SYMPTOMS - ENGINE CONTROLS
Document ID: 2655294
Diagnostic Instructions
Symptoms Description
Symptoms cover conditions that are not covered DTCs. Certain conditions can cause multiple symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause certain symptoms are listed separately under Additional Symptoms Test. Perform the Symptoms Testing before using the Additional Symptoms Tests. Poor Fuel Fill Quality test may be performed separately from the Symptoms Testing and Additional Symptoms Tests procedures.
Symptoms Definition
Backfire
Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.
Cuts Out, Misses
A steady pulsation or jerking that follows engine speed, which is usually more pronounced as the engine load increases. This condition is not normally felt above 1500 RPM or 48 km/h (30 mph). The exhaust has a steady spitting sound at idle or at low speed.
Engine Control Module (ECM) Reduced Engine Power
The ECM illuminates the Reduced engine Power lamp and will limit engine power under potential engine/vehicle damaging or emissions related conditions. A DTC may not be set.
Detonation/Spark Knock
A mild to severe ping which usually occurs worse while under acceleration. The engine makes sharp metallic knocks that change with throttle opening.
Dieseling, Run-On
The engine continues to run after the key is turned OFF, but runs very rough.
Hard Start
Engine cranks OK, but does not start for a long time. The vehicle does eventually run, or may start but immediately stalls.
Hesitation, Sag, Stumble
Momentary lack of response as the accelerator is pushed down. This condition can occur at any vehicle speed. This condition is usually more pronounced when first trying to make the vehicle move, as from a stop. This condition may cause the engine to stall in severe conditions.
Lack of Power, Sluggishness, or Sponginess
The engine delivers less than expected power. Little or no increase in speed when the accelerator pedal is pushed down part way.
Poor Fuel Economy
Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.
Rough, Unstable, or Incorrect Idle and Stalling
The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle may vary in speed. Either condition may be severe enough to stall the engine.
Surges/Chuggles
Engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator pedal position.
Symptoms Verification
Before using the Symptom tables, perform the following inspections:
- Verify the engine control module (ECM) and malfunction indicator lamp (MIL) are operating correctly.
- Verify there are no DTCs that are stored.
- Verify the scan tool data is within a normal operating range. Refer to Control Module References for scan tool information.
- Verify the customer concern.
- Perform the Visual/Physical Inspection in this section. The visual/physical inspection is extremely important, and can lead to correcting a condition without additional testing. It may also help reveal the cause of an intermittent condition.
Identifying Intermittent Conditions
Many intermittent conditions occur with harness or connector movement due to engine torque, rough pavement, vibration or physical movements of a component. Refer to the following for a list of issues that may cause an intermittent condition:
- Moisture and water intrusion in connectors, terminals, and components
- Incomplete connector mating
- Poor terminal contact
- High circuit or component resistance—High resistance can include any resistance, regardless of the amount, which can interrupt the operation of the component.
- Harness that is too short or tight
- Wire insulation that is chaffed or cut
- High or low ambient temperature
- High or low engine coolant temperatures
- High underhood temperatures
- Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load
- High or low system voltage
- High vehicle load conditions
- Rough road surfaces
- Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge
- Incorrect installation of aftermarket, add on accessories
- Visual/Physical Check
- Verify the control module grounds are clean, tight, and correctly located.
- Verify the vacuum hoses are not split or kinked, and are properly connected, as shown on the Vehicle Emission Control Information label.
- Verify the air filter is clean and free from restrictions.
- Verify there is no water intrusion in connectors terminals and components.
- Inspect the air intake ducts for the following conditions:
- Collapsed
- Damaged areas
- Looseness
- Incorrect installation
- Leaking
- Inspect for air leaks at the throttle body mounting area, the mass air flow (MAF) sensor and intake manifold sealing surfaces.
- Inspect the wiring harness for the following conditions:
- Poor connections
- Pinches
- Cuts
- Inspect for loose, damaged, unseated, or missing sensors/components.
- Inspect the terminals for corrosion and correct contact.
Symptoms Testing
- Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Engine Control Module (ECM) Commanded Reduced Engine Power, Fuel Odor, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles
- Test the fuel system for the following:
- Test the ignition system for the following conditions:
- Inspect for the following conditions:
- Incorrectly operating transmission torque converter clutch (TCC)—The scan tool should indicate an engine speed drop when the TCC is commanded ON.
- Incorrectly operating A/C compressor
- Items that can cause an engine to run rich or lean—Refer to DTC P0171, P0172, P0174, or P0175 for more information.
- Slow responding heated oxygen sensors (HO2S). The HO2S should respond quickly to different throttle positions—Refer to DTC P0133, P013A-P013F, P014A, P014B, P0153, P015A, P015B, P015C, or P015D for more information.
- Caution: Refer to Heated Oxygen and Oxygen Sensor Caution.
- Water intrusion in the HO2S connector
- Incorrec tlyoperating cylinder deactivation system, if equipped. Refer to Cylinder Deactivation (Active Fuel Management) System Diagnosis.
- Note: The embossed arrows on the mass airflow (MAF) sensor indicate the direction of the intake air flow. The arrows must point toward the engine.
- Incorrect MAF sensor installation. A MAF sensor that is incorrectly installed may cause a hard start. Install the MAF in the proper direction. Refer to Mass Airflow Sensor with Intake Air Temperature Sensor Replacement.
- Incorrect MAF sensor connections.
- Engine oil contaminated by fuel
- Split or kinked vacuum hoses—Verify that the routing and connections are as shown on the Vehicle Emission Control Information label.
- Excessive knock sensor (KS) system spark retard activity—Refer to DTC P0324, P0325, P0326, P0327, P0328, P0330, P0332, or P0333.
- Electromagnetic interference (EMI) on the reference circuit, which can cause a misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter with little change in actual engine speed indicates that EMI is present. Inspect the high voltage components near the ignition control circuit if a condition exists.
- Incorrectly operating crankcase ventilation valve—Refer to Crankcase Ventilation System Inspection/Diagnosis.
- A stuck open evaporative emission (EVAP) canister purge solenoid.
- Inspect the exhaust system components for the following conditions:
- Physical damage or possible internal failure
- Restricted three-way catalytic converters.
For more information, refer to Symptoms - Engine Exhaust.
- Inspect the engine cooling system for the following conditions:
- Inspect the engine for the following mechanical conditions:
- Excessive oil in the combustion chamber or leaking valve seals
- Incorrect cylinder compression
- Sticking or leaking valves
- Worn camshaft lobes
- Incorrect valve timing
- Worn rocker arms
- Broken valve springs
- Excessive carbon buildup in the combustion chambers—Clean the chambers with top engine cleaner, if necessary. Follow the instructions on the can.
- Incorrect engine parts
For more information, refer to Symptoms - Engine Mechanical.
- If the above conditions do not address the symptom, refer to the additional symptoms tests.
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 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 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. 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. 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 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 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 for fuel pressure specifications and testing procedures.
- Hesitation, Sag, Stumble
- Test the fuel pressure. Refer to Fuel System Diagnosis 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 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 Cleaning.
- Rough, Unstable, or Incorrect Idle and Stalling
- Observe the TB 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 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, P015A, P015B, P015C, or P015D for more information.
