Additional Symptoms Tests

Detonation/Spark Knock

• Test for an engine overheating condition. Refer to Engine Overheating .
• 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.

Engine Control Module (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, however a DTC may not be set.

A repair may not be necessary. Observe the scan tool Reduced Engine Power History parameter, if applicable, or refer to K20 Engine Control Module: Scan Tool Information 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 engine control module to reduce engine power - Refer to Cooling Fan Description and Operation , and Cooling System Description and Operation (LUV, LE2)Cooling System Description and Operation (2HO) to verify correct operation of the cooling fan.
• 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 to verify high pressure fuel system pressure and operation.
• 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.
• 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).
• Fuel temperature out of range too high
• 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 Calibration procedure must be performed.

Fuel Odor

• Inspect for leaking, damaged, or deteriorated fuel lines.
• Inspect for a saturated EVAP canister.
• Inspect for a condition with the internal components of the fuel tank assembly.

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.
• 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 - Engine Coolant Temperature Sensor 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.
• Verify that the fuel system operates correctly during engine start-up. The fuel pump should turn ON for 2 S when the ignition is turned to ON and fuel pressure should be as specified. For additional fuel system information, refer to Fuel System Description.

Hesitation, Sag, Stumble

• Test the fuel pressure.
• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage.
• Test the manifold absolute pressure (MAP) sensor.
• 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.
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 from 0 degrees to 20 degrees and back to zero while observing the scan tool Intake Camshaft Position Variance and Exhaust Camshaft 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.

Poor Fuel Economy

• 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.
• Inspect for heavy loads being carried or towed
• Inspect for acceleration rate too much or too often
• 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.
• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage.
• 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 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 Throttle Body Idle Air Flow Compensation Reset procedure must be performed.

• Inspect the engine mounts.
• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage.
• Inspect the intake and exhaust manifolds for casting flash.
• Inspect the exhaust system for damaged or collapsed pipes
• Inspect engine coolant for a slushy or frozen condition. Refer to Cooling System Draining and Filling

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.
• 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 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.
• 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.