Symptoms - Engine Controls
Symptoms - Engine Controls
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
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 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 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.
Engine Control Module (ECM) Commanded Reduced Engine Power
- The ECM illuminates the Reduced Engine Power lamp and will limit engine power under potential engine/vehicle damaging or emissions related conditions. A DTC may not be set.
Fuel Odor
- A noticeable smell of unburned fuel.
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:
* Verify that the malfunction indicator lamp (MIL) is operating correctly. Use the scan tool to command the lamp ON and OFF.
* Verify that there are no DTCs that are stored.
* Verify that the scan tool data is within a normal operating range. Refer to Control Module References 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
* Poor terminal contact
* 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
* Rough road surfaces
* Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge
* Incorrect installation of aftermarket, add on accessories
Visual/Physical Check
* Verify that the control module grounds are clean, tight, and correctly located.
* Verify that the vacuum hoses are not split or kinked, and are properly connected.
* Verify that the air filter is clean and free from restrictions.
* Verify that there is no water intrusion in connectors terminals and components.
* Inspect the air intake ducts for the following conditions:
- Collapsed
- Damaged areas
- Looseness
- Incorrect installation
- Leaking
* 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:
- Poor connections
- Pinches
- Cuts
* 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:
- Correct fuel system operation and fuel pressure-Refer to Fuel System Diagnosis Fuel System Diagnosis.
- Correct fuel injectors operation-Refer to Fuel Injector Solenoid Coil Test Fuel Injector Solenoid Coil Test.
- A fuel quality condition-Refer to Alcohol/Contaminants-in-Fuel Diagnosis Testing and Inspection.
* Test the ignition system for the following:
- Spark plugs for incorrect heat range or an abnormal condition-Refer to Spark Plug Inspection Testing and Inspection and Ignition System Specifications [1][2]Specifications.
- Coolant or oil fouled spark plugs. For diagnosis-Refer to Coolant in Combustion Chamber Coolant in Combustion Chamber or Oil Consumption Diagnosis Oil Consumption Diagnosis.
- Secondary ignition system susceptible to moisture. Wet down the secondary ignition system with water from a spray bottle to help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied.
- Weak spark using the J 26792 - spark tester-Refer to Electronic Ignition System Diagnosis Electronic Ignition (EI) System Diagnosis.
* Inspect for the following conditions:
- Correct operation of the transmission torque converter clutch (TCC) - The scan tool should indicate an engine speed drop when the TCC is commanded ON. Refer to Torque Converter Diagnosis Torque Converter Diagnosis.
- The correct operation of the A/C compressor
- An engine that runs lean or rich-Observe the Fuel Trim parameters with a scan tool.
- Slow responding heated oxygen sensors (HO2S)-The HO2S should respond quickly to different throttle positions.
Caution: Refer to Heated Oxygen and Oxygen Sensor Caution Heated Oxygen and Oxygen Sensor Caution
- Water intrusion in the HO2S connector.
Note: The embossed arrows on the mass air flow (MAF) sensor indicate the direction of the intake air flow. The arrows must point toward the engine.
- Inspect the MAF sensor installation. A MAF sensor that is incorrectly installed may cause a hard start. Install the MAF in the proper direction. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
- Loose or incorrect mass air flow (MAF) sensor connections.
- Engine oil contaminated by fuel
- Split or kinked vacuum hoses-Verify that the routing and connections are correct.
- Excessive knock sensor (KS) system spark retard activity-Observe the scan tool Knock Retard parameter for activity greater than 0°.
* Inspect the exhaust system components for the following:
- Physical damage or possible internal failure
- Restricted three-way catalytic converters
For more information, refer to Symptoms - Engine Exhaust Symptoms - Engine Exhaust.
* Electromagnetic interference (EMI) on a reference circuit, which 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.
* Inspect the crankcase ventilation valve for proper operation-Refer to Crankcase Ventilation System Inspection/Diagnosis Testing and Inspection.
* Inspect for a stuck open evaporative emission (EVAP) canister purge solenoid valve
* Inspect the engine cooling system for the following conditions:
- A thermostat with an incorrect heat range-Refer to Thermostat Diagnosis Thermostat Diagnosis.
- Improper engine coolant level-If the level is low, refer to Loss of Coolant Loss of Coolant.
* Inspect the crankshaft position (CKP) sensor for an intermittent condition-Observe the scan tool CKP Resync parameter. The Resync parameter should remain at 0 during all operating conditions, and when moving the related harnesses and connectors between the CKP sensor and the ECM.
* Inspect the engine for the following mechanical failures:
Refer to Symptoms - Engine Mechanical Symptoms - Engine Mechanical.
- Excessive oil in the combustion chamber or leaking valve seals
- Oil consumption
- Incorrect cylinder compression
- Sticking or leaking valves
- Worn camshaft lobes
- Incorrect valve timing
- Worn rocker arms
- Broken valve springs
- Excessive carbon buildup in the combustion chambers-Clean the chambers with top engine cleaner, if necessary. Follow the instructions on the can.
- Incorrect engine parts
* If the above conditions do not address the symptom, refer to the additional symptoms tests.
Additional Symptoms Tests
Detonation/Spark Knock
* Inspect for excessive carbon buildup in the combustion chambers. Clean the chambers with Top Engine Cleaner, if necessary. Follow the instructions on the can.
* Test for an engine overheating condition. Refer to Engine Overheating Engine Overheating.
* Verify that the engine coolant temperature (ECT) has not shifted in value. Allow the engine to run and reach operating temperature. Observe the ECT Sensor parameter with a scan tool and compare the reading to that parameter listed on the Engine Control Module Scan Tool Information (LLT) Scan Tool Testing and Procedures list. If the reading is not in the range specified in the list, test the resistance of the ECT sensor. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor Temperature Versus Resistance - Engine Coolant Temperature Sensor for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement Service and Repair. If the sensor is within the specification, test the ECT sensor circuits for high resistance.
* If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicle minimum octane requirements.
ECM Commanded Reduced Engine Power
Under certain conditions the ECM may limit engine power by reducing engine torque and, for some vehicles, fuel pressure as well. For most, but not all of the conditions, the ECM will illuminate the reduced engine power lamp on the instrument panel cluster, however a DTC may not be set. Observe the scan tool Reduced Engine Power History parameter, if applicable, or refer to Engine Control Module Scan Tool Information (LLT) Scan Tool Testing and Procedures to determine the reason for the reduced engine power event.
Verify or inspect for the following:
* Vehicle being operated at sustained high engine speeds, or, towing heavy loads up an incline for an extended period of time, which may cause the engine oil or coolant to overheat. A repair may not be necessary. Inspect the airflow passageways in front of the engine for obstructions and clear away any debris or foreign material that is found. If no obstructions are found, review approved driving habits with the customer. The customer may need to operate the vehicle at a higher engine speed to improve cooling system performance, or, at a slower engine speed to reduce engine load.
* A cooling fan condition which may cause the ECM to reduce engine power. Refer to Cooling Fan Description and Operation Cooling Fan and Cooling System Description and Operation Cooling System to verify correct operation of the cooling fan.
* Temporarily reduced engine power. Under extremely cold ambient temperatures some SIDI equipped vehicles may experience ECM commanded reduced engine power for a few minutes during engine warm-up. This would be a normal condition, noticeable only at wide open throttle, and the reduced engine power lamp would not be illuminated.
* If the vehicle is equipped with the OnStar(R) feature, reduced engine power may be due to OnStar(R) remote command. Verify the vehicle is not in the OnStar(R) initiated Stolen Vehicle Slowdown mode.
* Maximum recommended engine oil life exceeded. Refer to the Owners Manual for scheduled maintenance recommendations.
* Excessively high fuel pressure condition. Refer to Fuel System Description Fuel System Description and Fuel System Diagnosis Fuel System Diagnosis to verify fuel system operation
* Intake air temperature out of range too high. Observe the scan tool IAT Sensor 1 and IAT Sensor 2 parameters. The readings should be within 9°C (16°F) of a each other. Refer to DTC P0096 or P0111 for more information.
* Fuel temperature out of range too high. Observe the scan tool Fuel Temperature Sensor parameter and refer to DTC P0178 and DTC P0179 for more information.
* An incorrect learned brake pedal position (BPP) sensor. Verify the following actions have not occurred:
- An engine control module (ECM) has been replaced
- A BPP sensor has been replaced
- A BPP sensor is misaligned or incorrectly installed
If any of the above actions have occurred, the Brake Pedal Position Sensor Learn Programming and Relearning procedure must be performed.
Fuel Odor
* Inspect for a saturated EVAP canister-Refer to Evaporative Emission Control System Description Description and Operation.
* Inspect for a condition with the internal components of the fuel tank assembly-Refer to Fuel System Description Fuel System Description.
* Inspect for leaking, damaged, or deteriorated fuel lines.
Hard Start
* Observe the scan tool TBI Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning Procedures.
* 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 ± 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 - Engine Coolant Temperature Sensor Temperature Versus Resistance - Engine Coolant Temperature Sensor for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement Service and Repair. If the sensor is within the specification, test the ECT sensor circuits for high resistance.
* Verify that the fuel system has adequate pressure for engine start-up. With a scan tool, compare the Fuel Pressure Sensor parameter value to the Desired Fuel Pressure parameter value when the ignition key is first turned to ON. Refer to Fuel System Diagnosis Fuel System Diagnosis.
* Inspect for excessive crankshaft endplay that will cause the crankshaft position (CKP) sensor reluctor wheel to move out of alignment with the CKP sensor. Refer to Crankshaft and Bearing Cleaning and Inspection Crankshaft and Bearing Cleaning and Inspection and Engine Mechanical Specifications Service Limits & General Specifications.
Hesitation, Sag, Stumble
* Test the fuel pressure. Refer to Fuel System Diagnosis Fuel System Diagnosis.
* Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
* Test the generator. Refer to Symptoms - Engine Electrical Symptoms - Engine Electrical. Repair the charging system if the generator output voltage is less than 9 volts or more than 16 volts.
* Inspect for excessive crankshaft endplay that will cause the crankshaft position (CKP) sensor reluctor wheel to move out of alignment with the CKP sensor. Refer to Crankshaft and Bearing Cleaning and Inspection Crankshaft and Bearing Cleaning and Inspection and Engine Mechanical Specifications Service Limits & General Specifications.
Note: The following actions may need to be repeated in order to verify an intermittent intake or exhaust camshaft actuator condition.
* Engine warm and idling, verify the correct operation of the camshaft actuator system. Command the intake and exhaust camshaft actuators for each cylinder bank from 0 degrees to 25 degrees and back to zero while observing the appropriate scan tool Intake CMP Variance and Exhaust CMP Position Variance parameters. Each parameter should be less than 2 degrees in each of the commanded states.
• If any of the parameters is greater than 2 degrees, inspect the suspect camshaft actuator and camshaft actuator solenoid valve and valve bore for contamination, obstruction, and damage. Refer to Camshaft Timing Drive Components Cleaning and Inspection Camshaft Timing Drive Components Cleaning and Inspection and Camshaft Actuator System Description Description and Operation for additional information.
Lack of Power, Hesitation, or Cut Out
* Inspect the engine electrical system for correct operation. Refer to Symptoms - Engine Electrical Symptoms - Engine Electrical.
* Verify that each injector harness is connected to the correct injector.
* Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
Note: The following actions may need to be repeated in order to verify an intermittent intake or exhaust camshaft actuator condition.
* Engine warm and idling, verify the correct operation of the camshaft actuator system. Command the intake and exhaust camshaft actuators for each cylinder bank from 0 degrees to 25 degrees and back to zero while observing the appropriate scan tool Intake CMP Variance and Exhaust CMP Position Variance parameters. Each parameter should be less than 2 degrees in each of the commanded states.
• If any of the parameters is greater than 2 degrees, inspect the suspect camshaft actuator and camshaft actuator solenoid valve and valve bore for contamination, obstruction, and damage. Refer to Camshaft Timing Drive Components Cleaning and Inspection Camshaft Timing Drive Components Cleaning and Inspection and Camshaft Actuator System Description Description and Operation for additional information.
Poor Fuel Economy
* Inspect for heavy loads being carried or towed
* Inspect for acceleration rate too much or too often
* Inspect the brake system for brake drag.
* Inspect for incorrect operation of the speedometer.
* Verify that the engine coolant temperature (ECT) has not shifted in value. Allow the engine to run and reach operating temperature. Observe the ECT Sensor parameter with a scan tool and compare the reading to the parameter listed in the Engine Control Module Scan Tool Information (LLT) Scan Tool Testing and Procedures list. If the reading is not in the range specified in the list, test the resistance of the ECT sensor. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor Temperature Versus Resistance - Engine Coolant Temperature Sensor for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement Service and Repair. If the sensor is within the specification, test the ECT sensor circuits for high resistance.
* Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
* Observe the scan tool TBI Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning Procedures.
Rough, Unstable, or Incorrect Idle and Stalling
* An excessively high idle may be due to the floor mat interfering with the accelerator pedal. With this condition present, it may not be possible to shift the transmission into gear. Inspect the accelerator pedal for binding and verify that the floor mat is not interfering with the accelerator pedal movement.
* Observe the scan tool TBI Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning Procedures.
* Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
* Inspect the engine mounts. Refer to Engine Mount Inspection Testing and Inspection.
* Inspect the intake and exhaust manifolds for casting flash.
Note: The ECM uses the bank 1 intake camshaft position sensor to determine injector and ignition system synchronization as well as to determine camshaft to crankshaft relationship. If the bank 1 intake camshaft position sensor signal to the ECM is intermittent, a stalling condition will occur and a DTC will not set.
* Inspect all bank 1 intake camshaft position sensor wiring for poor connections. Start the engine and observe for engine stalling while wiggling the bank 1 intake camshaft position sensor wiring harness. Refer to Inducing Intermittent Fault Conditions Inducing Intermittent Fault Conditions for further diagnosis.
Surges/Chuggles
* Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair.
* Inspect for slow responding 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 PCM reduces the amount of fuel delivered to the engine, causing a driveability condition.
* Verify that each injector harness is connected to the correct injector.