Symptoms - Engine Controls
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Symptoms - Engine Controls
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The symptom comprises conditions not covered by diagnostic trouble codes (DTC). Some conditions can cause several symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause specific symptoms are listed separately under additional symptoms testing. Perform the symptoms testing before using the additional symptoms testing.
Backfire
The 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 1 500 RPM or 48 km/h (30 MPH). The exhaust has a steady spitting sound at idle or at low speed.
Detonation/Spark Knock
A mild to severe ping which usually gets 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.
Engine Control Module (ECM) Commanded 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.
Fuel Odor
A noticeable smell of unburned fuel.
Hard Start
The 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
A 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 vehicle 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 speed may vary. 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.
Before using the Symptom tables, perform the following inspections:
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Ensure that the malfunction indicator lamp (MIL) is operating correctly. Use the scan tool to command the lamp ON and OFF.
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Ensure that there are no DTCs that are stored.
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Ensure that the scan tool data is within a normal operating range. Refer to
Control Module References
for scan tool information.
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Verify the customer concern.
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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.
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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:
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Moisture and water intrusion in connectors, terminals, and components
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Incomplete connector mating
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High resistance in circuits or components. High resistance can be a resistance, irrespective of size, that disrupts the function of components.
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Harness that is too short or tight
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Wire insulation that is chaffed or cut
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High or low ambient temperature
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High or low engine coolant temperatures
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High underhood temperatures
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Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load
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High or low system voltage
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High vehicle load conditions
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Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge
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Incorrect installation of aftermarket, add on accessories
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Visual/Physical Inspection
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Ensure that the control module grounds are clean, tight, and correctly located.
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Ensure that the vacuum hoses are not split or kinked, and are properly connected.
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Ensure that the air filter is clean and free from restrictions.
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Ensure that there is no water intrusion in connectors terminals and components.
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Inspect the air intake ducts for the following conditions:
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Inspect for air leaks at the throttle body mounting area, the mass air flow (MAF) sensor and intake manifold sealing surfaces.
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Inspect the wiring harness for the following conditions:
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Inspect for loose, damaged, unseated, or missing sensors/components.
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Inspect the terminals for corrosion and correct contact.
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Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles
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Test the fuel system for the following:
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Test ignition system for the following:
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Wet down the secondary ignition system with water from a spray bottle-Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied.
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Inspect for the following conditions:
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The proper operation of the transmission torque converter clutch (TCC) - The scan tool should indicate an engine speed drop when the TCC is commanded ON. Refer to
Torque Converter Diagnosis
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The proper operation of the A/C compressor.
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Slow responding heated oxygen sensors (HO2S). The HO2S should respond quickly to different throttle positions.
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Note
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The embossed arrows on the mass air flow (MAF) sensor indicate the direction of the intake air flow. The arrows must point toward the engine.
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Inspect the 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 Replacement
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Inspect the mass air flow (MAF) sensor connections.
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Engine oil contaminated by fuel
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Vacuum hoses for splits or kinks-Verify that the routing and connections are correct.
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The exhaust system components for the following:
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Physical damage or possible internal malfunction
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Electromagnetic interference (EMI) on the reference circuit 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.
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Evaporative emission (EVAP) canister purge solenoid that is stuck open
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The engine cooling system for the following conditions:
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Improper engine coolant level-If the level is low, refer to
Loss of Coolant
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The crankshaft position sensor for an intermittent condition-Observe the scan tool crankshaft position Resync parameter. The Resync parameter should remain at 0 during all operating conditions, and when moving the related harnesses and connectors between the crankshaft position sensor and the ECM.
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Excessive oil in the combustion chamber or leaking valve seals
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Incorrect cylinder compression
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Sticking or leaking valves
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Excessive carbon buildup in the combustion chambers-Clean the chambers with top engine cleaner. Follow the instructions on the can.
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If the above conditions do not address the symptom, refer to the additional symptoms tests.
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Additional Symptoms Tests
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Hard Start
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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 - Engine Coolant Temperature Sensor
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 sensor circuits for high resistance.
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Check that the fuel system has sufficient pressure for engine start-up. See
Diagnostics fuel system
for fuel pressure specifications.
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Hesitation, Sag, Stumble
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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.
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Note
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The following actions may need to be repeated in order to verify an intermittent intake or exhaust camshaft actuator condition.
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Engine warm and idling, verify the correct operation of the camshaft actuator system. Command the intake and exhaust camshaft actuator for each cylinder bank from 0 to 25 degrees and back to 0 while observing the appropriate scan tool Intake Camshaft Position Variance and Exhaust Camshaft Position Variance parameters. Each parameter should be less than 2 degrees in each of the commanded states.
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If any of the parameters is greater than 2 degrees, inspect the suspect camshaft actuator and camshaft actuator solenoid valve and valve bore for contamination, obstruction, and damage. Refer to the appropriate link below:
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Poor Fuel Economy
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Heavy loads being carried or towed
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Acceleration rate too much or too often
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Inspect the brake system for brake drag.
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Inspect for incorrect operation of the speedometer.
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Detonation/Spark Knock
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Inspect for excessive carbon buildup in the combustion chambers. Clean the chambers with Top Engine Cleaner. Follow the instructions on the can.
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If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicle minimum octane requirements.
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Lack of Power, Sluggishness, or Sponginess
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Verify that each injector harness is connected to the correct injector.
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Rough, Unstable, or Incorrect Idle and Stalling
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Inspect the engine mounts.
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Inspect the intake and exhaust manifolds for casting flash.
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Note
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The ECM uses the cylinder 1 intake camshaft position sensor to determine camshaft to crankshaft relationship in order to control fuel injector and ignition system synchronization. If the cylinder 1 intake camshaft position sensor signal to the ECM is intermittent, a stalling condition will occur and a DTC will not set.
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Inspect all cylinder 1 intake camshaft position sensor wiring for poor connections. Start the engine and check whether the engine stops when you move the harness to the intake camshaft position sensor for cylinder 1. Refer to
Inducing Intermittent Failure Conditions
for further diagnosis.
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Surges/Chuggles
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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 that results in a high – but false – signal voltage, which gives a rich exhaust indication. The PCM reduces the amount of fuel delivered to the engine, causing a driveability problem.
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Verify that each injector harness is connected to the correct injector.
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Fuel Odor
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Inspect for damaged or deteriorated fuel lines.
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ECM Commanded Reduced Engine Power
Refer to
Engine Control Module Scan Tool Information (LCD, LBX or LBS)
Engine Control Module Scan Tool Information (LBY)
Engine Control Module Scan Tool Information (LHU)
Engine Control Module Scan Tool Information (LLU)
for various conditions that may cause reduced engine power.
