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 the following
- Hard Starting (Cranks Okay)
- Engine Surges And/Or Chuggles
- Lack Of Power, Sluggish Or Spongy
- Detonation/Spark Knock
- Hesitation, Sag Or Stumble
- Cuts Out, Misses
- Rough, Unstable Or Incorrect Idle
- Poor Fuel Economy
- Excessive Exhaust Emissions
- Engine Diesels
- Engine Backfires
Note. Systems and components listed under each symptom may not be applicable on all models.
INTERMITTENT PROBLEM DIAGNOSIS
Most intermittent problems are caused by faulty electrical connection or wiring. Ensure connector halves and terminals are fully seated and not backed out. Ensure terminals are not damaged, properly formed and there is a firm terminal-to-wire connection.
DO NOT use Diagnostic Trouble Code (DTC) procedures to diagnosis intermittent problems. 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, road test vehicle and monitor voltage or resistance values using a DVOM or Tech 1. While attempting to reproduce conditions causing intermittent fault, a status change on DVOM or Tech 1 indicates a fault has been located.
Use a DVOM or Tech 1 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 or Tech 1 during test procedures indicate area or component of fault.
An intermittent MIL, and NO DTCs may be caused by electrical system interference from a defective relay, an ECM driven solenoid or a switch. These defective components can cause a sharp electrical surge during system operation. Improper installation of add-on electrical options, such as lights and radios, improper routing of ignition wires to close to spark plugs, ignition components and alternator can also cause an intermittent MIL.