INTRODUCTION
Before attempting to diagnose any symptoms or intermittent faults, make sure test procedures in both the BASIC TESTING and TESTS W/CODES articles have been performed. Use this article to diagnose driveability problems that exist when a hard fault code is not present or vehicle is not equipped with a self-diagnostic system.
Symptom checks are intended to direct the technician to malfunctioning component(s) so that further diagnosis may be performed. A "symptom" should lead to further testing of specific components or systems, or verification of adjustment specifications.
Use intermittent test procedures to locate intermittent driveability problems that do not occur when the vehicle is being tested. These problems may cause a noticeable driveability problem or cause the "malfunction" warning light to illuminate on some vehicles.
It is also possible that certain driveability concerns have been rectified by the manufacturer through substitution of a revised calibration chip or computer control unit. Check with manufacturer for latest information on updated chips and control units.
Note. For specific component or system test procedures, see the SYSTEM/COMPONENT TESTS article. For specifications, refer to the SPECIFICATIONS article. For tune-up procedures, refer to the ADJUSTMENTS article.
SYMPTOM DIAGNOSIS
Symptom checks cannot be used properly unless the problem is actually happening while the vehicle is being tested. To reduce diagnostic time, make sure test procedures in the BASIC TESTING and TESTS W/CODES articles have been performed before attempting to diagnose a symptom. For symptoms available for diagnosis see the SYMPTOM-TO-DIAGNOSIS MENU .
| Symptom | Go To |
|---|---|
| Difficult or Hard to Start | DIFFICULT TO START |
| Rough or Unstable Idle | ROUGH/UNSTABLE IDLE |
| Engine Hesitates or Misses | HESITATES OR MISSES |
| Poor Fuel Economy/Mileage | POOR FUEL MILEAGE |
SYMPTOM-TO-DIAGNOSIS MENU
DIFFICULT TO START
- Ensure vacuum hoses are not disconnected or damaged.
- Ensure electrical harness, connectors and wires are not broken or loose.
- Check voltage to ECU
- Check ECU operation.
- Check control relay operation.
- Ensure there are no codes in memory.
- Check fuel pump control system operation.
- Check that fuel pressure meets specification.
- Ensure ignition timing is correct.
- Check injector operation.
| Symptom or Component | Malfunction or Possible Cause | Action To Be Taken |
|---|---|---|
| Electrical System | Low Voltage, Broken Wiring, Poor Connection Faulty Relay or Pressure Switch | Check for at least 11.5V at battery. Check system and fuel pump relay. On Turbo models, check pressure switch. |
| Fuel Pump | Pump Not Working Or Poor Delivery | Check fuel pump delivery and flow. |
| Fuel System | Leakage, Faulty Fuel Pressure Regulator Blocked Return Line | Check for leakage. Check fuel line pressure. Check Fuel Pressure Regulator. |
| Induction System | Air system leakage | Check for leakage, downstream of throttle housing with pressure tester and visually between air mass meter and throttle housing. |
| Injectors | Opening, Duration Out Of Spec, Leakage | Using multimeter, check injector duration. |
| Temperature Sensor | Not Functioning, Break In Wiring, Poor Connection | Check sensor with multimeter. |
| ECU | Poor Connection, No Input Signals, Faulty ECU | Using multimeter, check all ECU input signals. |
| Auxiliary Air Valve | Malfunction Or Air Leak In Air Valve | Using multimeter, check air valve. |
DIAGNOSTIC CHART FOR DIFFICULT TO START SYMPTOM
ROUGH/UNSTABLE IDLE
- Ensure vacuum hoses are not disconnected or damaged.
- Ensure electrical harness, connectors and wires are not broken or loose.
- Ensure there are no codes in memory.
- Check ECU operation.
- Check that fuel pressure meets specification.
- Check injector operation.
- Check air conditioner switch operation.
- Check automatic idle control (AIC) operation.
- Check ignition switch operation.
- Check airmass meter voltage output.
- Check engine coolant temperature sensor operation.
- Check Oxygen (O2) sensor voltage output.
- Check throttle switch operation.
- Check Hall Effect sensor operation.
| Symptom or Component | Malfunction or Possible Cause | Action To Be Taken |
|---|---|---|
| Temperature Sensor | Not Functioning, Break In Wiring, Poor Connection | Check sensor with multimeter. |
| Air Mass Meter/CO | CO Content Too High/Low, Filament Broken | Check air mass meter CO setting. Check for broken filament. |
| Induction System | Air system leakage | Check for leakage, downstream of throttle housing with pressure tester and visually between air mass meter and throttle housing. |
| Injectors | Opening, Duration Out Of Spec, Leakage | Using multimeter, check injector duration. |
| Fuel System | Leakage, Faulty Fuel Pressure Regulator Blocked Return Line | Check for leakage. Check fuel line pressure. Check Fuel Pressure Regulator. |
| ECU | Poor Connection, No Input Signals, Faulty ECU | Using multimeter, check all ECU input signals. |
| Auxiliary Air Valve | Malfunction Or Air Leak In Air Valve | Using multimeter, check air valve. |
| Electrical System | Broken Wiring, Poor Connection | Check wiring and connectors. Check ground and battery connections. |
| EGR Valve | Valve Not Working Or Valve Sticking | Check EGR system. |
DIAGNOSTIC CHART FOR ROUGH OR UNSTABLE IDLE SYMPTOMS
HESITATES OR MISSES
- Ensure vacuum hoses are not disconnected or damaged.
- Ensure electrical harness, connectors and wires are not broken or loose.
- Ensure there are no codes in memory.
- Check airmass meter voltage output.
- Check engine coolant temperature sensor operation.
- Check that fuel pressure meets specification.
- Check ignition switch operation.
- Check Turbo/APC operation.
- Check inhibitor switch operation (automatic transmission).
- Check injector operation.
- Check Oxygen (O2) sensor voltage output.
- Check throttle switch operation.
- Check Hall Effect sensor operation.
| Symptom or Component | Malfunction or Possible Cause | Action To Be Taken |
|---|---|---|
| ECU | Poor Connection, No Input Signals, Faulty ECU | Check ECU connector pins 9, 11, 13, 17 and 25. |
| Electrical System | Alternator, Broken Wiring, Poor Connection Poor Ground | Check alternator. Check ground and battery connections. Check battery. |
| Injectors | Opening, Duration Out Of Spec, Leakage | Using multimeter, check injector duration. |
| Turbo/APC System | Malfunction/Leakage | Check turbo operation. Check APC system operation. |
| Throttle Position | Incorrectly Adjusted Throttle Position Sensor | Using multimeter, check throttle position sensor. Check for sticking sensor. Check sensor adjustment. |
| Air Mass Meter | No Burn-Off Function, Filament Broken | Check air mass meter burn-off function. Check for a broken filament. |
| Induction System | Air system leakage | Check for leakage, downstream of throttle housing with pressure tester and visually between air mass meter and throttle housing. |
| Fuel System | No Fuel Pressure Pressure Too Low | Check fuel line pressure. Check delivery rate. |
| Temperature Sensor | Not Functioning, Break In Wiring, Poor Connection | Check sensor with multimeter. |
DIAGNOSTIC CHART FOR ENGINE HESITATING OR MISSING SYMPTOMS
POOR FUEL MILEAGE
- Ensure there are no codes in memory.
- Check air conditioner switch operation.
- Check airmass meter voltage output.
- Check engine coolant temperature sensor operation.
- Check that fuel pressure meets specification.
- Check ignition switch operation.
- Check injector operation.
- Check Oxygen (O2) sensor operation.
- Check throttle switch operation.
- Check Hall Effect sensor operation.
| Symptom or Component | Malfunction or Possible Cause | Action To Be Taken |
|---|---|---|
| Fuel System | No Fuel Pressure, Pressure Too Low | Check fuel line pressure. Check delivery rate. |
| Injectors | Opening, Duration Out Of Spec, Leakage | Using multimeter, check injector duration. |
| Temperature Sensor | Not Functioning, Break In Wiring, Poor Connection | Check sensor with multimeter. |
| Throttle Position Sensor | Incorrectly Adjusted Throttle Position Sensor | Using multimeter, check throttle position sensor. |
| Air Mass Meter High | Filament Broken | Check air mass meter burn-off function is not operating too high. Check for a broken filament. |
| Electrical System | Broken Wiring, Poor Connection | Check wiring and connectors. Check ground and battery connections. |
| ECU | Poor Connection, No Input Signals, Faulty ECU | Using multimeter, check all ECU input signals. |
DIAGNOSTIC CHART FOR POOR FUEL MILEAGE SYMPTOM
INTERMITTENT PROBLEM DIAGNOSIS
Use this procedure to locate intermittent driveability problems that do not occur when the vehicle is being tested. These problems may cause a noticeable driveability problem or cause the "malfunction" warning light to illuminate on some vehicles. Before attempting to diagnose an intermittent fault, ensure steps in BASIC TESTING and G - TESTS W/ CODES (if applicable) have been performed. Intermittent fault testing requires you to duplicate a circuit or component failure, in order to identify the fault. These procedures may lead to the computer recording a fault code (on some systems) which may help in diagnosis.
If problem vehicle does not produce fault codes, it will be necessary to monitor voltage or resistance values using a DVOM while attempting to produce conditions which will create an intermittent fault. A change in status on the DVOM will indicate a fault has been located.
Intermittent Simulation
To produce the conditions which create an intermittent fault so that it may be identified during testing, some of the following methods may be used
- Applying light vibration to components.
- Heating a component.
- Wiggling or bending a wiring harness.
- Applying humidity to a component.
- Remove or apply a vacuum supply source.
Monitor component voltage, or monitor for self-diagnostic codes while attempting to simulate intermittent. Use the results of these tests to identify a faulty component or an area which should be checked closely for the problem.
Note. Intermittent problems are often caused by poor harness connector. It is important to check harness connector contact.