM uses this information from the oxygen sensor, and other sensors, to modify fuel delivery to achieve, as near as possible, an ideal air/fuel ratio of 14.7:1. This air/fuel ratio allows the three-way catalytic converter to be more efficient in the conversion process of reducing exhaust emissions while at the same time providing acceptable levels of driveability and fuel economy.

The ECM program electronically signals the fuel injector in the TBI assembly to provide the correct quantity of fuel for a wide range of operating conditions. Several sensors are used to determine existing operating conditions and the ECM then signals the injector to provide the precise amount of fuel required.

The ECM used on EFI vehicles has a "learning" capability. If the battery is disconnected to clear diagnostic codes, or for repair, the "learning" process has to begin all over again. A change may be noted in vehicle performance. To "teach" the vehicle, make sure the vehicle is at operating temperature and drive at part throttle, under moderate acceleration and idle conditions, until performance returns.

With the EFI system the TBI assembly is centrally located on the intake manifold where air and fuel are distributed through a single bore in the throttle body, similar to a carbureted engine. Air for combustion is controlled by a single throttle valve which is connected to the accelerator pedal linkage by a throttle shaft and lever assembly. A special plate is located directly beneath the throttle valve to aid in mixture distribution.

Fuel for combustion is supplied by a single fuel injector, mounted on the TBI assembly, whose metering tip is located directly above the throttle valve. The injector is "pulsed" or "timed" open or closed by an electronic output signal received from the ECM. The ECM receives inputs concerning engine operating conditions from the various sensors (coolant temperature sensor, oxygen sensor, etc.). The ECM, using this information, performs high speed calculations of engine fuel requirements and "pulses" or "times" the injector , open or closed, thereby controlling fuel and air mixtures to achieve, as near as possible, ideal air/fuel mixture ratios.

When the ignition key is turned on, the ECM will initialize (start program running) and energize the fuel pump relay. The fuel pump pressurizes the system to approximately 10 psi. If the ECM does not receive a distributor reference pulse (telling the ECM the engine is turning) within two seconds, the ECM will then de-energize the fuel pump relay, turning off the fuel pump. If a distributor reference pulse is later received, the ECM will turn the fuel pump back on.

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

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 two different run modes. When the engine rpm is above 600, the system goes into open loop operation. In open loop operation, the ECM will ignore the signal from the oxygen (O 2 ) sensor and calculate the injector on-time based upon inputs from the coolant and MAP 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).
  2.  
  3. The coolant sensor must be above a specified temperature.
  4.  
  5. A specific amount of time must elapse after starting the engine. These values are stored in the PROM.
  6.  

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

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 MAP 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

Upon deceleration, a leaner fuel mixture is required to reduce emission of hydrocarbons (H) and carbon monoxide (CO). To adjust the injection on-time, the ECM uses the decrease in MAP 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.

Deceleration Fuel Cut-Off

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.

Battery Voltage Correction

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:



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

Fuel Cut-Off

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.

ELECTRONIC FUEL INJECTION SUBSYSTEMS



Electronic Fuel Injection (EFI) is the name given to the entire fuel injection system. Various subsystems are combined to form the overall system. These subsystems are:

  1. Fuel Supply System.
  2.  
  3. Throttle Body Injector Assembly (TBI).
  4.  
  5. Idle Air Control (IAC).
  6.  
  7. Electronic Control Module (ECM).
  8.  
  9. Data Sensors.
  10.  
  11. Electronic Spark Timing (EST).
  12.  
  13. Emission Controls.
  14.  

Each subsystem is described in the following paragraphs.

Fuel Supply System

Fuel, supplied by an electric fuel pump mounted in the fuel tank, passes through an inline fuel filter to the TBI assembly. To control fuel pump operation, a fuel pump rely is used.

When the ignition switch is turned to the ON position the fuel pump relay activates the electric fuel pump for 1.5-2.0 seconds to prime the injector. If the ECM does not receive reference pulses from the distributor after this time, the ECM signals the relay to turn the fuel pump off. The relay will once again activate the fuel pump when the ECM receives distributor reference pulses.

The oil pressure sender is the backup for the fuel pump relay. The sender has two circuits, one for the instrument cluster light or gauge, the other to activate the fuel pump if the relay fails. If the fuse relay has failed, the sender activates the fuel pump when oil pressure reaches 4 psi. Thus a failed fuel pump relay would cause a longer crank, especially in cold weather. If the fuel pump fails, a no start condition exists.

Throttle Body Injector (TBI) Assembly

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. A device to control idle speed (IAC) and a device to provide information about throttle valve position (TPS) are included as part of the TBI unit.

The throttle body portion of the TBI unit 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 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 turns on 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 the injector pressure on one side, and the air cleaner pressure on the other. The function of the regulator is to maintain constant pressure (approximately 11 psi) to the injector throughout the operating loads and speed ranges of the engine. If the regulator pressure is too low, below 9 psi, it can cause poor performance. Too high a pressure could cause detonation and a strong fuel odor.

Idle Air Control (IAC)

The purpose of the idle air control (IAC) system is to control engine idle speeds while preventing stalls due to changes in engine load. The IAC assembly, mounted on the throttle body, controls bypass air around the throttle plate. By extending or retracting a conical valve, a controlled amount of air can move around the throttle plate. If rpm is too low, more air is diverted around the throttle plate to increase rpm.

During idle, the proper position of the IAC valve is calculated by the ECM based on battery voltage, coolant temperature, engine load, and engine rpm. If the rpm drops below a specified rate, the throttle plate is closed. The ECM will then calculate a new valve position.

Three different designs are used for the IAC conical valve. The first design used is single 35 taper while the second design used is a dual taper. The third design is a blunt valve. Care should be taken to insure use of the correct design when service replacement is required.

The IAC motor has 255 different positions or steps. The zero, or reference position, is the fully extended position at which the pintle is seated in the air bypass seat and no air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate.

The ECM always monitors how many steps it has extended or retracted the pintle from the zero or reference position; thus, it always calculates the exact position of the motor. Once the engine has started and the vehicle has reached approximately 40 mph, the ECM will extend the motor 255 steps from whatever position it is in. This will bottom out the pintle against the seat. The ECM will call this position "0" and thus keep its zero reference updated.

The IAC only affects the engine's idle characteristics. If it is stuck fully open, idle speed is too high (too much air enters the throttle bore) If it is stuck closed, idle speed is too low (not enough air entering). If it is stuck somewhere in the middle, idle may be rough, and the engine won't respond to load changes.

Idle Speed Control

Incorrect diagnosis and/or misunderstanding of the idle speed control systems used on EFI engines may lead to unnecessary replacement of the IAC valve. Engine idle speed is controlled by the ECM which changes the idle speed by moving the IAC valve. The ECM adjusts idle speed in response to fluctuations in engine load (A/C, power steering, electrical loads, etc.) to maintain acceptable idle quality and proper exhaust emission performance.

The following is provided to assist the technician to better understand the system and correctly respond to the following customer concerns:

  1. Rough Idle/Low Idle Speed.
  2.  
  3. High Idle Speed/Warm-up Idle Speed; No "Kickdown".
  4.  

Rough Idle/Low Idle Speed

The ECM will respond to increases in engine load, which would cause a drop in idle speed, by moving the IAC valve to maintain proper idle speed. After the induced load is removed the ECM will return the idle speed to the proper level.

During A/C compressor operation, (MAX, BI-LEVEL, NORM or DEFROST mode) the ECM will increase idle speed in response to an "A/C-ON" signal, thereby compensating for any drop in idle speed due to compressor load. The ECM will also increase the idle speed models in response to high power steering loads.

During periods of especially heavy loads (A/C-ON plus parking maneuvers) significant effects on idle quality may be experienced. Abnormally low idle, rough idle and idle shake may occur if the ECM does not receive the proper signals from the monitored systems.

High Idle Speed/Warm-Up Idle Speed No "Kickdown"

Engine idle speeds as high as 2100 rpm may be experienced during cold starts to quickly raise the catalytic converter to operating temperature for proper exhaust emissions performance. The idle speed attained after a cold start is ECM-controlled and will not drop for 45 seconds regardless of diver attempts to "kickdown".

It is important to recognize the EFI engines have no accelerator pump or choke. Idle speed during warm-up is entirely ECM-controlled and cannot be changed by accelerator "kickdown" or "pumping".

Diagnosis

Abnormally low idle speeds are usually caused by an ECM system-controlled or system-monitored irregularity, while the most common cause for abnormally high idle speed is an induction (intake air) leak. The idle air control valve may occasionally lose its memory function, and it has an ECM-programmed method of "relearning" the correct idle position. This reset, when required, will occur the next time the car exceeds 35 mph. At this time the ECM seats the pintle of the IAC valve in the throttle body to determine a reference point. Then it backs out a fixed distance to maintain proper idle speed.

Electronic Control Module (ECM)

The ECM, located in the passenger compartment, is the control center of the fuel injection system. The ECM constantly monitors the input information, processes this information from various sensors, and generates output commands to the various systems that affect vehicle performance.

The ability of the ECM to recognize and adjust for vehicle variations (engine transmission, vehicle weight, axle ratio, etc.) is provided by a removable calibration unit (PROM) that is programmed to tailor the ECM for the particular vehicle. There is a specific ECM/PROM combination for each specific vehicle, and the combinations are not interchangeable with those of other vehicles.

The ECM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the "CHECK ENGINE" light, and store a code or codes which identify the problem areas to aid the technician in making repairs.

Data Sensors

A variety of sensors provide information to the ECM regarding engine operating characteristics. These sensors and their functions are described below.

ENGINE COOLANT TEMPERATURE SENSOR

The coolant sensor is a thermistor (a resistor which changes value based on temperature) mounted on the engine coolant stream. As the temperature of the engine coolant changes, the resistance of the coolant sensor changes. Low coolant temperature produces a high resistance (100,000 ohms at &-40ºC/&-40ºF), while high temperature causes low resistance (70 ohms at 130ºC/266ºF).

The ECM supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ECM determines the engine coolant temperature. This information is used to control fuel management, IAC, spark timing, EGR, canister purge and other engine operating conditions.

OXYGEN SENSOR

The exhaust oxygen sensor is mounted in the exhaust system where it can monitor the oxygen content of the exhaust gas stream. The oxygen content in the exhaust reacts with the oxygen sensor to produce a voltage output. This voltage ranges from approximately 100 millivolts (high oxygen - lean mixture) to 900 millivolts (low oxygen - rich mixture).

By monitoring the voltage output of the oxygen sensor, the ECM will determine what fuel mixture command to give to the injector (lean mixture-low voltage-rich command, rich mixture-high voltage lean command).

Remember that the oxygen sensor indicates to the ECM what is happening in the exhaust. It does not cause things to happen. It is a type of gauge: high oxygen content = lean mixture; low oxygen content = rich mixture. The ECM adjust fuel to keep the system working.

MAP SENSOR

The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). A closed throttle engine coastdown would produce a relatively low MAP value (approximately 20-35 kPa), while wide-open throttle would produce a high value (100 kPa). This high value is produced when the pressure inside the manifold is the same as outside the manifold, and 100% of outside air (or 100 kPa) is being measured. This MAP output is the opposite of what you would measure on a vacuum gauge. The use of this sensor also allows the ECM to adjust automatically for different altitude.

The ECM sends a 5 volt reference signal to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage the ECM can determine the manifold pressure. A higher pressure, lower vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.

VEHICLE SPEED SENSOR (VSS)

Vehicle should not be driven without a VSS as idle quality may be affected.

The vehicle speed sensor (VSS) is mounted behind the speedometer in the instrument cluster. It provides electrical pulses to the ECM from the speedometer head. The pulses indicate the road speed. The ECM uses this information to operate the IAC, canister purge, and TCC.

Some vehicles equipped with digital instrument clusters use a Permanent Magnet (PM) generator to provide the VSS signal. The PM generator is located in the transmission and replaces the speedometer cable. The signal from the PM generator drives a stepper motor which drives the odometer.

THROTTLE POSITION SENSOR (TPS)

The Throttle Position Sensor (TPS) is connected to the throttle shaft and is controlled by the throttle mechanism. A 5 volt reference signal is sent to the TPS from the ECM. As the throttle valve angle is changed (accelerator pedal moved), the resistance of the TPS also changes. At a closed throttle position, the resistance of the TPS is high, so the output voltage to the ECM will be low (approximately 0.5 volt). As the throttle plate opens, the resistance decreases so that, at wide open throttle, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TPS, the ECM can determine fuel delivery based on throttle valve angle (driver demand). The TPS can either be misadjusted, shorted, open or loose. Misadjustment might result in poor idle or poor wide-open throttle performance. An open TPS signals the ECM that the throttle is always closed, resulting in poor performance. This usually sets a Code 22. A shorted TPS gives the ECM a constant wide-open throttle signal and should set a Code 21. A loose TPS indicates to the ECM that the throttle is moving. This causes intermittent bursts of fuel from the injector and an unstable idle.

PARK/NEUTRAL SWITCH

Vehicle should not be driven with the Park/Neutral switch disconnected as idle quality may be affected in Park or Neutral.

This switch indicates to the ECM when the transmission is in Park or Neutral.

 
label.common.footer.alt.autozoneLogo