Additional Symptoms Tests

Detonation/Spark Knock

• Test the engine for an 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.

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.

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 to determine the reason for the reduced engine power.

Verify or inspect for the following:

• Vehicle being driven inappropriately. Towing heavy loads up an incline for an extended period of time or operating the vehicle at sustained, excessively high engine speeds 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.
• 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.
• A cooling system condition which may cause the engine coolant to overheat. Refer to Hybrid/EV Cooling System Description and Operation to verify correct operation of the cooling system.
• High pressure fuel system condition. Refer to Fuel System Diagnosis , and Fuel System Description to verify high pressure fuel system operation.
• Intake air temperature 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
• Fuel temperature too high.
• A hybrid/EV propulsion system malfunction. Inspect for the following conditions:
  • Hybrid/EV battery pack charge too low. Review the scan tool Hybrid/EV Battery Pack Charge Remaining parameter for a Battery Power Low condition.
  • Drive motor or inverter temperature too high.
  • Engine powered Hybrid/EV battery charging unavailable. The Internal combustion engine (ICE) may be unable to charge the Hybrid/EV battery. Refer to the conditions listed below for a possible cause:
    • ICE has been run out of fuel. Review the scan tool Remaining Fuel in Tank parameter to verify that the engine has an adequate supply of fuel in the fuel tank.
    • Any ICE performance malfunction, such as an incorrectly operating ignition or fuel system. Review the appropriate scan tool parameters to help diagnose the condition.
    • A Hybrid/EV drive motor 1 (A) or 2 (B) malfunction.
    • An incorrectly operating transmission clutch.
  • An incorrectly operating transmission fluid pump. Inspect for the following conditions:
    • Incorrect transmission fluid level
    • Plugged or restricted transmission fluid coolers or pipes
  • For additional information, refer to Hybrid Modes of Operation Description .

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 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 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 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.
• Test the fuel pump operation. The fuel pump should turn ON for 2 s when attempting to start the engine. Refer to Fuel System Diagnosis .

Poor Fuel Economy

• Inspect for heavy loads being carried or towed.
• 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 bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning .

Rough, Unstable Idle and/or Stalling

• 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 engine mounts for looseness, wear, and damage.