Checks
| Action
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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.
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Preliminary Checks
| Refer to Important Preliminary Checks in
Symptoms
.
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Sensor Checks
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| • | Check for proper operation and performance of the oxygen sensor. |
| | Inspect the oxygen sensor for silicon contamination. Caused by fuel
or the use of improper engine sealants, the oxygen sensor will have a white,
powdery coating. The sensor will output a high, but false signal voltage.
This false rich condition will cause the control module (AF ECU, VCM/PCM
or ECM) to reduce the amount of fuel to the engine. |
| • | Check the ECT sensor for proper operation by performing the following: |
| 1. | Allow the engine temperature to reach ambient air temperature
by leaving the engine OFF for several hours (overnight for most accurate results). |
| 2. | Turn ON the ignition, with the engine OFF. |
| 3. | Using a scan tool compare the ECT and IAT sensors displayed temperature. |
The sensors should be within 3°C (5°F) of each other.
| 4. | Replace the ECT sensor or repair the ECT sensor circuit if the
ECT is not near the expected value. |
| • | Check for proper MAP sensor operation. A MAP sensor that does
not respond quickly and accurately to manifold pressure changes may cause
this symptom. |
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Fuel System Checks
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| • | Determine if the condition is rich or lean by performing the following: |
| - | Operate the vehicle under the conditions of the concern. |
| - | Monitor the oxygen sensor output. |
| • | Check for leaking fuel injectors. |
| • | Check for fuel (gasoline) in the fuel pressure regulator vacuum
hose (if equipped). This indicates a faulty gasoline fuel pressure regulator. |
| • | Check for proper EVAP Purge system operation. |
| • | Check for proper installation of the gas distribution adaptor
(GDA). |
| • | Check for air leaks at the GDA. |
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Additional Checks
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| • | Check for manifold or air induction system leaks. |
| • | Check for proper EGR valve operation. A stuck open valve may cause
this symptom. |
| • | Check the PCV system for proper operation and valve type. |
| • | Check for proper engine cylinder compression. |
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