SYMPTOMS
Before proceeding with any symptom diagnosis, perform all steps under PRELIMINARY CHECKS .
SYMPTOM DIAGNOSIS
Symptom checks should not be used unless symptom occurs while vehicle is being tested. To reduce diagnostic time, ensure steps in appropriate BASIC DIAGNOSTIC PROCEDURES and SELF-DIAGNOSTICS articles were performed before diagnosing a symptom. Symptoms available for diagnosis include
- Hard Start
- Hesitation, Sag Or Stumble
- Vehicle Surges Or Chuggles
- Lack Of Power, Sluggish Or Spongy
- Engine Backfires
- Cuts Out, Misses
- Rough, Unstable Or Incorrect Idle, Stalling
- Poor Fuel Economy
- Engine Dieseling/Run-On
- Detonation/Spark Knock
- Excessive Exhaust Emissions Or Odors
Symptom Definition
Engine cranks okay, but does not start for a long time. Engine eventually starts, and may die immediately or run okay.
Momentary lack of response when accelerator is pushed down. Condition occurs at all vehicle speeds or usually occurs when taking off from a stop. If severe enough, may cause engine to stall.
Engine power varies under steady throttle or cruise. Vehicle speeds up and slows down without changing position of accelerator pedal.
Engine delivers less power than expected. Little or no increase in speed when accelerator is pushed down.
Fuel ignites in intake manifold or in exhaust system, making a loud popping noise.
Cuts out or misses is a steady pulsation or jerking that follows engine speed and is usually more pronounced as engine load increases. Exhaust may have a steady spitting sound at idle or low speed. Perform a careful visual inspection as described in appropriate BASIC DIAGNOSTIC PROCEDURES article.
Engine runs unevenly at idle. If severe enough, vehicle will shake. Idle may vary in RPM. Either problem may cause stalling. Engine idles at incorrect RPM.
Fuel economy, as measured by an actual road test, is noticeably lower than expected. Fuel economy is noticeably lower than was on this vehicle at one time.
Engine continues to run after ignition is turned off but runs very rough. If engine runs smoothly, check ignition switch.
A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that change with amount of acceleration.
Vehicle fails emission test. Vehicle may also have excessive "rotten egg" smell (hydrogen sulfide) being emitted from tail pipe. Excessive odors DO NOT necessarily indicate exhaust emissions are high.
PROBLEM DIAGNOSIS
Intermittent fault testing requires duplicating circuit or component failure to identify fault. These procedures may lead to computer setting a diagnostic trouble code which may help in diagnosis.
If problem vehicle does not produce diagnostic trouble codes, monitor voltage or resistance values using a scan tool or DVOM while attempting to reproduce conditions causing the intermittent fault. A status change on scan tool or DVOM indicates a fault has been located.
Use scan tool or DVOM to pinpoint faults. When monitoring voltage, ensure ignition is in ON position or engine is running. When monitoring resistance, ensure ignition is in OFF position or negative battery cable is disconnected. A status change on scan tool or DVOM while performing test procedures indicates area of fault. See TEST PROCEDURES .
Intermittent Symptom Definition
Malfunction Indicator Light (MIL) illuminates but does not remain on. A stored diagnostic trouble code may or may not exist.