Fuel Injection System - Bosch Afc Mazda MX-6 V

DESCRIPTION

Note. For specifications on Throttle Position Sensor (TPS), idle speed and mixture, and fuel pump output volume, see appropriate article in TUNE-UP section.

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. For computer control information, see the appropriate article in 1988 COMPUTERIZED ENGINE CONTROLS section.

The Bosch Airflow Controlled (AFC) fuel injection system is an electronically controlled system operated by incoming airflow. Some vehicles are equipped with a potentiometer to measure incoming air flow, while other vehicles use a hot wire type airflow sensor.

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, cold start injector, 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. Not all models use all components.

ELECTRIC FUEL PUMP(S)

Fuel under pressure from electric fuel pump flows through a fuel damper, fuel filter, injector fuel rail and fuel pressure regulator. Fuel pump(s) may be located on frame rail, in fuel tank or both. Electrical power for fuel pump operation during cranking mode is provided from starter relay via the fuel pump relay (if equipped) and ECU.

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 (both sides on Subaru 1.8L) from the injector fuel rail. The spring chamber is connected to intake manifold vacuum.

At idle, intake manifold vacuum is high. The diaphragm is pulled down by intake manifold vacuum. Any excessive fuel is returned to the fuel tank. As the throttle is depressed, intake manifold vacuum decreases. The regulator spring overcomes manifold vacuum increasing fuel pressure.

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.

ELECTRONIC CONTROL UNIT (ECU)

All components of the control system are electrically connected to the ECU. 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 "on" 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.

Hot Wire Type

The airflow meter continually measures temperature, amount, density, and speed of air entering engine intake system. The meter consists of a platinum wire filament located within intake air stream.

The wire filament is kept at a constant temperature above that of air entering engine regardless of composition of air entering engine. The airflow meter sends a temperature related signal to be processed by the ECU. (Scheme 84)

Scheme 84

Potentiometer Type

This airflow meter uses a movable vane connected to a potent- iometer. As air entering the engine moves the vane, the potentiometer is moved informing the ECU on the amount of air entering the engine. Some potentiometer airflow meters use an air temperature sensor located inside the airflow meter air passage. (Scheme 85)

Scheme 85

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 POSITION SENSOR (TPS)

A contact-type TPS is installed on the throttle chamber. 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 86) Some models send a specific signal to ECU, depending on throttle angle. The open contacts prevent loss of power during sudden acceleration/deceleration by signaling ECU of the required fuel enrichment.

Scheme 86

COOLANT TEMPERATURE SENSOR

This sensor provides ECU with engine temperature information relating to warm-up enrichment operation. Some models use a dual-sensor element which also signals the ignition computer (if equipped).

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 AAV supplies additional air into the intake system which increases engine RPM during a cold start.

The AAV 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 87)

Scheme 87

COLD START INJECTOR

Most models use a cold start injector which delivers additional fuel, and a start injector time switch which controls operation of the cold start injector. 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.

All 929 models use 2 intake air temperature sensors. One sensor is located inside the airflow meter, the other is located near the intake runner on the cylinder head. The sensor located inside airflow meter detects air temperature ENTERING engine, while engine air temperature sensor measures air temperature INSIDE engine.

By-Pass Air Control (BAC) Valve

The BAC valve contains an air valve and Idle Speed Control (ISC) valve. Engine coolant is directed around the air valve warming up the thermo wax. When engine coolant temperature is less than 122°F (50°C), the wax is contracted and the engine idles fast. When coolant temperature is more than 122°F (50°C), the wax is fully expanded, closing valve.

The Idle Speed Control (ISC) valve controls air by-pass amount during cold and warm engine operation. During cold engine operation, the ISC valve opens, raising fast idle speed to a predetermined RPM. The ISC valve also compensates for all engine loads during warm engine operation to maintain a preset idle RPM. The ISC valve is controlled by the ECU.

ADJUSTMENTS

Note. For specifications on Throttle Position Sensor (TPS), idle speed and mixture, and fuel pump output volume, see TUNE-UP article.

TESTING

Note. For testing of fuel system components not covered in this article, refer to appropriate article in 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.

FUEL INJECTORS & RESISTORS

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 resistance or injection signal from the computer.
2. Disconnect electrical connector from each injector. Measure injector resistance. See INJECTOR RESISTANCE SPECIFICATIONS table. If injector is not to specification, replace injector.

INJECTOR RESISTANCE SPECIFICATIONS

FUEL PRESSURE

1. Relieve fuel system pressure. Disconnect negative battery cable. Disconnect fuel filter hose from high pressure side. Connect Multi-Pressure Tester (49-9200-750A) between fuel hose and pulsation damper.
2. Disconnect vacuum line from pressure regulator control solenoid valve. Using a "T" connector, connect tester vacuum hose to solenoid valve. Connect negative battery cable. Connect tester to vehicle battery.
3. Connect a jumper wire across Yellow test connector terminals located near airflow sensor on 929 and near windshield wiper motor on all other models. Turn ignition on. Check connections for leaks. Turn ignition off. Disconnect jumper wire from Yellow test connector.
4. Start engine and warm to operating temperature. Position lever on adapter (in fuel line) so handle is parallel with fuel line. Operate engine at various speeds. On 323, 626 and MX-6, fuel pressure should be 34-40 psi (2.4-2.8 kg/cm 2 ) and on 929, pressure should be 38-45 psi (2.7-3.2 kg/cm 2 ). If fuel pressure is not to specification, check fuel pump, fuel lines and injectors.
5. Turn ignition off. Connect a jumper wire across Yellow test connector terminals. Turn ignition on. Position lever on adapter (in fuel line) so handle is perpendicular to fuel line. Fuel pressure should be 64-85 psi (4.5-6.0 kg/cm 2 ). If fuel pressure is not to specification, check fuel pump volume. If fuel pressure is more than specified, replace fuel pump.
6. Disconnect jumper wire from Yellow test connector. Start engine. Position lever on adapter so handle is parallel to fuel line. With engine idling, fuel pressure should be 24-31 psi (1.7-2.2 kg/cm 2 ) on 323, 27-33 psi (1.9-2.3 kg/cm 2 ) on 626 and MX-6, 31-38 psi (2.2-2.7 kg/cm 2 ) on 929. If fuel pressure is not within specification, check pressure regulator and pressure regulator control solenoid valve vacuum hose.
7. Disconnect fuel pressure regulator vacuum hose. Fuel pressure should be 34-40 psi (2.4-2.8 kg/cm 2 ) on 323, 626 and MX-6 models and 38-45 psi (2.7-3.2 kg/cm 2 ) on 929. If fuel pressure is within specification, replace fuel pressure regulator.

929 (Engine Air Temperature Sensor)

Disconnect engine air temperature sensor connector (located on cylinder head). Using an ohmmeter, measure resistance across sensor ter-minals. See MAZDA 929 ENGINE AIR TEMPERATURE SENSOR RESISTANCE table. For testing of airflow meter air temperature sensor, see AIRFLOW METER in this article.

929 ENGINE AIR TEMPERATURE SENSOR RESISTANCE

626 & MX-6

Disconnect airflow meter connector. Using an ohmmeter, check resistance across meter terminals. (Scheme 88) See MAZDA 626 & MX-6 AIRFLOW METER RESISTANCE table.

Scheme 88

MAZDA 626 & MX-6 AIRFLOW METER RESISTANCE

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.

Disconnect coolant temperature sensor connector. Using an ohm meter, measure resistance between sensor terminals. See COOLANT TEMPERATURE SENSOR RESISTANCE table.

COOLANT TEMPERATURE SENSOR RESISTANCE

COOLANT TEMPERATURE SWITCH

Remove switch from radiator. Using an ohmmeter, check for continuity across switch terminals. Switch should have continuity at temperatures greater than 63°F (17°C).

Wiring Diagram for 323 EGI Non-Turbo. Scheme 89

Wiring Diagram for 323 EGI Turbo. Scheme 90

Wiring Diagram for MX-6 & 626 EGI Non-Turbo. Scheme 91

Wiring Diagram for MX-6 & 626 EGI Turbo. Scheme 92

Wiring Diagram for 929 EGI. Scheme 93