INTRODUCTION
Before diagnosing symptoms or intermittent faults, follow proper workflow. See TESTING PROCEDURE under SELF-DIAGNOSTICS - EXCEPT 7-SERIES or SELF-DIAGNOSTICS - 7 SERIES article. If no Diagnostic Trouble Codes (DTCs) are present and a no start condition exists, proceed to BASIC DIAGNOSTIC PROCEDURES article. If no DTCs are present and a driveability condition exists, diagnosis by symptom (i.e., ROUGH IDLE, ENGINE STALLS, etc.).
Note. Some driveability problems may have been corrected by manufacturer with a revised computer calibration chip or computer control unit. Check with manufacturer for latest chip or computer application.
Use intermittent test procedures to locate driveability problems that do not occur when the vehicle is being tested. These test procedures should also be used if a soft (intermittent) trouble code was present, but no problem was found during self-diagnostic testing.
Always personally verify customer complaint. Always verify that complaint is truly a system malfunction. Perform a quick test to determine if the vehicle systems have logged fault codes. Call up the faulted system or appropriate test schedule to verify the correct control module is installed in the car. Follow Diagnostic Information System (DIS or MoDiC) on screen instructions and perform all tests as specified. Use DIS/MoDiC and fault symptom diagnostic procedures as necessary. Follow appropriate test module procedures for systems that malfunction but fail to set faults in memory.
Note. For specific testing procedures, see SYSTEM & COMPONENT TESTING article. For specifications, see ON-VEHICLE ADJUSTMENTS or SERVICE & ADJUSTMENT SPECIFICATIONS article.
SYMPTOM DIAGNOSIS
| CAUTION | When battery is disconnected, vehicle computer may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION before disconnecting the battery. |
Symptom checks cannot be used properly unless the problem occurs while the vehicle is being tested. To reduce diagnostic time, ensure steps in BASIC DIAGNOSTIC PROCEDURES , SELF-DIAGNOSTICS - EXCEPT 7-SERIES or SELF DIAGNOSTICS - 7-SERIES articles were performed before diagnosing a symptom. Following symptoms are available for diagnosis
- Engine Does Not Start (Will Not Crank)
- Engine Cranks, But Fails To Start
- Engine Backfires
- Loss Of Power
- Detonation (Engine Knock)
- Stalls At Idle
- Engine Runs Rough (Off Idle)
- Rough Or Unstable Idle
- Engine Misfires All Conditions
- Poor Fuel Economy
- Excessive Emissions
ENGINE DOES NOT START (WILL NOT CRANK)
- Loose or corroded battery terminals.
- Ensure battery is fully charged.
- Starter motor inoperative.
ENGINE CRANKS, BUT FAILS TO START
- Starter motor rotating too slowly.
- Ignition system malfunction.
- DME/Motronic control unit malfunction.
- Insufficient fuel pressure.
ENGINE BACKFIRES
- Air leak into/from exhaust system.
- Intake manifold vacuum leak.
- Burned valve.
- Lean air/fuel mixture.
- Incorrect ignition timing.
- Incorrect valve timing.
LOSS OF POWER
- Insufficient fuel pressure.
- Restricted fuel filter.
- Restricted air filter.
- Ensure engine compression is okay.
- Ignition system malfunction.
- Burned or sticking valves.
- Restricted exhaust system.
- DME/Motronic control unit malfunction.
- Incorrect valve timing.
DETONATION (ENGINE KNOCK)
- Ignition timing too far advanced.
- Blown head gasket.
- Stuck thermostat (engine overheating).
- Lean fuel mixture.
- DME/Motronic control unit malfunction.
- No coolant in radiator/engine (engine overheating).
- Wrong octane fuel.
STALLS AT IDLE
- Insufficient fuel pressure.
- DME/Motronic control unit malfunction.
- Incorrect ignition timing.
- Blocked or restricted exhaust system.
ENGINE RUNS ROUGH (OFF IDLE)
- Fouled spark plugs.
- Burned or sticking valve.
- Ignition system malfunction.
- Intake manifold air leak.
- Fuel injector not operating.
- DME/Motronic control unit malfunction.
- Blown head gasket.
ROUGH OR UNSTABLE IDLE
- Fouled spark plugs.
- Burned or sticking valves.
- Intake manifold air leak.
- Incorrect valve timing.
- Ignition system malfunction.
- Fuel injector not operating.
- DME/Motronic control unit malfunction.
- Blown head gasket.
ENGINE MISFIRES - ALL CONDITIONS
Note. (Scheme 24)- (Scheme 28).
- Fouled spark plugs.
- Burned or sticking valves.
- Ignition system malfunction.
- Fuel injector not operating.
- DME/Motronic control unit malfunction.
- Blown head gasket.
Scheme 24
Scheme 25
Scheme 26
Scheme 27
Scheme 28
POOR FUEL ECONOMY
- Leaking fuel injectors.
- Faulty oxygen sensor.
- DME/Motronic control unit malfunction.
EXCESSIVE EMISSIONS
- Leaking fuel injectors.
- DME/Motronic control unit malfunction.
- Faulty oxygen sensor.
- Faulty mass airflow sensor.
- Faulty temperature sensor.
PROBLEM DIAGNOSIS
Intermittent fault testing requires duplicating circuit or component failure to identify the problem. These procedures may lead to the computer setting a fault code (on some systems) which may help in diagnosis. If problem vehicle does not produce fault codes, monitor voltage or resistance values using a DVOM while attempting to reproduce conditions causing intermittent fault. A status change on DVOM indicates a fault has been located.
Use a DVOM to pinpoint faults. When monitoring voltage, ensure ignition is in ON position, or engine is running. Ensure ignition is in OFF position or negative battery cable is disconnected when monitoring circuit resistance. Status changes on DVOM during test procedures indicate area of fault.
Intermittent Simulation
To reproduce the conditions creating an intermittent fault, use the following methods
- Lightly vibrate component.
- Heat component.
- Wiggle or bend wiring harness.
- Spray component with water.
- Remove/apply vacuum source.
Monitor circuit/component voltage or resistance while simulating intermittent. If engine is running, monitor for self-diagnostic codes. Use test results to identify a faulty component or circuit.