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 panel cluster, however a DTC may not be set. 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. 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 engine control module to reduce engine power-Refer to Cooling Fan Description and Operation (LWE, LUW) and Cooling System Description and Operation 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 out of range-Refer to Fuel System Diagnosis , and Fuel System Description to verify high pressure fuel system pressure and operation.
• Maximum recommended engine oil life exceeded-Refer to the Owners Manual for scheduled maintenance recommendations.
• 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). Refer to DTC P0112, P0113, or P0114 for additional information.
• 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 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

• 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 (ECT) 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 in Fuel System Diagnosis , procedure. For additional fuel system information, refer to Fuel System Description .

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 Replacement .
• Test the manifold absolute pressure (MAP) sensor. Refer to DTC P0106 .
• 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.

Poor Fuel Economy

• 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. Refer to Mass Airflow Sensor Replacement .
• Inspect the brake system for brake drag.

Poor Fuel Fill Quality

• Inspect for restricted vent lines
• Inspect for high fuel temperature
• Inspect for a condition with the internal components of the fuel tank assembly
• For more information refer to the following:
  • Engine Controls Schematics
  • Fuel System Description
  • Evaporative Emission Control System Description

Rough, Unstable, or Incorrect Idle and Stalling

• Inspect the engine mounts Engine Mount Replacement .
• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor Replacement .
• Inspect the intake and exhaust manifolds for casting flash.
• Inspect the exhaust system for damaged or collapsed pipes
• 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.
• 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.

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 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.