Symptoms - Engine Controls
Diagnostic Instructions
Symptoms Description
Symptoms cover 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: 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 1 500 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 gets worse while under acceleration. The engine makes sharp metallic knocks that change with throttle opening.
Dieseling, Run-On: Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.
Hard Start: Engine cranks OK, but does not start for a long time. The vehicle does eventually run, or may start but immediately stalls.
Hesitation, Sag, Stumble: 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 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.
Poor Fuel Fill Quality: Difficulty when refueling the vehicle.
Rough, Unstable, or Incorrect Idle and Stalling: The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle may vary in speed. Either condition may be severe enough to stall the engine.
Surges/Chuggles: 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
| • | The engine control module (ECM) and the malfunction indicator lamp (MIL) are operating correctly. |
| • | There are no DTCs stored. |
| • | The scan tool data is within the normal operating range. |
| • | Bulletins are not available for the current symptom. |
| • | The ECM grounds are clean, tight, and in the correct locations. |
| • | The vehicle tires are correctly inflated and meet original equipment specifications. |
| • | The air filter element is not restricted. |
| • | 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 determine the cause of an intermittent condition. |
Identifying Intermittent Conditions
Many intermittent conditions occur with harness or connector move 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. |
| • | Electromagnetic 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 for the following conditions:
| • | For the correct installation of the Manifold Absolute Pressure Sensor. |
| • | The fuel system for the following: |
| • | The ignition system for the following: |
| - | For diagnosis of coolant or oil fouled spark plugs. |
| - | Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied. |
| - | Weak spark using the EL 26792 HEI spark tester . For equivalent regional tools, refer to Special Tools . |
| • | The operation of the transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON. |
| • | The operation of the A/C compressor. |
| • | The crankshaft position sensor for the correct resistance. The crankshaft position sensor resistance may be out of range after a hot soak. The resistance should be between 700-1 200 Ω. |
| - | 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. |
| • | Vacuum hoses for splits or kinks. Verify that the routing and connections are as shown on the vehicle emission control information label. |
| • | Knock sensor system for excessive spark retard activity. Refer to DTC P0325 . |
| • | Fuel octane switch setting. Verify the fuel octane switch is set to the proper fuel octane rating for the fuel being used in the vehicle. |
| • | Turbocharger system for proper operation, if equipped. |
| • | Turbocharger charge air cooler system has no leaks, if equipped. |
| • | The exhaust system components for the following: |
| - | Physical damage or possible internal malfunction |
| • | Electromagnetic interference on the reference circuit can cause a misfire condition. You can usually detect electromagnetic interference 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 electromagnetic interference is present. Inspect the high voltage components near the ignition control circuit if a condition exists. |
| • | Inspect the positive crankcase ventilation system and all connections for leaks or restrictions. |
| • | Evaporative Emission (EVAP) Canister Purge Solenoid Valve that is stuck open. |
| • | The engine cooling system for the following conditions: |
| - | The engine coolant thermostat heater is working proper. |
- If the above conditions do not address the symptom, refer to the additional symptoms tests.
Additional Symptoms Tests
Detonation/Spark Knock: Test the engine for an overheating condition. Refer to Symptoms - Engine Cooling .
Poor Fuel Economy: 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.
Rough, Unstable, or Incorrect Idle and Stalling: Inspect the engine mounts.
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 room temperature vulcanizing 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.
Hard Start
| • | Test the engine coolant temperature (ECT) sensor. Compare the ECT sensor value to the intake air temperature (IAT) sensor value on a cold engine. The ECT and IAT sensor values should be within approximately 3°C (5°F). If the ECT sensor is out of range with the IAT sensor, test the resistance of the ECT sensor. Refer to Temperature Versus Resistance table for resistance specifications. Replace the ECT sensor if the resistance is not within specification. If the sensor is within the specification, test the ECT circuits for a high resistance. |
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 for foreign material accumulation in the throttle bore, and for carbon deposits on the throttle plate and shaft. Also inspect for throttle body tampering. |
Poor Fuel Fill Quality
Difficult to fill
| • | A condition with the internal components of the fuel tank assembly. |
For more information, refer to the following:
Fuel Odor
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