Checks
| Action
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DEFINITION: The
engine runs unevenly at idle. The engine or vehicle may shake if the condition
is severe enough. The engine idle speed may vary in RPM. The
condition may be severe enough to stall the engine. The engine
idles at an incorrect speed.
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Preliminary Check
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Sensor Checks
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| • | Check the performance of the mass air flow (MAF) sensor. Refer
to
DTC P0101
Diagnostic Aids. |
| • | Check the throttle position (TP) sensor. A sticking throttle shaft
or a binding throttle linkage will cause a high TP sensor voltage or an open
throttle indication. Under these conditions the powertrain
control module (PCM) may not control the idle. Monitor the
TP sensor voltage with the scan tool or a digital multimeter. The
indicated voltage should be less than 1.25 volts with the
throttle closed. |
| • | Inspect for a shifted or fixed engine coolant temperature (ECT)
sensor. Test the 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 (9°F) more
or less than the ambient air temperature, check for a high resistance
in the coolant sensor circuit or the sensor itself. Refer to
Temperature Versus Resistance
. |
| • | Inspect the performance of the heated oxygen sensors (HO2S) 1.
Refer to: |
| • | Check the camshaft position (CMP) sensor engine reference signal
with a scan tool. Observe the Engine Speed parameter while running the engine
at idle. The scan tool should indicate steady and linear RPM readings
while running. If the erratic values, such as sudden spikes or
drops in engine speed are displayed, the engine reference signal
is not stable enough for the engine to run properly. |
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Fuel System Checks
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| • | Determine whether a rich or lean fueling condition can cause the
concern. Drive the vehicle at the speed of the concern. Monitor the Fuel Trim
parameters on a scan tool in order to identify the fuel system
status. |
| - | Lean -- The long term fuel trim value will be approaching
20 percent. Refer to
DTC P0171
Diagnostic Aids. |
| - | Rich -- The long term fuel trim value will be approaching -20 percent.
Refer to
DTC P0172
Diagnostic Aids. |
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Ignition System Checks
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Engine
Mechanical Check
| Check for any of the following engine mechanical concerns:
| • | An incorrect valve lash |
| • | Any broken or weak valve springs |
| • | An incorrect valve timing |
| • | Any sticking or leaking valves |
| • | An incorrect or worn camshaft |
Refer to
Engine Compression Test
in
Engine Mechanical.
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Additional Checks
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| • | Check for vacuum leaks. Vacuum leaks can cause higher than normal
and erratic idle speeds. |
| • | Check the air inlet system for any leaks or restrictions. |
| • | Check the throttle body for blockage and a damaged or binding
throttle valve. |
| • | Check for binding of the accelerator cable or pedal. Inspect the
accelerator cable for the proper adjustment. Refer to
Accelerator Cable Adjustment
. |
| • | Verify that the battery cables and ground straps are clean and
secure. |
| • | Inspect the PCM grounds and engine grounds for clean and tight
connections in the correct locations. Refer to
Engine Controls Schematics
. |
| • | Check for a DTC C1215 in the ABS idle-up circuit. Refer
to
DTC C1215
. |
| • | Check the idle air control (IAC) system for proper operation.
Use a scan tool in order to command the IAC valve to increase and/or decrease
the engine idle speed. Refer to
Idle Air Control (IAC) System Diagnosis
. |
| • | An idle concern that occurs when the A/C is On may indicate a
fault in the A/C idle-up system. The idle speed should increase with the A/C
On. Check the scan tool in order to determine if the PCM is
receiving an A/C signal. |
| • | A rough running or stalling condition that occurs when the A/C
is On may be caused by A/C refrigerant pressure that is too high or a faulty
high pressure switch. |
| • | Inspect the throttle body and the IAC valve for excessive carbon
deposits. A buildup of deposits in the passages, on the IAC pintle, and around
the throttle valve and the throttle bore can affect IAC system performance.
Remove the excess deposits as necessary. Refer to
Throttle Body Inspection and Cleaning
. |
| • | Check the positive crankcase ventilation (PCV) valve for the correct
part and proper operation. |
| • | Check the power steering pressure (PSP) switch for proper operation.
Refer to
Power Steering System Test
in Power Steering System. |
| • | Check for proper adjustment of the transmission range (TR) switch.
Observe the park/neutral position (PNP) parameter on the scan tool for the
correct position display, when shifting the transmission through the
PRNDL pattern. Service the TR switch as necessary. Refer to
DTC P0705
. |
| • | A malfunctioning air limiting valve in the throttle body could
cause the following conditions: |
| - | A slightly higher than normal idle speed on a hot engine when
the air limiting valve is stuck open. |
| - | A lower than normal idle speed on a cold engine when the air limiting
valve is stuck closed. |
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