Symptoms - Engine Controls
Symptoms - Engine ControlsImportant Preliminary Inspections Before Starting
Perform Diagnostic System Check - Vehicle Initial Inspection and Diagnostic Overview before using the symptom tables, and verify that all of the following are true:
- The engine control module (ECM) and malfunction indicator lamp (MIL) are operating correctly.
- There are no DTCs stored.
- The scan tool data is within the normal operating range, Scan Tool Data List
- Verify the customer concern and locate the correct symptom in the table of contents. Inspect the items indicated under that symptom.
Visual/Physical Inspection
Several of the symptom procedures ask for a careful visual and physical inspection. This step is extremely important. The visual and physical inspection can lead to correcting a problem without further inspections, and can save valuable time. Ensure that:
- The ECM grounds are clean, tight, and in the proper location.
- The vacuum hoses are not split or kinked, and properly connected, as shown on the Vehicle Emission Control Information label. Inspect thoroughly for any type of leak or restriction.
- The mass air flow (MAF) sensor is properly installed. The arrows on the plastic portion of the sensor must point toward the engine.
- The air intake ducts are not collapsed or damaged.
- There are no leaks at the throttle body mounting area, the MAF sensor, or the intake manifold sealing surfaces.
- The ignition wires are not cracked, brittle, or carbon tracked.
- The engine harness wiring and terminals are properly connected and are not pinched or cut.
Intermittent
IMPORTANT:
- Inspect for improper installation of electrical components if an intermittent condition exists. Inspect for aftermarket theft deterrent devices, lights, and cellular phones. Verify that no aftermarket equipment is connected to the class 2 circuit. If you cannot locate an intermittent condition, a cellular phone communication signal may cause the condition.
- The problem may or may not turn ON the MIL or store a DTC.
Faulty electrical connections or wiring cause most intermittent problems. Perform a careful visual and physical inspection of the suspect connectors for the following conditions:
- Improperly mated connector halves
- Terminals that are not seated
- Terminals that are damaged or improperly formed
Reform or replace connector terminals in the problem circuit in order to ensure proper contact tension. Connector Repairs Remove the terminal from the connector body in order to inspect for poor terminal wire connection. Testing For Intermittent Conditions and Poor Connections
Road test the vehicle with the DMM connected to the suspected circuit. An abnormal reading that occurs when the malfunction occurs is a good indication that there is a malfunction in the circuit being monitored.
Use a scan tool in order to help detect intermittent conditions. Useful features of the scan tool include the following:
- Trigger the Snapshot feature in order to capture and store engine parameters when the malfunction occurs. Review this stored information in order to see the specific running conditions that caused the malfunction.
- Freeze Frame/Failure Records can also aid in locating an intermittent condition. Review and capture the information in the Freeze Frame/Failure Record associated with the intermittent DTC being diagnosed. Drive the vehicle within the conditions that were present when the DTC originally set.
- Use the Plot Function on the scan tool in order to plot selected data parameters. Review this stored information to aid in locating an intermittent problem.
IMPORTANT: If the intermittent condition exists as a start and then stall, test for DTCs relating to the vehicle theft deterrent system. Test for improper installation of electrical options such as lights, cellular phones, etc.
Any of the following may cause an intermittent MIL with no stored DTC:
- The ignition coils are shorted to a ground or arcing at the ignition wires or the spark plugs.
- The ECM grounds are loose or dirty. Engine Controls Schematics
- The ignition control (IC) wires are routed too close to the secondary ignition wires, coils, or the generator. Ensure that all of the circuits from the ECM to the ignition coils have good connections.
- There is an open diode across the A/C compressor clutch or any other open diodes.
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.
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.
Poor Fuel Fill Quality: Difficulty when refueling the vehicle. 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.
Use the following tables when diagnosing a symptom complaint:
- Testing for Intermittent Conditions and Poor Connections
- Hard Start
- Surges/Chuggles
- Lack of Power, Sluggishness, or Sponginess
- Detonation/Spark Knock
- Hesitation, Sag, Stumble
- Cuts Out, Misses
- Poor Fuel Economy
- Poor Fuel Fill Quality
- Rough, Unstable, or Incorrect Idle and Stalling
- Dieseling, Run-On
- Backfire