Fuel Injection System - Bosch Afc Toyota 4Runner I

DESCRIPTION

Note. The Bosch AFC (L-Jetronic) Fuel Injection system is used on all models; however, variations may exist between model applications. This article covers the Bosch AFC system in general, with manufacturers' differences noted under SPECIAL FEATURES .

The Bosch Airflow Controlled (AFC) fuel injection system is an electronically controlled system operated by incoming airflow. The AFC fuel injection system also contains a feedback system which measures oxygen content of exhaust gases and maintains the air/fuel ratio at about 14.7:1.

The fuel injection system consists of an electric fuel pump, fuel pressure regulator, fuel damper, fuel injectors, Electronic Control Unit (ECU), and airflow meter. In addition, an air temperature sensor, throttle position sensor, coolant temperature sensor, oxygen sensor, catalytic converter, auxiliary air valve, idle speed control valve, throttle body, and electrical relays are used.

Note. Only primary sub-systems which affect fuel system operation will be covered in this article. For further information, see TUNE-UP article in the TUNE-UP section.

ELECTRIC FUEL PUMP

Fuel under pressure from electric fuel pump flows through a fuel damper, fuel filter, injector fuel rail and fuel pressure regulator. Electrical power for fuel pump operation during cranking mode is provided from starter relay via the fuel pump relay and ECU.

At idle, the airflow meter measuring flap opens slightly (about 5°) which closes the fuel pump contacts and provides power to fuel pump after engine has started.

With engine stopped, no airflow is present, measuring flap closes and fuel pump contacts are opened to cut power to fuel pump. This method reduces the risk of fire, in case of a collision.

FUEL PRESSURE REGULATOR

The pressure regulator is a sealed unit which is divided by a diaphragm into 2 chambers (fuel and spring chambers). The fuel chamber receives fuel through the inlet side from the injector fuel rail. The spring chamber is connected to intake manifold vacuum.

At idle, when fuel requirements are low, the diaphragm is pulled down by intake manifold vacuum to allow excessive fuel to return to the fuel tank. As the throttle is depressed and fuel requirements increase, intake manifold vacuum decreases and diaphragm moves up shutting off the fuel return line to maintain constant fuel pressure.

On Toyota 4WD trucks and 4Runner, enrichment is necessary during 4WD operation. The ECU sends a signal to the fuel pressure regulator controlling solenoid to cut off the intake vacuum signal to the pressure regulator and increase fuel pressure. MR2 models use a similar system during air conditioner operation. Automatic transmission equipped models use this system to compensate for the load created by the torque converter.

FUEL INJECTORS

A fuel rail links the fuel pressure regulator with the fuel injectors. Each cylinder is provided with a solenoid-operated injector which sprays fuel toward the back of each intake valve.

Toyota models are energized through the ignition switch. All models are grounded through the ECU. The injectors on Toyota models are linked to resistors to reduce operating voltage to 3 volts and to protect injectors from power surges.

The ECU controls the injectors and the length of time they are open. The "open" time of the injector governs the amount of fuel delivered. The injectors deliver 1/2 the amount of fuel required once every crankshaft revolution.

ELECTRONIC CONTROL UNIT (ECU)

All components of the control system are electrically connected to the ECU. (Scheme 46) The ECU is a pre-programmed computer which receives and interprets data from various sensors to calculate the amount of fuel required by the engine to maintain efficiency with minimum exhaust emissions. The oxygen sensor informs the ECU of oxygen content of exhaust gases and the ECU constantly adjusts the air/fuel ratio by controlling the injector "open" time.

An automatic function of the ECU is to provide fuel enrichment whenever engine is cranked, regardless of engine temperature. This is activated by a direct electrical connection from the starter circuit to the ECU (most models). The ECU is a sealed unit, and no service is required.

Electronic Control Unit (ECU). Scheme 46

AIRFLOW METER

Airflow meter measures the amount of air aspirated and provides a signal to the control unit. The aspirated air applies force to the flap so that its opening angle is balanced with the return torque of the helical spring. The flap shaft is connected to a potentiometer which, corresponding with its opening angle, provides a voltage signal to the control unit.

After engine start-up, a set of contacts are closed to signal fuel pump operation. An air temperature sensor is an integral component of the airflow meter and is unserviceable. (Scheme 47)

Bosch AFC Airflow Meter. Scheme 47

AIR TEMPERATURE SENSOR

The air temperature sensor is an integral component of the airflow meter which converts temperature of incoming air into electrical signals. These signals are received by the ECU and processed to adjust the amount of fuel delivered by the injectors. The air temperature sensor is not serviceable.

THROTTLE BODY

The throttle body controls the intake airflow in response to accelerator pedal movement. Located between the airflow meter and the intake manifold, the throttle body shaft is connected to throttle position sensor.

Most models have a dashpot mounted on the throttle body. The dashpot is used to decrease the intake air gradually when throttle valve is closed.

Note. Many models are equipped with a dashpot to prevent abrupt closing of throttle valve during deceleration and shifting.

THROTTLE POSITION SENSOR

A contact-type throttle position sensor is installed on the throttle chamber of all models. It converts throttle position into electrical signals to inform ECU of throttle position. Signals are sent to ECU when throttle is fully open or at idle. (Scheme 48) The potentiometer prevents loss of power during sudden acceleration/deceleration by signaling the ECU of necessary fuel enrichment requirements.

Contact-Type Throttle Position Sensor. Scheme 48

COOLANT TEMPERATURE SENSOR

This sensor provides ECU with engine temperature information relating to warm-up enrichment operation. During warm-up period after a cold engine start, additional fuel is required to maintain engine performance. As engine temperature increases, the ECU decreases fuel enrichment until engine reaches normal operating temperature.

ELECTRICAL RELAYS

The various relays used with the electronic controls of the AFC injection system control power to injectors, fuel pump, ECU, and cold start system. The electrical relays may consist of one component for all relays or a combination of individual relays.

AUXILIARY AIR VALVE

Most models with Bosch AFC fuel injection use an Auxiliary Air Valve (AAV) to shorten engine warm-up time. The auxiliary air valve supplies additional air into the intake system which increases engine RPM during a cold start.

The auxiliary air valve consists of an electrically heated bi-metallic strip, movable disc, and air by-pass channel. The heater coil on the bi-metallic strip is energized by the fuel pump relay. Control of the valve is based upon engine temperature; the air by-pass channel is open when engine is cold and gradually closes as temperature rises. At predetermined temperatures, air by-pass channel is blocked and additional airflow stops. (Scheme 49)

Auxiliary Air Valve. Scheme 49

COLD START INJECTOR

Most Toyota models use a cold start injector which delivers additional fuel, and a start injector time switch which controls operation of the cold start injector. Some models use an Idle Speed Control (ISC) valve, while others use an Auxiliary Air Valve (AAV) to provide additional air during cold engine starts and warm-up.

The start injector time switch limits cold start injection to 1-12 seconds, depending upon engine coolant temperature. When engine coolant temperature rises above a specified point, bi-metallic contact breaks ground circuit of cold start injector and cold start enrichment is by-passed.

The ISC valve performs the same function as the auxiliary air valve. Mounted on the throttle body, the ISC is a solenoid-actuated valve which provides additional air during cold engine operation. It also maintains idle speed during air conditioner operation. The ISC is controlled directly by the ECU.

The cold start injector provides an extra amount of fuel for initial cold engine start-up. It is controlled by a thermo/time switch which receives a signal from the ECU.

START INJECTOR TIME SWITCH

The start injector time switch is located in the engine cooling system. During cold engine operation, the switch provides a signal to the cold start injector to open, providing additional fuel. As engine coolant temperature rises, the signal to the cold start injector is no longer needed and the system does not function.

Camry, Cressida, MR2, Supra & Van

The ISC performs the same function as the auxiliary air valve on other vehicles. It controls airflow through the throttle body air by-pass passage to maintain a fast idle speed when engine is cold. It also provides by-pass air to maintain idle speed after engine warm-up and during air conditioner operation.

Truck & 4Runner

During 4WD operation it is necessary to enrichen the fuel mixture to compensate for engine load. This is accomplished by the use of a 4WD indicator switch, the ECU and a vacuum switching valve.

When the vehicle is shifted into 4WD mode, the 4WD indicator switch is closed, sending a signal to the ECU. The ECU then signals the vacuum switching valve to cut off the vacuum signal to the fuel pressure regulator. This action increases the amount of fuel available to the fuel injectors.

AUTOMATIC TRANSMISSION ENRICHMENT

When an automatic transmission is shifted into DRIVE, it is necessary to enrichen the fuel mixture to compensate for the engine load produced by the torque converter.

This is accomplished by the use of the neutral safety switch and the ECU. When the transmission is shifted to DRIVE, the neutral safety switch provides a signal to the ECU which in turn makes adjustments to enrichen air/fuel mixture.

FUEL ENRICHMENT SYSTEM (MR2)

Because of the added engine load which occurs during air conditioner operation, it is necessary to enrichen the fuel mixture. The ECU controls a vacuum switching valve, which cuts off the intake manifold vacuum signal to the fuel pressure regulator. This increases fuel pressure at the injectors.

EGR VACUUM SWITCHING VALVE (SUPRA & CRESSIDA)

Controlled by the ECU, the EGR vacuum switching valve opens and closes the vacuum signal to the EGR valve in accordance to engine load.

TESTING

Note. See COMPUTER CONTROL SYSTEM articles in the COMPUTER CONTROLS section.

Do not attempt to test ECU as permanent damage could result. It is possible to check wires for continuity. The ECU should only by judged faulty after compression is checked, ignition system has been tested and found problem-free, and all other fuel injection components have been thoroughly tested (including wiring).

Note. AFC electrical systems can be checked by using Electronic Fuel Injection testers prescribed by the manufacturer. Instructions for use of testers must be followed carefully to prevent damage to system.

1. Connect tachometer to engine. Start engine and run at idle. Remove harness connector from injectors one at a time. Engine idle speed should drop 100-300 RPM as each injector is disconnected. If engine idle speed does not drop, check the wiring connector, injector or injection signal from the computer.

FUEL PRESSURE

1. Disconnect negative battery cable. Unplug cold start injector electrical connector. Place container under fuel rail at union bolt of cold start injector hose.
2. Remove union bolt and drain fuel rail. Remove cold start injector hose from union bolt and install fuel pressure gauge with hose to union bolt. Install gauge and bolt to fuel rail with a gasket on each side of gauge fitting.
3. Connect battery cable and start engine. Disconnect vacuum line from fuel pressure regulator and pinch line closed. Gauge reading should be 33-38 psi (2.3-2.7 kg/cm 2 ). If pressure is too high, replace fuel pressure regulator; if too low, check fuel system for leaks.
4. Connect vacuum line to pressure regulator. Pressure reading should decrease to 28 psi (2.0 kg/cm 2 ) with engine at idle speed. If not, replace fuel pressure regulator. Stop engine. If pressure drops quickly, check fuel pump, pressure regulator, and injectors.

FUEL PUMP CIRCUIT

1. Turn ignition on. Remove cap from fuel pump test connector. Using a jumper wire, jump both terminals of connector. Connector is located below airflow meter on Camry, Cressida, and Supra models. On Celica models it is located next to the throttle body.
2. Fuel pressure should be felt at cold start injector hose and noise of fuel returning to tank should be heard at fuel pressure regulator.
3. Remove jumper wire, install connector cap and turn ignition off. If none of the above conditions were met, check fusible link, engine fuse, fuel injection fuse, circuit opening relay, fuel pump, and all electrical connections.

See the TOYOTA COMPUTER CONTROL SYSTEM and TOYOTA EFI ELECTRONIC CONTROL SYSTEM articles in the COMPUTER CONTROLS section.

AIRFLOW METER POTENTIOMETER

See the TOYOTA COMPUTER CONTROL SYSTEM and TOYOTA EFI ELECTRONIC CONTROL SYSTEM articles in the COMPUTER CONTROLS section.

Ensure engine is cold, then start engine. Pinch rubber hose between air valve and throttle chamber. Engine speed should decrease. After engine reaches operating temperature, pinch hose again. Engine speed should not decrease more than 50 RPM. If valve does not operate as outlined, replace auxiliary air valve.

ISC VALVE

See TOYOTA EFI COMPUTER CONTROL SYSTEM in the COMPUTER CONTROLS section.

Note. To avoid damage to injector, perform this test within the shortest possible time.

1. Disconnect negative battery cable, and cold start injector connector. Remove cold start injector (with fuel supply connected) and place over glass container.
2. Connect battery power to injector terminals. Fuel should spray from injector. If not, replace cold start injector.
3. Disconnect test harness from cold start injector and check for fuel leakage from injector. Maximum leakage is less than one drop of fuel per minute. Resistance between cold start injector terminals should be 3-5 ohms.

Camry, Celica (2S-E), Celica Supra, Corolla, Cressida, MR2, Pickup, 4Runner

Points open above 95°F. Connect ohmmeter between Black and Green terminals with connector disconnected at switch. Resistance should be 20-40 ohms less than 86°F and 40-60 ohms greater than 104°F. When checking between black terminal and ground, resistance should be 20-80 ohms.

Van

Points open above 95°F. Connect ohmmeter between Black and Green terminals with connector disconnected at switch. Resistance should be 20-40 ohms less than 86°F and 40-60 ohms greater than 104°F. When checking between black terminal and ground, resistance should be 20-40 ohms less than 86°F and 20-80 ohms greater than 104°F.

Supra (1986 1/2)

Points open above 95°F. Connect ohmmeter between Black and Green terminals with connector disconnected at switch. Resistance should be 25-50 ohms less than 59°F and 60-85 ohms greater than 86°F. When checking between black terminal and ground, resistance should be 25-85 ohms.

1. Warm engine to normal operating temperature and stop engine. Using a thermometer, measure temperature of coolant. Disconnect negative battery cable and sensor electrical connector.
2. Connect ohmmeter leads to both terminals of sensor. Readings should be as specified for corresponding temperatures. See COOLANT TEMPERATURE SENSOR RESISTANCE chart. If not, replace coolant temperature sensor.

COOLANT TEMPERATURE SENSOR RESISTANCE

REMOVAL & INSTALLATION

Note. For removal and installation procedures, see TOYOTA COMPUTER CONTROL SYSTEMS articles in the COMPUTER CONTROLS section.

Disconnect negative battery cable. Clear area for access to ECU. Disconnect electrical connector lock lever (if used) and carefully remove connector. Remove ECU retaining screws and remove ECU. To install, reverse removal procedure.

Disconnect negative battery cable. Disconnect air ducts and hoses connecting air cleaner and airflow meter. Remove air cleaner cover, if required. Remove airflow meter retaining bolts. Unplug airflow meter electrical connector and remove airflow meter. To install, reverse removal procedure.

1. Disconnect negative battery cable. Unplug throttle position sensor electrical connector. Remove 2 screws securing throttle position sensor to housing. Remove by slowly pulling sensor off throttle shaft.
2. To install, reverse removal procedure. Make sure sensor is aligned on throttle shaft and after replacement, perform throttle position sensor adjustment. See «ADJUSTMENTS»(/toyota/4runner/i-1984-1989/remont/testing-diagnostics/#fuel-injection-system-bosch-afc) .

Disconnect negative battery cable and remove electrical connector from cold start injector. Relieve fuel system pressure and remove fuel supply line from injector. Remove injector retaining bolts and remove injector. To install, reverse removal procedure.

Note. Replacement of auxiliary air valve on Toyota models requires that immediate replacement be available or cooling system be drained to below level of valve.

Disconnect negative battery cable and remove electrical connector from air valve. Drain engine coolant, if required. Remove air hoses and coolant hoses (if equipped). Remove retaining bolts and remove air valve. To install, reverse removal procedure.

Note. Replacement of temperature sensor on Toyota models requires that immediate replacement be available or cooling system be drained below level of sensor.

Disconnect negative battery cable and remove electrical connector from coolant sensor. Drain engine coolant, if required. Remove sensor. To install, reverse removal procedure.

THERMO TIME SWITCH

Note. Thermo time switch removal should be done only when engine is cold. Removal of switch requires having replacement switch ready for immediate installation or cooling system be drained below level of switch.

Disconnect negative battery cable and electrical connector from switch. Drain cooling system as required. Remove switch. To install, reverse removal procedure.

Disconnect negative battery cable and relieve fuel system pressure. Disconnect fuel lines and vacuum line at regulator. Remove pressure regulator. To install, reverse removal procedure.

Removal

1. Release fuel system pressure and disconnect negative battery cable. Drain cooling system. Clear fuel rail and intake air chamber by disconnecting all air hoses, coolant hoses, vacuum hoses, and fuel hoses.
2. Remove EGR valve and pipe and intake air duct. Remove intake air chamber and support bracket. Disconnect fuel injection wiring harness from all connectors near fuel rail and place harness on top of engine.
3. Remove fuel rail retaining bolts. Remove fuel rail, injectors and fuel pressure regulator as an assembly.
4. Separate fuel injectors from fuel rail by pulling injectors. Discard sealing grommet and "O" ring. Remove insulators from injector holes in intake manifold.

Installation

1. To install, reverse removal procedure. Install new insulators in injector holes in intake manifold. Install new grommets and "O" rings on fuel injectors.
2. Coat grommets and "O" rings with gasoline and push injectors onto fuel rail. Coat insulators and injector tips with gasoline prior to installation of injectors. Ensure injectors rotate freely.

IDLE ADJUSTMENTS

See TUNE-UP article in the TUNE-UP section.

See the COMPUTER CONTROL SYSTEM and EFI ELECTRONIC CONTROL SYSTEM articles in the COMPUTER CONTROLS section.

EFI/TCCS Wiring Diagram (Camry). Scheme 50

EFI/TCCS Wiring Diagram (Celica - 2S-E). Scheme 51

EFI/TCCS Wiring Diagram (Celica - 3S-GE). Scheme 52

EFI/TCCS Wiring Diagram (Corolla - RWD). Scheme 53

EFI/TCCS Wiring Diagram (Cressida). Scheme 54

EFI/TCCS Wiring Diagram (MR2). Scheme 55

EFI/TCCS Wiring Diagram (Pickup/4-Runner). Scheme 56

EFI/TCCS Wiring Diagram (1986 Supra). Scheme 57

EFI/TCCS Wiring Diagram (1986 1/2 Supra). Scheme 58

EFI/TCCS Wiring Diagram (Van). Scheme 59

See also:
• SPECIAL FEATURES