Fuel Injection System - Tbi Chevrolet Cab & Chassis C20

COMPONENT TESTING

Note. Specific component testing information not available. Refer to any tests performed during REMOVAL & INSTALLATION or OVERHAUL procedures

APPLICATION

ROCHESTER THROTTLE BODY NO.

IDENTIFICATION

The throttle body injection (TBI) identification number is stamped on TBI mounting flange (throttle lever side). (Scheme 61) Alphabetical code letters are stamped on the throttle body at external tube locations to identify vacuum hose connections.

Throttle Body Identification Location. Scheme 61

DESCRIPTION

The single unit electronic fuel injection (EFI) system consists of 7 major sub-assemblies: Fuel supply system, throttle body injector (TBI) assembly, Idle Air Control System (IAC), Electronic Control Module (ECM), Electronic Spark Timing (EST), data sensors and emission controls. Fuel is supplied to the engine through an electronically pulsed (timed) injector valve located in the throttle body unit on top of the intake manifold. The ECM controls the amount of fuel metered through the injector valve based upon engine demand and efficiency information. The ECM is a digital electronic computer which receives and computes signals from various data sensors.

Note. Primary sub-systems which affect fuel system operation will be covered in this article: Fuel supply system, TBI assembly, IAC system, ECM and data sensors.

FUEL SUPPLY SYSTEM

An electric fuel pump (located inside fuel tank as an integral part of the fuel gauge sending unit) supplies fuel under pressure to the throttle body assembly. A fuel pump relay controls fuel pump operation. When the ignition switch is turned on, the fuel pump relay activates the fuel pump for 1 1/2 seconds to prime the injector. If the ECM does not receive reference pulses (engine cranking) from the distributor after this period, the ECM deactivates the fuel pump relay. The fuel pump relay will be activated again when the ECM receives distributor reference pulses.

As a back-up system to the fuel pump relay, the fuel pump can also be activated by oil pressure sending unit. The sending unit has 2 internal circuits. One circuit operates the oil pressure indicator in the instrument panel. The second circuit is normally an open switch which closes when the oil pressure reaches about 4 psi (.3 kg/cm 2 ). If fuel pump relay fails, oil pressure sending unit will close, and supply voltage to fuel pump.

THROTTLE BODY INJECTOR ASSEMBLY

The throttle body injector (TBI) assembly is composed of 2 castings: a throttle body with a valve to control air flow, and a fuel body with an integral pressure regulator and fuel injector. The throttle body casting may contain ports to generate vacuum signals for EGR valve, MAP sensor and canister purge system.

The pressure regulator is a diaphragm-operated relief valve with injector pressure acting on one side of the valve and air cleaner pressure acting on the other side of the valve. The pressure regulator maintains a constant pressure drop of about 10 psi (.7 kg/cm 2 ) across the injector, throughout all engine operating conditions. (Scheme 62)

Sectional View of Throttle Body Assembly. Scheme 62

The fuel injector is a solenoid-operated device controlled by the ECM. Fuel is supplied at the lower end of the injector by the fuel supply system. The ECM activates the solenoid which lifts a normally closed ball valve off its seat. Fuel under pressure is injected in a conical spray pattern at the walls of the throttle bore, above the throttle valve. Excess fuel passes through the pressure regulator and is returned to the fuel tank.

During engine cranking, the fuel injector is activated (pulsed) once for each distributor reference pulse received by the ECM. This is referred to as the synchronized mode. In the non-synchronized mode, the injector is pulsed once every 6.25-12.5 milliseconds, depending upon engine calibration and operating conditions. In this mode, the pulse is totally independent of distributor reference pulses.

IDLE AIR CONTROL (IAC) SYSTEM

The IAC system consists of an electrically controlled motor (actuator) which positions the IAC valve in the air by-pass channel around the throttle valve. The IAC valve is a part of the throttle body casting. The ECM calculates the desired position of the IAC valve based upon battery voltage, coolant temperature, engine load and engine speed to control idle speed while preventing stalls due to engine load changes.

If engine speed is lower than desired, the ECM activates the IAC motor to retract the IAC valve. When the IAC valve is retracted, more air is diverted around the throttle valve to increase engine speed. If engine speed is higher than desired, the ECM activates the IAC motor to extend the IAC valve. When the IAC valve is extended, less air is diverted around the throttle valve, decreasing engine speed. If engine speed falls below a preset level and the throttle valve is closed, the ECM senses a near stall condition. To prevent stalling, the ECM will calculate an IAC valve position based upon barometric pressure.

ELECTRONIC CONTROL MODULE

The electronic control module (ECM) is located in the passenger compartment and is the "brain" of both the EFI and Computer Command Control systems. Locations vary, but the ECM is generally located under the instrument panel behind the glove compartment or behind the passenger footwell kick panel. Information from all data sensors is received and processed by the ECM to produce the proper pulse duration ("on" time) for the injector, correct idle speed and proper spark timing. The ECM performs calculations to control the following EFI operating conditions: Engine start, engine running, fuel enrichment during acceleration, lean fuel mixture during deceleration, fuel cut-off and battery voltage correction.

During engine starts, the ECM delivers an injector pulse for each distributor reference pulse received (synchronized mode). The injector pulse width is based upon coolant temperature and throttle position. The air/fuel ratio is determined by the ECM when the throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 220°F (104°C). The lower the coolant temperature, the longer the injector pulse width (richer air/fuel mixture ratio). The higher the coolant temperature, the shorter the injector pulse width (leaner air/fuel ratio).

If the engine is flooded, the driver must depress the accelerator pedal enough to set the wide open throttle position. At this position, the ECM calculates injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio will be maintained as long as the throttle remains in the wide open position and engine speed is below 600 RPM. If the throttle position becomes less than 80 percent open and/or the engine speed exceeds 600 RPM, the ECM changes the injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).

When the engine is running above 600 RPM, the ECM operates in the open loop mode. In open loop, the ECM calculates injector pulse width based upon coolant temperature and manifold absolute pressure (MAP). The engine will remain in open loop operation until the oxygen sensor reaches operating temperature, the coolant temperature reaches a preset temperature, and a specific period of time has elapsed after the engine starts. When all these conditions are met, the ECM operates in the closed loop mode. In closed loop, the ECM controls injector pulse width based upon oxygen sensor signals to maintain the air/fuel mixture ratio close to 14.7:1.

Fuel enrichment during acceleration is provided by the ECM. Sudden opening of the throttle valve causes a rapid increase in MAP. Pulse width is directly related to MAP, throttle position and coolant temperature. The higher the MAP and the wider the throttle angle, the wider the pulse width (richer mixture). During enrichment, the injector pulses are not in proportion to distributor reference signals (non-synchronized). Any reduction in throttle angle cancels fuel enrichment.

During normal deceleration, the air/fuel mixture must be leaner. The ECM calculates the injector pulse width in a manner similar to that used for fuel enrichment, and fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut-off completely to remove fuel from the engine. This deceleration fuel cut-off overrides the normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.

Battery voltage corrections by the ECM are performed during all operating modes of the EFI system. As battery voltage decreases, the ECM increases the injector pulse width with a correction factor stored in the ECM's memory.

The ECM used on EFI vehicles has a "learning" capacity. If the battery is disconnected, the "learning" process must begin all over again. During this period, a change may be noted in vehicle performance. To "teach" the vehicle, ensure the vehicle is at normal operating temperature. The vehicle should then be driven the vehicle at part throttle, moderate acceleration and idle until performance returns.

DATA SENSORS

Each sensor furnishes an electrical signal to the ECM, modifying injector pulse width to conform to engine operating conditions. These sensors are as follows

Coolant Temperature Sensor (CTS)

The CTS is located in the thermostat housing. It is a variable resistor (thermistor) type sensor, and transmits an electrical signal to the ECM proportionate to engine temperature. Low coolant temperature produces high resistance while high coolant temperature produces low resistance.

The ECM supplies a 5-volt signal to the CTS and measures the voltage that returns. By measuring the voltage drop between the 2 readings, the ECM is informed of engine coolant temperature. Coolant temperature is used for fuel management, idle air control, spark timing, EGR operation, canister purge operation and other engine operating functions.

Oxygen Sensor

The oxygen sensor used in the EFI system is a closed-end Zirconia sensor placed in the exhaust gas stream. The sensor is constructed in such a way that the exhaust gases pass by the bottom of the sensor and atmospheric air is admitted at the top of the sensor. The Zirconia produces an electrical voltage when exposed to oxygen, similar to a small battery. By comparing the amount of oxygen present in the exhaust gases to the amount of oxygen in the atmosphere, the sensor produces a signal which is proportional to the oxygen concentration in the exhaust gases.

As the oxygen content of the exhaust gases increases relative to the surrounding atmosphere, a lean fuel mixture is indicated by a low voltage output. As the oxygen content decreases, a rich fuel mixture is indicated by a higher voltage output. The ECM interprets the electrical signal and adjusts the injector pulse width to maintain the air/fuel ratio close to 14.7 to 1.

Note. No attempt should be made to measure oxygen sensor voltage output. Current drain of conventional voltmeter could permanently damage sensor, shift sensor calibration and/or render sensor unusable. DO NOT connect jumper wire, test leads or other electrical connectors to sensor.

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor is mounted on the right side of the engine compartment. This sensor is a variable resistance type which 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 (atmospheric air) and manifold pressure (vacuum). A closed throttle condition (engine coast down) would produce a low MAP reading while a wide open throttle condition (engine acceleration) would produce a high MAP reading. The high value is produced because the pressure inside the intake manifold (vacuum) is the same as the pressure outside the manifold (atmospheric air).

The ECM supplies a 5-volt reference signal to the MAP sensor. As MAP changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage (similar to the CTS), the ECM is informed of intake manifold pressure. A higher pressure (high voltage) requires more fuel, while a lower pressure (low voltage) requires less fuel.

Vehicle Speed Sensor (VSS)

This sensor is mounted behind the speedometer in the instrument cluster. It provides the ECM with pulses to determine vehicle speed. This information is used by the ECM to control the IAC motor.

Note. The vehicle should not be driven without the vehicle speed sensor installed.

Throttle Position Sensor (TPS)

The TPS is mounted on the side of the throttle body and is connected to the throttle shaft. As the throttle valve angle changes (accelerator pedal moved), the resistance of the sensor also changes. The ECM supplies a 5-volt reference signal to the TPS. A closed throttle condition produces high resistance at the sensor and the output signal to the ECM will be low (about .5 volts). A wide open throttle condition produces low resistance at the sensor. The output signal to the ECM will be high (about 5 volts).

By monitoring the output voltage of the TPS and comparing that value to the reference signal, the ECM can calculate fuel requirements based upon throttle valve angle (driver demand).

Engine Speed Sensor

The engine speed signal comes from the Hall Effect Unit mounted above the distributor.Pulses from the distributor are sent to the ECM where the time between these pulses is used to calculate the engine speed. The ECM adds spark advance modifications to the signal and sends the signal back to the distributor.

Note. For information on other sensors that are used by the ECM to control engine performance and other systems, refer to GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROL section.

Component Location. Scheme 63

PRELIMINARY CHECKS

The following systems and components must be in good condition and operating properly before beginning diagnosis of the fuel injection system

1. All support systems and wiring.
2. Battery connections and specific gravity.
3. Ignition system.
4. Compression pressure.
5. Fuel supply system pressure and flow.
6. All electrical connections and terminals.
7. Vacuum line, fuel hose and pipe connections.

Note. Trouble shooting and diagnosis of fuel system should begin with determining fuel system pressure. Before performing any test on the fuel system, pressure must be released from the system.

HESITATES, SLUGGISH, SAGS OR POOR MILEAGE

1. Visually check MAP sensor hose for leaks or restrictions (water in hose). Replace hose if required. Check TPS for sticking or binding and repair or replace if required. Make sure fuel pressure is a steady 9-13 psi (.6-.9 kg/cm 2 ) at all operating ranges. If pressure is incorrect, go to FUEL SYSTEM DIAGNOSIS chart. Ensure base engine timing is correct.
2. With fuel injector electrical connector disconnected, check for fuel leakage from injector while cranking. If leakage occurs, replace injector. Check fuel injector fuel filter for blockage and replace if necessary. Check for an open in the HEI ground circuit and repair as required.
3. Check the operation of the A/C compressor control, torque converter clutch (TCC) and cooling fan control circuit systems, and repair as required.

CUTS OUT OR STALLS

1. Check for intermittent open or short to ground in the following circuits: 5-volt reference (416), HEI reference (430), fuel pump circuit (120), injector drive circuits (467 and 468), IAC drive circuits (441, 442, 443 or 444). (Scheme 64)
2. Check for restricted fuel filter. Make sure fuel pressure is 9-13 psi (.6-.9 kg/cm 2 ) at all operating ranges. Inspect fuel injector "O" rings for damage. Ensure steel back-up washer is located beneath large "O" ring of fuel injector assembly.

SURGE

Check for intermittent open or short to ground in the following circuits: transmission converter clutch (420 and 422), HEI by-pass (424), EST (423). (Scheme 64) See GENERAL MOTORS COMPUTER COMMAND CONTROL SYSTEM article in COMPUTERIZED ENGINE CONTROLS section.

HARD STARTING (HOT OR COLD)

1. Test for high resistance in coolant temperature sensor circuit. Visually check TPS for sticking or binding. Ensure fuel pressure is 9-13 psi (.6-.9 kg/cm 2 ) at all operating ranges.
2. Fuel pressure leakdown after ignition is turned off should be gradual. An instant drop in pressure indicates a leaking in-tank fuel pump coupling, hose or check valve.
3. Check fuel pump relay. Disconnect oil pressure switch. If engine cranks but will not start, perform fuel system diagnosis (to point where fuel pump fuse proves okay).
4. Check injector. With injector harness connector disconnected, check for fuel leakage while cranking. Check cranking circuit. See GENERAL MOTORS COMPUTER COMMAND CONTROL SYSTEM article in COMPUTERIZED ENGINE CONTROLS section.

FUEL SYSTEM PRESSURE TEST

1. Remove "FUEL PUMP" fuse from fuse block in passenger compartment. Crank engine. Engine will start and run until fuel supply remaining in fuel lines is used. Engage starter again for about 3 seconds to make sure all fuel is out of lines. Turn ignition off and replace fuse.
2. Remove air cleaner and plug air cleaner (THERMAC) vacuum port on throttle body. Remove steel fuel line between throttle body and fuel filter. When removing fuel line, always use 2 wrenches to prevent damage. Install a fuel pressure gauge (J-29658) between throttle body and fuel filter.
3. Start vehicle and observe fuel pressure reading. Fuel pressure should be 9-13 psi (.6-.9 kg/cm 2 ). If not, see FUEL SYSTEM DIAGNOSIS chart. Go to INJECTOR SYSTEM DIAGNOSIS chart if pressure is correct.
4. Depressurize fuel system as described in step 1. Remove fuel pressure gauge and reinstall steel line between filter and throttle body. Start vehicle and watch for leaks. Remove plug from throttle body thermal vacuum port and reinstall air cleaner.

ELECTRONIC CONTROL MODULE (ECM)

Note. Location of ECM varies between model application. ECM is located in passenger compartment either behind right kick panel or under instrument panel.

Removal & Installation

Disconnect battery negative cable. Disconnect 2 electrical connectors from ECM. Remove ECM mounting hardware and ECM. To install, reverse removal procedure.

Fuel pump relay is located on left or right side of engine compartment. On left side, relay is mounted in area of brake master cylinder and is closest relay to fender. If relay is mounted on right side, relay is nearest firewall. Remove electrical connector, mounting screws and relay. To install, reverse removal procedure.

Removal and Installation

MAP sensor is located in engine compartment. Location varies between application, but is generally mounted on firewall. Remove vacuum hose, mounting screws and MAP sensor. To install, reverse removal procedure.

Remove instrument cluster and speedometer assembly. Disconnect VSS from speedometer. Disconnect VSS electrical connector and remove VSS. To install, reverse removal procedure.

Disconnect battery negative cable. Disconnect electrical connector and remove CTS. To install, reverse removal procedure.

Note. Handle CTS with care to prevent damage to sensor calibration.

Note. Oxygen sensor may be difficult to remove when engine temperature is below 120°F (49°C). Excessive force may damage threads.

Disconnect battery negative cable. Disconnect electrical connector. DO NOT attempt to remove single wire from oxygen sensor. Carefully back sensor out of exhaust manifold. Handle sensor with care and do not allow dirt or other foreign matter to contact louvered end of sensor. To install, reverse removal procedure.

Note. Prior to reinstalling a serviceable sensor, coat threads with liquid graphite compound containing glass beads (special anti-seize compound).

Fuel Injection Wiring Diagram. Scheme 64