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.

Engine Control Module (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. Observe the scan tool Reduced Engine Power History parameter or refer to Engine Control Module Scan Tool Information (LZ1 w/Tech 2) , Engine Control Module Scan Tool Information (V8 Gasoline) to determine the reason for the reduced engine power event.

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. 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 or conditions 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.
• Reduced engine power due to OnStar® remote command. A repair may not be necessary. Verify the vehicle is not in the OnStar® initiated Stolen Vehicle Slowdown mode. Refer to Remote Vehicle Speed Limiting Description and Operation , and OnStar Stolen Vehicle Slowdown Active .

Fuel Odor

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

Hard Start

• Observe the TB Idle Airflow Compensation parameter with a scan tool. A value greater than 80% 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 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 , 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.
• Verify that the fuel pump operates and provides adequate pressure for engine startup. The fuel pump should turn ON for 2 s when the ignition is turned to ON. Refer to Fuel System Diagnosis (Without FPCM) , Fuel System Diagnosis (FPCM)  for fuel pressure specifications and testing procedures.

Hesitation, Sag, Stumble

• Test the fuel pressure. Refer to Fuel System Diagnosis (Without FPCM) , Fuel System Diagnosis (FPCM)  for fuel pressure specifications and testing procedures..
• 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.
• Test the manifold absolute pressure (MAP) sensor. Refer to DTC P0106 .

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.
• Observe the TB Idle Airflow Compensation parameter with a scan tool. A value greater than 80% 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 Cleaning .

Poor Fuel Fill Quality

• Inspect for restricted vent lines
• Inspect for a stuck closed evaporative emission (EVAP) vent valve
• Inspect for high fuel temperature
• Inspect for a condition with the internal components of the fuel tank assembly

  For more information, refer to:

  • Fuel System Description
  • Evaporative Emission Control System Description .

Rough, Unstable, or Incorrect Idle and Stalling

• Observe the TB Idle Airflow Compensation parameter with a scan tool. A value greater than 80% 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 Cleaning .
• Inspect the engine mounts-Refer to Engine Mount Inspection .

Surges/Chuggles

• Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage-Refer to DTC P0101 , and DTC P0102 or P0103 for more information.
• 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 P0133, P013A-P013F, P014A, P014B, P0153, P1133, or P1153 (LC9, LMG, L96) , or DTC P0133, P013A-P013F, P014A, P014B, P0153, P015A, P015B, P015C, or P015D (L9H, L94) for more information.