Fuel Injection System - LH (16-valve) Saab 900 I

DESCRIPTION

The Bosch LH-Jetronic fuel injection system integrates the fuel injection and idle speed control functions into one system. One electronic control unit (ECU) controls all system functions.

The LH Jetronic fuel injection system operates with moderate fuel pressure, held constant by a line pressure regulator. Fuel is metered into the cylinders by electrically controlled solenoid valves in the injectors. The injectors are installed just ahead of each cylinder intake valve.

Actual duration of injection (2-12 milliseconds), is determined by the ECU. The ECU receives several signals on various driving, engine and outside conditions from a set of sensors and an air mass meter. (Scheme 3)

Cutaway View of Air Mass Meter. Scheme 3

Signals from air mass meter and exhaust gas oxygen sensor provide most of the information used by the electronic control unit to maintain proper air/fuel ratios. An idle speed adjustment screw is located on throttle housing.

ELECTRONIC CONTROL UNIT (ECU) OPERATION

The ECU is a microprocessor based computer that receives and evaluates readings from various sensors. The control unit then produces a signal that is sent to each injector to ensure the correct amount of fuel delivery for all operating conditions. Located in the right kick panel, it also controls the Automatic Idle Control system.

Note. The LH system on the Saab 900S is the same type as the turbo except with a Blue/Red lead wire connecting Terminal No. 24 of the ECU to terminal NO. 8 of the EZK ignition control unit.

FUEL SYSTEM OPERATION

The fuel system consists of a fuel tank, a fuel feed pump, fuel lines, filter, and a line pressure regulator. Fuel is supplied to each injector at a constant pressure. Only fuel injection duration is varied (to meet changing engine requirements).

The fuel feed pump is an electrically operated vane pump. The pump is integral with main fuel pump on Saab models. Fuel is strained through a mesh screen before entering fuel pump. There is a check valve on outlet side of pump to prevent fuel supply from draining back into tank.

The in-tank fuel pump assembly is comprised of a main fuel pump, a pressurized container and a fuel feed pump. (Scheme 4)

Cutaway View of Fuel Pump Assembly. Scheme 4

The fuel feed pump supplies fuel from the tank to a pressurized container. The main, electrically-driven rotary pump draws pressurized fuel from the container and delivers it to the fuel injection manifold. Because the main pump always draws pressurized fuel, air is unable to enter fuel system. The fuel filter is attached to a bracket located underneath the vehicle.

The pump is equipped with a check valve to prevent back flow. It also have an overpressure relief valve which opens if fuel system becomes clogged and pressure rises too high.

INJECTORS OPERATION

The injectors, located in intake manifold, point directly toward intake valve. All injectors are supplied with fuel from a common fuel manifold. The injection pulse is controlled electrically by a needle valve in lower section of injector. The needle valve is operated by an electric solenoid which is switched on and off by ECU.

FUEL PUMP RELAY OPERATION

The fuel pump relay is located behind right side kick panel. The system relay and fuel pump relay are located together on a holder near ECU. This relay energizes fuel feed pump and main fuel pump.

AUTOMATIC IDLING CONTROL SENSOR OPERATION

The automatic idle control (AIC) system is comprised of a valve, solenoid and an electric motor. (Scheme 6) The AIC system is used to provide a more regular idle speed.

This system determines the volume of air that will enter engine intake system. Airflow volume is determined by size of opening in valve. When airflow drops below a specified volume, the ECU signals the AIC system to modify size of valve opening.

AIR MASS METER SENSOR OPERATION

The air mass 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, and a circuit board with a CO adjustment potentiometer.

The wire filament sends a temperature related signal to the circuit board for processing. Once processed, airflow signal is sent to the ECU.

Over a period of time, the filament can become coated with dirt or other contaminants. To prevent this, the filament is automatically heated to a high temperature for about one second, each time engine is shut off.

OXYGEN SENSOR OPERATION

The oxygen sensor checks exhaust gas for presence of unburned oxygen. Unburned oxygen in exhaust gas indicates incomplete combustion due to improper mixture, spark timing or other conditions. On turbo models, sensor is located in bend of turbo outlet. On non-turbo model, sensor is located on top of front section of exhaust pipe.

THROTTLE SWITCH SENSOR OPERATION

The throttle switch consists of built-in contacts which are opened and closed by the throttle cam. The contacts are closed at idle and open at WOT. (Scheme 6) The switch is located on side of the throttle housing.

Cutaway View of AIC System. Scheme 5

COOLANT TEMPERATURE SENSOR OPERATION

The coolant temperature sensor is located in the cylinder head. This sensor provides ECU with an engine temperature signal. If signal from temperature sensor is not reaching ECU, an engine temperature of 86°F (30°C) will be assumed by ECU.

If sensor reads a high engine temperature (with cooling system operating properly), this would indicate combustion temperature that is too high. The control unit would then richen the mixture (to the point allowed by the oxygen sensor and air mass meter signals) to lower combustion and engine temperature.

ENGINE SPEED SENSOR OPERATION

The engine speed signal is generated by an electronic (Hall Effect) ignition distributor. The low voltage ignition primary pulses are monitored at low tension side of ignition coil. Injection timing is regulated by this signal.

Cutaway View of Throttle Switch. Scheme 6

SYSTEM RELAY OPERATION

The system relay is located behind right side kick panel. The system relay and fuel pump relay are located together on a holder near ECU. This relay provides current to the ECU. It is energized during filament burn off after ignition has been switched off.

HOT (SLOW) IDLE RPM & IDLE MIXTURE ADJUSTMENT

See the TUNE-UP - 4-CYL article in the ENGINE PERFORMANCE section.

THROTTLE VALVE ADJUSTMENT

See THROTTLE VALVE TEST (OPEN POSITION) and THROTTLE VALVE TEST (CLOSED POSITION).

FUEL INJECTION BASE LINE VALUE ADJUSTMENT

Note. The following is to only be performed if ECU, air mass meter, or temperature sensor is replaced, or if a major part(s) of engine (cylinder head, engine block, timing chain, etc.) is replaced or rebuilt.

1. Remove sealing plug covering potentiometer screw (located on side of air mass meter). Remove harness connector from air mass meter. Connect ohmmeter between pins No. 3 and 6 of air mass meter. Resistance should be 380 ohms. If resistance is incorrect, adjust potentiometer screw. Reconnect air mass meter connector.
2. Connect Pulse Ratio Meter (85 93 597) and Adaptor Harness (021 80 701) to 3-terminal limp-home test connector (located near A/C evaporator). Start engine and run until cooling fan cycles.
3. If base line value is correct, ratio meter reading will oscillate slowly and evenly between both ends of scale. A pause between ratio meter fluctuations is acceptable. If okay, stop engine, disconnect ratio meter and install sealing plug. If fluctuation is uneven or mostly on high or low side of scale, go to next step.
4. If needle is most often at high side of scale, turn potentiometer screw counterclockwise to adjust. If needle is most often at low side of scale, turn potentiometer screw clockwise to adjust. After adjustment, stop engine, disconnect ratio meter and install sealing plug.

TESTING PRECAUTION

FUEL LINE PRESSURE TEST

1. Disconnect banjo coupling at inlet to fuel injection manifold. Connect a fuel pressure gauge using existing seals. Ensure pressure gauge hangs vertically.
2. Turn ignition off. Remove fuse No. 30 on 900 models. Remove fuse No. 14 from 9000 Turbo. Connect a jumper lead between fuse No. 27, 28, or 29 and slot of fuse No. 30 for 900 models. Connect the jumper between fuse No. 22 and slot of fuse No. 14 on 9000 Turbo. This will provide current to the fuel pump. Turn ignition on.
3. Line pressure should rise to about 36.3 psi (2.57 kg/cm 2 ). If line pressure is too high, go to step 4). If line pressure is too low, got to step 5).
4. Turn ignition off. Disconnect fuel return line from pressure regulator and blow in line. If line is open, pressure regulator is defective and should be replaced. If line is blocked, unblock, or replace with a new return line. Check one-way valve at tank, and retest system.
5. Check fuel level, do not rely on fuel gauge. Check for fuel system leaks. Check fuel pressure regulator. See FUEL PRESSURE REGULATOR test.

Note. If proceeding to FUEL PRESSURE REGULATOR TEST, do not remove fuel pressure gauge.

FUEL PRESSURE REGULATOR TEST

1. If fuel pressure gauge is already attached, go to step 2). If fuel pressure gauge is not attached, disconnect banjo coupling at inlet to fuel injection manifold. Connect fuel pressure gauge at this point, using existing seals. Ensure pressure gauge hangs vertically.
2. Connect Vacuum Gauge (83 93 514), with "Y" connection, to vacuum port of fuel pressure regulator. Hang pressure gauge vertically. Connect cooling system pressure tester to "Y" connection.
3. Turn ignition off. Remove fuse No. 30 on 900 models. Remove fuse No. 14 from 9000 Turbo. Connect a jumper lead between fuse No. 27, 28, or 29 and slot of fuse No. 30 for 900 models. Connect the jumper between fuse No. 22 and slot of fuse No. 14 on 9000 Turbo. This will provide current to the fuel pump. Turn ignition on.
4. Line pressure should rise to about 35.0 psi (2.5 kg/cm 2 ). Using a pressure tester, raise system pressure to various test pressures, and compare line pressure to specification. See APPLIED PRESSURE REGULATOR SPECIFICATIONS table. APPLIED PRESSURE REGULATOR SPECIFICATIONS Applied Pressure: psi (kg/cm 2 ) Line Pressure: psi (kg/cm 2 ) 0 (0) 35.0 (2.5) 2.8 (.2) 37.8 (2.7) 5.6 (.4) 40.6 (2.9) 8.4 (.6) 43.4 (3.1)
5. Remove pressure tester. Install vacuum pump to "Y" connection. Line pressure should be about 35.0 psi (2.5 kg/cm 2 ). Use vacuum pump to create various test vacuums. Compare vacuum readings to specification. See APPLIED VACUUM REGULATOR SPECIFICATIONS table. APPLIED VACUUM REGULATOR SPECIFICATIONS Applied Vacuum: In. Hg Line Pressure: psi (kg/cm 2 ) 0 (0) 35.0 (2.5) -2.8 32.2 (2.3) -5.6 29.4 (2.1) -8.4 26.6 (1.9)
6. Turn ignition on to operate fuel pump. Verify line pressure of 35.0 psi (2.5 kg/cm 2 ). Turn ignition off. Check pressure gauge reading. Pressure should drop 1.4-2.8 psi (.1-.2 kg/cm 2 ), and then hold steady for at least 10 minutes.
7. If pressure drops more than specified, check for leakage in pressure regulator and fuel pump non-return valve.

FUEL PUMP DELIVERY PRESSURE TEST

1. Turn ignition on. Pinch off fuel return line, and check line pressure. When line is plugged, there should be an immediate rise in line pressure. CAUTION: DO NOT allow pressure to rise above 84 psi (6 kg/cm 2 ).
2. If line pressure does not increase, check for the following problems: Less than 11.5 volts across pump terminals Obstructed fuel lines Clogged fuel filter Inoperative pressure reducing pump valve (replace pump if defective) Fuel filter installed backward NOTE: If fuel filter has been installed backward, it must be replaced.
3. After checking fuel pump pressure, turn ignition off. Go to FUEL PUMP DELIVERY FLOW test procedure (do not disconnect pressure gauge).

FUEL PUMP DELIVERY FLOW TEST

1. Disconnect fuel return line at pressure regulator. Connect a section of test hose to return line at pressure regulator. Place other end of test hose in graduated container of about 2 quarts.
2. Activate fuel pump by turning ignition on. Run fuel pump for 30 seconds. Check volume of fuel flow and fuel pressure drop. Minimum allowed volume is .95 quart (.9 l), and maximum allowed fuel pressure drop is 1.4-2.8 psi (.1-.2 kg/cm 2 ). If fuel volume or pressure drop does not meet specification, replace fuel pressure regulator.

SYSTEM CIRCUIT TEST

1. Remove fuel pump fuse. This will be fuse No. 30 on 900 models and fuse No. 14 on 9000 Turbo. Locate 2-pin connector for oxygen sensor preheating circuit (connector is attached to right-hand inner fender). Connect voltmeter leads across connector terminals and set to 20-volt scale. Voltmeter should read 12 volts. CAUTION: Engine may start and run due to residual fuel in system.
2. If voltage reading is incorrect, perform ELECTRONIC CONTROL UNIT test. If voltage reading is correct, go to next step.
3. Remove fuel pump fuse. This will be fuse No. 30 on 900 models and fuse No. 14 on 9000 Turbo. Attach Fuel Pump Switch Harness (83 93 886), or jumper wire, between left connection of fuse No. 30 and right connection of fuse No. 27 on 900 models. Attach Fuel Pump Switch Harness (83 93 836) between left connection of fuse No. 14 and right connection of fuse No. 22 on 9000 Turbo.
4. Turn ignition on. If fuel system is operating properly, fuel flow through lines and injection manifold should be heard. If fuel system is not operating properly, perform FUEL PUMP DELIVERY PRESSURE test. NOTE: When testing injectors, leave connections intact. Test by inserting voltmeter lead through back of connector.
5. Connect voltmeter positive lead to Blue/Red wire of injector No. 1. Connect negative lead to Green/Red wire of injector No. 1. While cranking engine, voltage reading should be 3-5 volts. Repeat test for each injector. If no reading is indicated, go to ELECTRONIC CONTROL UNIT (ECU) test.

ELECTRONIC CONTROL UNIT (ECU) TEST

Note. Ignition must be off when connecting or disconnecting the ECU. Remove wiring harness connector from ECU, prior to testing. Repair any problems as they are found, before proceeding to next step in test procedure. Always probe from rear of connector to avoid enlarging connector pin holes.

Scheme 7

1. Connect negative lead of voltmeter to a good ground near the ECU. Check for battery voltage at pin No. 18 of ECU harness connector with ignition on. (Scheme 7) Battery voltage should be present. If not, check fuse No. 22. (Scheme 7): ECU Harness Connector Pin Locations Locations are shown as viewed from rear of connector.
2. Check ECU ground locations by connecting a test light between pin No. 18 and pins No. 5, 11 and 25 of ECU harness connector. Test light should light at all 3 pins.
3. Connect test light between pin No. 1 of ECU harness connector and vehicle ground. Test light should pulse when engine is cranking. To check power output to ECU, ground ECU harness connector pin No. 21 and check voltage at pin No. 9. Twelve volts should be present and relay should be heard to "pull down".
4. Ground pins No. 21 and 17 at ECU harness connector. Check voltage at Green/Red wire of 3-terminal limp-home test connector (connector is located near evaporator housing). Battery voltage should be present. If voltage is okay, pump and system relays are operating properly. Proceed to TEMPERATURE SENSOR test. If voltage is not okay, go to SYSTEM RELAY TERMINALS test.

SYSTEM RELAY TERMINALS TEST

Note. During normal operation, relay grounds are provided by the ECU. Ground connections are simulated within the following steps.

Scheme 8

1. Remove system/fuel pump relay holder from inside inner fender (near ECU). When holder is viewed looking forward, system relay is on left side of holder. Fuel pump relay is on right side of holder.
2. Using a voltmeter, check for battery voltage between system relay terminals No. 30 and 86. (Scheme 8) Check that there is continuity between system relay terminal No. 85 and ECU harness connector pin No. 21. (Scheme 8): Fuel Pump & System Relay Terminals Do not test terminals of the relay itself.
3. Check for continuity between system relay pin No. 87B and ECU harness connector pin No. 2 (mass meter). Check for continuity between system relay terminal No. 87 and ECU harness connector pin No. 9. Check for continuity between system relay terminal No. 87 and fuel pump relay terminal 86. If all connections are okay, go to FUEL PUMP RELAY TERMINALS test.

FUEL PUMP RELAY TERMINALS TEST

Note. During normal operation, relay grounds are provided by the ECU. Ground connections are simulated within the following steps.

1. Remove system/fuel pump relay holder from inside inner fender (near ECU). System relay is on left side of holder and fuel pump relay is on right side of holder.
2. Check for battery voltage at fuel pump relay terminal No. 30 (voltage should be always present at this point). (Scheme 9) Check for continuity between fuel pump relay terminal No. 85 and ECU harness connector pin No. 17. Check for continuity between fuel pump relay terminal No. 87 and Blue/Red wire of each injector plug wire.
3. Check for continuity between fuel pump relay terminal No. 87 and ECU harness connector pin No. 2 (Blue/Red AIC actuator wire). Check for continuity between fuel pump relay terminal No. 87 and Green wire of oxygen sensor heater.
4. Check for continuity between fuel pump relay terminal No. 87B and Gray/Red wire at 3-terminal limp-home test connector (connector is located near evaporator housing). Check for continuity between fuel pump relay terminal No. 87B and right side contact of fuel pump fuse No. 30 on 900 models. Check for continuity between fuel pump relay terminal No. 87B and right side contact of fuel pump fuse No. 14 on 9000 Turbo.

TEMPERATURE SENSOR TEST

1. Unplug sensor wiring connector at sensor. Set ohmmeter to 20K scale. Connect ohmmeter across sensor terminals. With coolant at 32°F (0°C), resistance should be 5800 ohms. With coolant about 68°F (20°C), resistance should be about 2600 ohms. Set ohmmeter to 2K scale. With coolant at 212°F (100°C), resistance should be 180 ohms. NOTE: Ignition must be off when connecting or disconnecting the ECU. Remove wiring harness connector from ECU prior to testing. Always probe from rear of connector to avoid enlarging connector pin holes.
2. Reconnect sensor wiring connector. Disconnect ECU harness connector from ECU. Repeat temperature sensor test. Readings should be the same.

Note. If temperature sensor is replaced, proceed to FUEL INJECTION BASE LINE adjustment.

OXYGEN SENSOR TEST

1. Remove 2-pin oxygen sensor connector from wiring harness socket. Connect ohmmeter across White leads of oxygen sensor to check resistance of oxygen sensor heating element. Resistance should be about 4 ohms.
2. Connect voltmeter (set on 2 volt scale) between Black lead of sensor and ground. Start engine. As engine temperature begins to rise, sensor will generate a low voltage signal.
3. A lean mixture (caused by a small vacuum leak) will generate a voltage reading close to zero volts. A rich mixture will generate a voltage reading close to one volt. Reconnect sensor leads.

Scheme 9

1. Ensure throttle is able to fully close when dashpot plunger is depressed. Remove harness connector from throttle switch. Ensure throttle is fully closed.
2. Connect ohmmeter across throttle switch terminals No. 1 and 2. (Scheme 9) Resistance should be present when throttle is closed. Move throttle to off-idle position. Resistance should not be present. (Scheme 9): Throttle Switch Terminal Locations
3. If resistance is correct, proceed to THROTTLE CONTROL TEST (OPEN POSITION) test. If resistance is incorrect, loosen throttle switch adjusting screws and rotate switch to correct switch position.

THROTTLE CONTROL TEST (OPEN POSITION)

1. Ensure throttle is not binding on floor mats, and that throttle cable is tight. Remove harness connector from throttle switch. Connect ohmmeter across throttle switch terminals No. 2 and 3. (Scheme 10)
2. Open throttle slowly. Resistance should be present just before throttle reaches WOT position. Move throttle to WOT position. Resistance should be present. If resistance is not present, loosen throttle switch adjusting screws, rotate switch to correct switch position and repeat THROTTLE CONTROL TEST (CLOSED POSITION) test.

AUTOMATIC IDLE CONTROL (AIC) TEST

Scheme 10

1. Ensure ECU and all control systems are properly connected. Start engine and run at normal operating temperature. Connect an earth to the Green/Red terminal test connector to a known good ground (connector is located near evaporator housing).
2. Remove ground wire. When wire is removed, engine RPM should increase then return to idle. If engine RPM does not respond, go to step 3). If engine RPM responds correctly, reconnect ground wire, set basic idle speed, and disconnect ground wire.
3. Pull back rubber boot from AIC actuator plug (at actuator). Connect positive voltmeter lead to terminal No. 2 of connector. (Scheme 10) (Scheme 10): AIC Switch Terminal Locations
4. Connect negative lead of voltmeter to grounding point at engine lifting hook. Start engine and check voltage reading. Meter should indicate battery voltage.
5. Connect positive voltmeter lead to terminal No. 2 of connector. Connect negative lead to terminal No. 3 of connector. Verify that voltage reading and idle speed change as A/C system is turned on and off.
6. Turn off engine. If voltage and idle speed changed as described, system is okay. If voltage changes but idle speed does not, go to next step. If voltage does not change, go to step 8.
7. Check all AIC system hoses for cracks and/or leaks. Disconnect AIC actuator plug from actuator. Check resistance through actuator windings by measuring resistance between actuator terminals No. 1 and 3. Resistance should be 36-44 ohms. Replace actuator if defective, and retest system. Reconnect harness plug to AIC actuator. NOTE: Ignition must be off when connecting or disconnecting the ECU. Remove wiring harness connector from ECU, prior to testing. Repair any problems as they are found, before proceeding to next step in test procedure. Always probe from rear of connector to avoid enlarging connector pin holes.
8. Disconnect ECU harness connector. Check continuity between ECU harness connector terminals No. 10 and 23. (Scheme 8) Resistance should be 36-44 ohms. If resistance is incorrect, go to step 10).
9. If resistance is correct and problem still exists, substitute a new ECU, connect ECU harness connector, recheck system operation and go to FUEL INJECTION BASE LINE VALUE adjustment.
10. Check for battery voltage at ECU harness connector pin No. 16 when A/C compressor is engaged. If resistance is correct and problem still exists, substitute a new ECU, connect ECU harness connector, and recheck system operation.

Scheme 11

1. Connect voltmeter between Gray/Red and Violet/White terminals of limp-home test connector. These will be terminals No. 1 and 2 on the 900 models and Nos. 2 and 3 on the 9000 Turbo. (Scheme 11) Start engine. If system is operating properly, meter will read less than one volt. (Scheme 11): Limp-Home Test Connector Connector is located near A/C evaporator housing.
2. If system is operating in limp-home mode, meter will indicate battery voltage. If battery voltage is indicated, proceed to AIR MASS METER test.

AIR MASS METER TEST

Scheme 12

1. Start and run engine until cooling fan cycles. Stop engine and disconnect air duct outlet from air mass meter. Start engine and increase engine speed to over 2500 RPM.
2. Turn ignition off. After 4 seconds, observe air mass meter inlet for a one second glow of hot wire. If glow does not occur, go to step 3). If glow occurs, but system is still operating in limp-home mode, the air meter and ECU should be checked by substituting known good units, one at a time. Proceed to FUEL INJECTION BASE LINE VALUE adjustment.
3. Pull air mass meter connector boot back (do not unplug connector). Connect voltmeter positive lead to pin No. 1 and negative lead to pin No. 4, from rear of connector. (Scheme 12) (Scheme 12): Air Mass Meter Test Connector Do not unplug connector to test.
4. Start and run engine until cooling fan cycles. Start engine and increase engine speed to over 2500 RPM. Allow engine to return to idle. Stop engine and observe voltmeter for a 1-second, 4-volt reading.
5. If voltage is okay, replace air mass meter and proceed to FUEL INJECTION BASE LINE VALUE adjustment. If voltage is not present, verify correct wiring between air mass meter and ECU by checking for continuity between air mass meter pin No. 4 and engine ground at lifting hook. Also verify that continuity is present between air mass meter pin No. 1 and ECU harness connector pin No. 8.
6. If wiring is okay, start engine and check for battery voltage between pin No. 2 (positive lead) and pin No. 4 (negative lead) of air mass meter. Stop engine. Repair wiring if necessary. NOTE: The following checks are to be made with the air mass meter and ECU harness connectors unplugged. The ignition must be off when connecting or disconnecting the ECU. Repair any problems as they are found before proceeding to next step in test procedure. Always probe from rear of each connector to avoid enlarging connector pin holes.
7. Place a jumper wire across pins No. 3 and 6 of air mass meter connector, and check for continuity between pins No. 6 and 7 of ECU harness connector. Place a jumper wire across pins No. 5 and 6 of air mass meter connector, and check for continuity between pins No. 6 and 14 of ECU harness connector.
8. If wiring checks okay, air meter and ECU should be checked by substituting known good units, one at a time. If any unit is found defective, and replaced, proceed to FUEL INJECTION BASE LINE VALUE adjustment.

TROUBLE SHOOTING PRELIMINARY CHECKS

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

1. Battery connections and specific gravity
2. Ignition system
3. Emission control system
4. Air intake system leakage
5. Engine compression pressure
6. Fuel supply system pressure and flow
7. Fuel and vacuum lines
8. Injection system component and sensor electrical connections
9. Air filter
10. Vacuum line, fuel hose and pipe connections

ENGINE DIFFICULT TO START OR WILL NOT START COLD

If cold engine is hard to start, or turns over but will not start, test the following: cold start injector, thermal time switch, temperature sensors, air mass meter, pump relay, and system relay.

ENGINE DIFFICULT TO START OR WILL NOT START HOT

If hot engine is difficult to start, or cranks but will not fire, test the cylinder injectors for leakage and proper operation.

LH Jetronic Fuel Injection Schematic. Scheme 13

POOR COLD IDLE/WARM-UP

If engine runs poorly only during warm-up, test the following items: cold start injector, blocked air control hose, cylinder injectors, thermal time switch, and air mass meter.

POOR HOT IDLE

If engine runs poorly after warm-up, test the following items: cold start injector, throttle valve, cylinder injectors, blocked air control hose, air mass meter and air temperature sensor.

CO LEVEL TOO HIGH

If the exhaust gas CO level is too high, test the following items: cold start injector, oxygen sensor, air mass meter, and temperature sensors.

CO LEVEL TOO LOW AT IDLE

If the exhaust gas CO level is too low at idle speeds, adjust CO mixture setting.

POOR ENGINE PERFORMANCE

If engine operates poorly at all speeds, test the following items: cold start injector (for leaks), throttle valve, CO adjustment, and throttle valve switch.

EXCESSIVE FUEL CONSUMPTION

If fuel consumption is high, test the following items: cold start injector, air mass meter, and temperature sensors.

ENGINE RUN-ON

If engine continues to run after ignition is shut off, test the following items: cylinder injectors, cold start injector, air mass meter, and temperature sensors.

ENGINE BACKFIRES INTO MANIFOLD

Test the air mass meter.

LH Jetronic Fuel Injection System Wiring Diagram. Scheme 14