Additional Symptoms Tests

Detonation/Spark Knock

• Test the engine for an overheating condition. Refer to Symptoms - Engine Cooling .
• Inspect for excessive carbon buildup in the combustion chambers. Clean the chambers with Top Engine Cleaner, if necessary. Follow the instructions on the can.
• 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 (Without Diesel) 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 .
• Temporary 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.
• Fuel temperature out of range too high.
• If equipped with manual transmission, reduced engine power may be due to overheated transmission oil. Allow the transmission oil to cool down. Refer to the Owners Manual for scheduled maintenance recommendations to determine if the transmission is due for servicing.
• High pressure fuel system condition. Refer to Fuel System Diagnosis , and Fuel System Description to verify high pressure fuel system pressure and operation.
• Reduced engine power due to OnStar® remote command. Verify the vehicle is not in the OnStar® initiated Stolen Vehicle Slowdown mode. Refer to OnStar Stolen Vehicle Slowdown Active , and Remote Vehicle Speed Limiting Description and Operation for additional information.
• Intake air temperature out of range too high. Engine running, observe the scan tool IAT sensor parameter. The readings should be between -40° C and +149° C (-40° F and +300° F. Refer to DTC P0112, P0113, or P0114 for additional information.
• An incorrect learned brake pedal position (BPP) sensor. Verify the following actions have not occurred :
  • An engine control module 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 Calibration procedure must be performed.

Fuel Odor

• Inspect for leaking damaged, or deteriorated fuel lines and components.
• 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 Throttle Body Idle Airflow Compensation parameter with a scan tool. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and clean, if necessary. Refer to Throttle Body 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 circuits for a high resistance.
• Test the idle air control (IAC) system.
• Verify that the fuel pump provides the correct pressure for engine start-up and operation. The fuel pump should turn ON for 2 seconds when the ignition is turned ON. Refer to Fuel System Diagnosis .
• Inspect for excessive crankshaft endplay that will cause the crankshaft position sensor reluctor wheel to move out of alignment with the crankshaft position sensor. Refer to Crankshaft and Bearing Cleaning and Inspection .

Hesitation, Sag, Stumble

• Test the fuel pressure. Refer to Fuel System Diagnosis .
• Test the manifold absolute pressure (MAP) sensor. Refer to DTC P0106, P0107, or P0108 .
• 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.

Poor Fuel Economy

• Inspect for heavy loads being carried or towed.
• Inspect for acceleration rate too much or too often.
• Observe the Throttle Body Idle Airflow Compensation parameter with a scan tool. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and clean, if necessary. Refer to Throttle Body Cleaning .
• Inspect for incorrect operation of the speedometer.
• Inspect the brake system for brake drag.

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 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 Throttle Body Idle Air Flow Compensation Reset procedure must be performed.

• Inspect the engine mounts looseness, wear, and damage.

Surges/Chuggles

• 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 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 ECM reduces the amount of fuel delivered to the engine, causing a driveability condition. Refer to DTC P0131, P0132, P0137, P0138, P0151, P0152, P0157, or P0158 , and Refer to DTC P013A-P013F, P014A, P014B, P015A-P015D, or P2270-P2273