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 .
• 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 K20 Engine Control Module: Scan Tool Information (LLT) , 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 , for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement . 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 engine control module 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 engine control module will illuminate the reduced engine power lamp on the instrument cluster. If equipped with the driver information center feature, a reduced engine power message may be displayed as well. A DTC may not be set.

A repair may not be necessary. Observe the scan tool Reduced Engine Power History parameter or refer to K20 Engine Control Module: Scan Tool Information (LLT) 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. 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 , and Cooling System Description and Operation 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® feature, reduced engine power may be due to OnStar® remote command. Verify the vehicle is not in the OnStar® initiated Stolen Vehicle Slowdown mode.
• High pressure fuel system condition. Refer to Fuel System Description , and Fuel System Diagnosis  to verify high pressure 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 procedure must be performed.

Fuel Odor

• Inspect for leaking, damaged, or deteriorated fuel lines.
• Inspect for a saturated EVAP canister-Refer to Evaporative Emission Control System Description .
• Inspect for a condition with the internal components of the fuel tank assembly-Refer to Fuel System Description .

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 .
• 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 , for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement . 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 .
• 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 , and Engine Mechanical Specifications .

Hesitation, Sag, Stumble

• Test the fuel pressure. Refer to 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 .
• Test the generator. Refer to 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 , and Engine Mechanical 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 20 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 , and Camshaft Actuator System Description for additional information.

Lack of Power, Hesitation, or Cut Out

• Inspect the engine electrical system for correct operation. Refer to 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 .
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 20 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 , and Camshaft Actuator System Description 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 K20 Engine Control Module: Scan Tool Information (LLT) , 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 , for resistance specifications. Replace the ECT sensor if the resistance is not within specification. Refer to Engine Coolant Temperature Sensor Replacement . 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 .
• 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 .

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.
• Engine idle speed may be unstable or the engine may stall if the ECM has learned an incorrect idle/airflow compensation value. A DTC may also set. Observe the scan tool Throttle Body Idle Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. If the throttle body needs cleaning, refer to Throttle Body Inspection and Cleaning .

  The actions listed below may also cause the ECM to learn an incorrect idle value.

  • The engine control module has been replaced
  • The throttle body has been replaced
  • The throttle body has been cleaned but the idle learn procedure was not performed after completing the cleaning
  • The air induction system was leaking, allowing non-metered air to enter the combustion chamber. The leak was repaired but the idle learn procedure was not performed after repairing the leak

    If any of the actions listed above have occurred, the Q38 Throttle Body: Throttle/Idle Learn procedure must be performed.

• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement .
• Inspect the engine mounts. Refer to Engine Mount 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 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 .
• 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.