Additional Symptoms Tests

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 (LM2) 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 , or Cooling Fan Description and Operation , and Cooling System Description and Operation (LM2) to verify correct operation of the cooling fan.
• 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 .
• Maximum recommended engine oil life exceeded. Refer to the Owners manual for scheduled maintenance recommendations.
• High pressure fuel system fuel pressure condition. Refer to Fuel System Diagnosis , Fuel System Description , and Fuel System Specifications to verify high pressure fuel system operation.
• Turbocharger boost pressure out of range. Refer to Compressor Air Intake Turbocharger System Description .
• A diesel exhaust after treatment system condition. Refer to Exhaust Aftertreatment System Description .
• A diesel exhaust after treatment reductant fluid condition. Inspect for any of the conditions listed below:
  • Reductant system malfunction. Verify the correct operation of the system and inspect for tampering. Refer to Diesel Exhaust Fluid (DEF) Warning Strategy (Diesel Exhaust Fluid (DEF) Level) , or Diesel Exhaust Fluid (DEF) Warning Strategy (Anti-Tempering) , or Diesel Exhaust Fluid (DEF) Warning Strategy (Diesel Exhaust Fluid (DEF) Quality) , or Diesel Exhaust Fluid (DEF) Warning Strategy (Emission Control) , and Reductant System Tamper Service Bay Test .
  • Reductant fluid contaminated, poor quality, or level too low. Refer to Contaminants-in-Diesel Exhaust Fluid Diagnosis .

Fuel Knock/Combustion Noise

• Inspect for obvious overheating problems. Refer to Engine Overheating (LM2) .
• Inspect for a low engine coolant level.
• Inspect for any restricted air flow through the radiator. Refer to Engine Radiator Cleaning .
• Inspect for a malfunctioning or incorrect thermostat.
• Inspect for a correct coolant solution. The solution should be a 50/50 mix of anti-freeze and water.

Hard Start

• Inspect for a slow cranking speed. Refer to Symptoms - Engine Electrical .
• Verify the engine electrical system for correct operation. Refer to Symptoms - Engine Electrical .
• Verify that the driver is using the correct starting procedure as described in the owners manual.
• Inspect for a restriction in turbocharger inlet duct.
• Inspect for a restriction or a leak in the intake manifolds.
• Inspect the exhaust system for a possible restriction. Refer to Restricted Exhaust .
• Inspect for excessive crankshaft endplay that will cause the CKP sensor reluctor wheel to move out of alignment with the CKP sensor.

Hesitation, Sag, Stumble

• 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.
• Inspect for excessive crankshaft endplay that will cause the CKP sensor reluctor wheel to move out of alignment with the CKP sensor.

Lack of Power, Hesitation, or Cut Out

• Compare the vehicle with a similar unit. Ensure the vehicle has an actual problem.
• Inspect the engine oil level and quality.
• Inspect for auxiliary fuel filters. Aftermarket fuel filters may restrict fuel flow.
• Inspect for a skewed mass air flow (MAF) sensor. Refer to Diagnostic Aids in DTC P0101 .
• Inspect for proper transmission operation.
• Inspect for a restriction in the charge air cooler. Refer to Charge Air Cooler Diagnosis 
• Inspect for an air leak in the charge air cooler. Refer to Charge Air Cooler Diagnosis .
• Inspect the engine electrical system for correct operation. Refer to Symptoms - Engine Electrical .
• Inspect for a worn or damaged turbocharger turbine wheel, shaft or compressor wheel.
• Inspect that the exhaust gas recirculation (EGR) valve is fully closing. Remove the EGR valve and visually inspect that there is no light seen between the valves and the valve seats.
• Verify that the EGR valve does not bind when manually moving the valve.
• Inspect the torque converter clutch (TCC) operation.
• Inspect the air conditioning system for proper operation. Refer to Air Conditioning (A/C) System Performance Test (LM2) .
• Inspect the exhaust system for a possible restriction.

Poor Fuel Economy

• Suggest to the driver to read the Important Facts on Fuel Economy in the Owner Manual.
• Confirm proper interpretation of the Average Fuel Economy value on the DIC. The Fuel Economy value should be reset periodically. Particularly if driving patterns, including load, terrain, etc., have changed significantly since the last Fuel Economy Reset.
• Suggest to the owner to fill the fuel tank and inspect the fuel economy.
• The following conditions can reduce fuel economy:
  • Aftermarket body accessories that can affect wind flow over the vehicle. Including air deflectors, flares, etc.
  • Heavy loads being carried or towed, or the shape of large loads
  • Extended idle time
  • Acceleration rate too much or too often
  • Incorrect tire size or air pressure
  • Improper fuel quality and rating - Refer to Contaminants-in-Fuel Diagnosis , and Fuel System Specifications .
  • Aftermarket ECM calibrations
  • Certain aftermarket air filters or air induction systems
  • An air leak in the induction or engine systems - Refer to full system air leak test in Charge Air Cooler Diagnosis .
  • Fuel system issues - Inspect the injector balance rates. Balance rates should be between +4 mm³ and -6.9 mm³ in Park or Neutral.
  • Brake system drag
  • Air conditioning system improper operation
  • Incorrect operation of the speedometer

Rough, Unstable, or Incorrect Idle and Stalling

• Inspect the engine mounts.
• Inspect the intake and exhaust manifolds for casting flash.
• The exhaust system for damaged or collapsed pipes
• The exhaust manifold for a collapsed inner wall
• The mufflers for heat distress or possible internal failure
• EMI on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM, with little change in actual engine RPM change, indicates that EMI is present. If a problem exists, inspect routing of high voltage components, such as fuel injector wiring, near the sensor circuits.
• Inspect the park neutral position (PNP) switch circuit.

Surges/Chuggles

• Verify the driver understands the torque converter clutch (TCC) operation.
• Verify the driver understands the A/C compressor operation.
• Use the scan tool to ensure the vehicle speed sensor (VSS) reading matches the speedometer. This excludes vehicles with electronic transmissions where some variation between VSS and the speedometer is normal.

Excessive Smoke (Black)

• The DPF is damaged and must be replaced. Inspect the following possible causes that may have damaged the DPF.
• Inspect for an air leak in the charge air cooler or the air ducts between the turbocharger and the intake manifold. Refer to Charge Air Cooler Diagnosis .
• Inspect for a restriction in the turbocharger charge air cooler.
• Inspect for a restriction in the intake manifold.
• Inspect for excessive oil entering the combustion chamber. Refer to Oil Consumption Diagnosis .
• Inspect for a stuck open EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement - Exhaust Manifold .
  • Visually inspect the EGR valve and verify the valve is in the closed position.
  • Verify that the EGR valve does not bind when manually moving the valve.

Excessive Smoke (White)

• Check the coolant level in the reservoir. White coolant smoke may be mistaken for blue/gray smoke. If the coolant level is low, refer to Loss of Coolant .
• If excessive smoke is present, inspect for a stuck open fuel injector. Refer to: Fuel System Diagnosis 
• If the ECM glow plug control module (GPCM) or a fuel injector has recently been replaced, it may be necessary to confirm the fuel injector flow rate values. Refer to Fuel Injector Flow Rate Programming .

Excessive Smoke (Blue, Gray, or White)

Blue, gray, or white smoke during regeneration may indicate a fuel with high sulfur content.