Checks
| Action
|
DEFINITION: The engine runs unevenly at idle. If severe
enough, the engine or vehicle may shake. The engine idle speed may
vary in RPM. Either condition may be severe enough to stall the
engine.
|
Preliminary Check
| Refer to
Symptoms - Engine Controls
.
|
Sensor Checks
|
| • | Check the heated oxygen sensor (HO2S). |
| | Check for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The sensor will result in a high but false signal voltage, or rich exhaust indication. The control module will reduce the amount of fuel delivered to the engine causing a severe driveability problem. |
| • | Check the engine coolant temperature (ECT) sensor using the scan tool in order to compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more than or less than the ambient air temperature on a cold engine, check for a high resistance in the coolant sensor circuit or the sensor itself. |
| • | Check the manifold absolute pressure (MAP) sensor response and accuracy. |
|
Fuel System Checks
|
| • | Check to determine if a rich or lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the fuel trim will help identify the problem. |
| • | Check the fuel injector driver circuit. |
| 1. | Disconnect the injector harness connector at the injectors. |
| 2. | Connect an injector test lamp between the terminals of each injector connector and note the lamp while cranking. |
| 3. | If the test lamp fails to blink at any connector, it is a faulty injector drive circuit harness, connector, or terminal. |
|
Ignition System Checks
|
| • | Check for the following conditions: |
| • | Check the spark plug wires by connecting a DMM to the ends of each wire in question. If the meter reads over 3,000 ohms per foot, then replace the wires. |
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Additional Checks
|
| • | Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low idle air control (IAC) counts. |
| • | Check the control module grounds for being clean, tight, and in their proper locations. |
| • | Check for the exhaust gas recirculation (EGR) being ON while idling which will cause roughness, stalling, and hard starting. |
| • | Check the battery cables and ground straps. They should be clean and secure. |
| • | Check the crankcase ventilation valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Refer to
Crankcase Ventilation System Inspection/Diagnosis
in Engine Mechanical. |
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Engine Mechanical Check
| Check for the following:
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