SYMPTOMS
Before proceeding with any symptom diagnosis, perform all steps under PRELIMINARY CHECKS.
Note. If PCM displays data but engine fails to start, see NO-START DIAGNOSIS in the BASIC TESTING article.
SYMPTOM DIAGNOSIS
Symptom checks should not be used unless symptom occurs while vehicle is being tested. To reduce diagnostic time, ensure basic diagnostic procedures in the BASIC TESTING and self-diagnostic steps in the TESTS W/CODES articles were performed before diagnosing a symptom. Symptoms available for diagnosis include
- Hard Start
- Stalls After Starting
- Hesitation, Sag Or Stumble
- Vehicle Surges Or Chuggles
- Lack Of Power Or Sluggish
- Engine Backfires
- Cuts Out, Misses
- Rough, Unstable Or Incorrect Idle
- Engine Will Not Idle
- Poor Fuel Economy
- Engine Dieseling/Run-On
- Detonation/Spark Knock
- Excessive Exhaust Emissions Or Odors
Symptom Definition
Engine cranks okay, but does not start immediately. Engine eventually starts and may run okay or soon die.
Engine starts okay but dies after brief idle, as soon as any load is placed on engine (such as turning on air conditioner or engaging transmission) or on initial drive-away.
Momentary lack of response when accelerator is pushed down. Can occur at any vehicle speed. Usually occurs after a stop. If severe enough, may cause engine to stall.
Engine power varies under steady throttle or cruise. Feels like vehicle speeds up and slows down even though accelerator pedal position remains constant.
Engine delivers less power than expected. Little or no increase in speed when accelerator is applied.
Fuel ignites within intake manifold or in exhaust system, making a loud popping noise.
Cuts out, misses is defined as a steady pulsation or jerking following engine speed, usually more pronounced as engine load increases. Exhaust may have a steady spitting sound at idle or low speed. Perform visual inspection as described in BASIC TESTING article.
| CAUTION | Grounding spark plug wire for extended periods may cause catalytic converter overheating. |
Engine runs unevenly at idle. If bad enough, vehicle will shake. Idle RPM may vary. Engine idles at incorrect RPM.
Engine starts but will not run at idle. Engine runs if accelerator is held at part throttle.
Fuel economy, as measured by an actual road test, is noticeably lower than expected. Current fuel economy is noticeably lower than previous fuel economy.
Engine continues to run (but very rough) after ignition is turned off. If engine runs smoothly, check ignition switch.
A mild to severe ping, usually worse during acceleration. Engine makes sharp metallic knocks which change with degree of acceleration.
Vehicle fails emission test. Vehicle may also emit "rotten egg" smell (hydrogen sulfide) from exhaust pipe.
INTERMITTENT PROBLEM DIAGNOSIS
Intermittent problem diagnosis requires duplication of circuit or component failure in order to identify problem. These procedures may lead to computer recording a trouble code which may help diagnosis.
If problem vehicle does not produce a trouble code, use a scan tool or DVOM to monitor voltage or resistance values while attempting to reproduce conditions which will create an intermittent problem. A change in status on scan tool or DVOM indicates a problem has been located.
When monitoring voltage, ensure ignition switch is in ON position or engine is running. When monitoring resistance (ohms), ensure ignition switch 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 problem.
Malfunction Indicator Light (MIL) comes on but does not stay on. A stored trouble code may or may not exist.
See also:
• ADJUSTMENTS