Rough, Unstable, or Incorrect Idle, Stalling
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. The engine idles at
the incorrect speed.
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Preliminary Check
| Refer to
Symptoms
for the Important Preliminary Checks.
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Sensor Checks
|
| • | Check the Throttle Position (TP) sensor -- If a sticking
throttle shaft or binding linkage causes a high TP sensor voltage open throttle
indication, the ECM will not control the idle. Monitor the TP sensor voltage.
A scan tool and/or voltmeter should read less than 1.25 volts with
a throttle closed. Refer to
Fuel Metering System Diagnosis
. |
| • | Check the Engine Coolant Temperature (ECT) sensor -- Using
the scan tool, compare the engine coolant temperature with the ambient air
temperature on a cold engine. If the engine coolant temperature is more
than 5°C (41°F) greater or less than the ambient air temperature,
check for a high resistance in the coolant sensor circuit or the sensor
itself. Refer to the Diagnostic Aids in
DTC 15 ECT Sensor Circuit High
. |
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Fuel System Checks
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| • | Check in order 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 injectors for leakage. |
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Ignition System Checks
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| • | Check the spark plugs. Remove the spark plugs and check for the
following items: |
| • | Check the spark plug cables by connecting an ohmmeter to the ends
of each cable in question. If the meter reads over 30,000 ohms, replace
the cables. |
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Additional Checks
|
| • | Check for vacuum leaks. Vacuum leaks can cause higher than normal
idle and low Idle Air Control (IAC) counts. |
| • | Check the ECM grounds for clean, tight, and proper routing. |
| • | Check the scan tool in order to determine if the ECM is receiving
an A/C signal. Refer to
Engine Control Module Diagnosis
. If a problem exists with the A/C ON, check the
A/C system operation. Refer to HVAC. |
| • | Check the battery cables and ground straps. They should be clean
and secure. Erratic voltage will cause the IAC to change its position, resulting
in poor idle quality. |
| • | Check the A/C refrigerant pressure for being too high or for a
faulty high pressure switch. |
| • | Check the Crankcase Ventilation Valve for proper operation by
starting the engine and placing a finger over inlet hole in valve end several
times. The valve should snap back. If not, replace the valve. Refer to
Crankcase Ventilation System Diagnosis
. |
| • | Check the High Idle system operation. Refer to High Idle Diagnosis
. |
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Engine Mechanical Check
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| • | Check the engine for the following items: |
| - | Broken or weak valve springs |
| • | Refer to Engine Mechanical. |
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