Diagnostic Instructions
Symptoms Description
Symptoms covers conditions that are not covered by DTCs. Certain conditions can cause multiple symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause specific symptoms are listed separately under Additional
Symptoms Testing. Perform the Symptoms Testing before using the Additional Symptoms Testing.
Symptoms Definition
Backfire: The fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.
Cuts Out, Misses: A steady pulsation or jerking that follows engine speed, which is usually more pronounced as the engine load increases. This condition is not normally felt above 1500 RPM or 48 km/h (30 mph). The exhaust
has a steady spitting sound at idle or at low speed.
Detonation/Spark Knock: A mild to severe ping which usually occurs worse while under acceleration. The engine makes sharp metallic knocks that change with throttle opening.
Dieseling, Run-On: The engine continues to run after the key is turned OFF, but runs very rough.
Hard Start: The engine cranks OK, but does not start for a long time. The vehicle does eventually run, or may start but immediately stall.
Hesitation, Sag, Stumble: A momentary lack of response as the accelerator is pushed down. This condition can occur at any vehicle speed. This condition is usually more pronounced when first trying to make the vehicle move, as from a stop.
This condition may cause the engine to stall in severe conditions.
Lack of Power, Sluggishness, or Sponginess: The engine delivers less than expected power. Little or no increase in vehicle speed when the accelerator pedal is pushed down part way.
Poor Fuel Economy: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.
Rough, Unstable, or Incorrect Idle and Stalling: The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle speed may vary. Either condition may be severe enough to stall the engine.
Surges/Chuggles: An engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator pedal position.
Symptoms Verification
Before using the Symptom tables, perform the following inspections:
| • | Ensure that the engine control module (ECM) and malfunction indicator lamp (MIL) are operating correctly. |
| • | Ensure that there are no diagnostic trouble codes (DTCs) that are stored. |
| • | Ensure that the scan tool data is within a normal operating range. Refer to
Control Module References for scan tool information. |
| • | Verify the customer concern. |
| • | Perform the Visual/Physical Inspection in this section. The visual/physical inspection is extremely important, and can lead to correcting a condition without additional testing. It may also help reveal the cause of an intermittent condition. |
Identifying Intermittent Conditions
Many intermittent conditions occur with harness or connector movement due to engine torque, rough pavement, vibration or physical movements of a component. Refer to the following for a list of issues that may cause an intermittent condition:
| • | Moisture and water intrusion in connectors, terminals, and components |
| • | Incomplete connector mating |
| • | High circuit or component resistance--High resistance can include any resistance, regardless of the amount, which can interrupt the operation of the component. |
| • | Harness that is too short or tight. |
| • | Wire insulation that is chaffed or cut. |
| • | High or low ambient temperature |
| • | High or low engine coolant temperatures |
| • | High underhood temperatures |
| • | Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load |
| • | High or low system voltage |
| • | High vehicle load conditions |
| • | Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge |
| • | Incorrect installation of aftermarket, add on accessories |
Visual/Physical Check
| • | Ensure that the control module grounds are clean, tight, and correctly located. |
| • | Ensure that the vacuum hoses are not split, kinked, and properly connected, as shown on the Vehicle Emission Control Information label. |
| • | Ensure that the air filter is clean and free from restrictions. |
| • | Ensure that there is no water intrusion in connectors terminals and components. |
| • | Inspect the air intake ducts for the following conditions: |
| • | Inspect for air leaks at the throttle body mounting area, the mass air flow (MAF) sensor and intake manifold sealing surfaces. |
| • | Inspect the wiring harness for the following conditions: |
| • | Inspect for loose, damaged, unseated, or missing sensors/components. |
| • | Inspect the terminals for corrosion and correct contact. |
Symptoms Testing
Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles
| • | Test the fuel system for the following: |
| • | Test ignition system for the following: |
| • | Inspect for the following conditions |
| - | The proper operation of the transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON. |
| - | The proper operation of the A/C compressor. |
| - | Items that can cause an engine to run lean or rich. |
| - | The crankshaft position (CKP) sensor for proper operation |
| - | The proper operation of the knock sensor (KS) system |
| - | Electromagnetic interference (EMI) on the reference circuit can cause a misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter with little change in
actual engine speed indicates that EMI is present. Inspect the high voltage components near the ignition control circuit if a condition exists. |
| - | The crankcase ventilation valve for proper operation. |
| - | The proper operation of the evaporative emission (EVAP) canister purge solenoid. |
| • | Inspect the engine for the following mechanical failures: |
| - | Excessive oil in the combustion chamber or leaking valve seals |
| - | Incorrect cylinder compression |
| - | Sticking or leaking valves |
| - | Excessive carbon buildup in the combustion chambers--Clean the chambers with top engine cleaner. Follow the instructions on the can. |
| • | If the above conditions do not address the symptom, refer to the additional symptoms tests. |
Additional Symptoms Tests
Hard Start
| • | Verify that the driver is using the correct starting procedure as described in the owners manual. |
| • | Verify that there is adequate fuel. |
Hesitation, Sag, Stumble
| • | Test the generator. Refer to
Symptoms - Engine Electrical. Repair the charging system if the generator output voltage is less than 9 V or more than 16 V. |
Poor Fuel Economy
| • | Heavy loads being carried or towed |
| • | Acceleration rate too much or too often |
| • | Inspect the brake system for brake drag. |
| • | Inspect for incorrect operation of the speedometer. |
| • | Inspect for foreign material accumulation in the throttle bore, and for carbon deposits on the throttle plate and shaft. Also inspect for throttle body tampering. |
Detonaton/Spark Knock
| • | If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicle minimum octane requirements. |
Lack of Power, Hesitation, or Cut Out
| • | Verify that each injector harness is connected to the correct injector. |
Rough, Unstable, or Incorrect Idle and Stalling
| • | Inspect the engine mounts. |
| • | Inspect the intake and exhaust manifolds for casting flash. |
Surges/Chuggles
| • | Test the heated oxygen sensors (HO2S). The HO2S should respond quickly to a change in throttle position. If the HO2S do not respond to different throttle positions, inspect for contamination from fuel, silicon, or the incorrect use of RTV sealant.
The sensors may have a white powdery coating and result in a high, but false, signal voltage, which gives a rich exhaust indication. The ECM reduces the amount of fuel delivered to the engine, causing a driveability condition. |
| • | Verify that each injector harness is connected to the correct injector. |
