Engine Controls - Tests W/o Codes: Diagnosis Mazda 626 GD

SYMPTOM DIAGNOSIS

Symptom checks cannot be used properly unless the problem occurs while the vehicle is being tested. To reduce diagnostic time, ensure steps in BASIC TESTING and TESTS W/CODES articles were performed before diagnosing a symptom. Symptoms available for diagnosis include

1. Difficult or no start (cranks okay)
2. Rough or unstable idle
3. Excessive fast idle
4. Engine stalls
5. Engine misfires or has lack of power
6. Engine runs rough on deceleration
7. Afterburn in exhaust system
8. Poor fuel mileage
9. Excessive oil consumption
10. Abnormal noise or knocking

FAILS EMISSION TEST

1. Ensure exhaust system is not restricted.
2. Check EGR valve and solenoid for correct operation.
3. Check fuel-cut system for malfunction. Fuel-cut system includes: throttle body dashpot unit, throttle position sensor and coolant thermosensor.
4. Check No. 1 and No. 2 purge control valve operation.
5. Check vacuum switching valve operation.
6. Check secondary air injection system (RX7).
7. Check airflow meter for cracks.
8. Verify correct airflow meter operation and resistance value.
9. Check atmospheric pressure sensor.
10. Check air valve and ISC switch.
11. Ensure fuel system pressure is correct.
12. Verify intake air system and throttle body electrical connections.
13. Check pressure regulator and pressure regulator solenoid.
14. Verify water and intake air thermosensor.
15. Verify throttle position sensor adjustment.

INTERMITTENT 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 faults. A status change on DVOM indicates a fault has been located.

Use a DVOM to pinpoint faults. When monitoring voltage, ensure ignition switch is in ON position or engine is running. Ensure ignition switch 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.

TEST PROCEDURES INTERMITTENT SIMULATION

To reproduce the conditions creating an intermittent fault use the following methods

1. Lightly vibrate component(s).
2. Heat component.
3. Wiggle or bending wiring harness.
4. Spray component with water.
5. 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.