Checks
| Action
|
DEFINITION: The
engine runs unevenly at idle, or idle is at incorrect speed. The engine or
vehicle may shake if the condition is severe, and the engine may stall.
|
Preliminary Check
|
|
Sensor Checks
|
| • | 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 DMM. 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. Compare the engine coolant temperature of a cold engine with the
ambient air temperature. If the temperatures differ by more than 5°C
(9°F), check for high resistance in the coolant sensor circuit
or the sensor. 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. |
|
Fuel System Checks
|
| • | 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 Diagnostic Aids in
DTC P0171
and
DTC P0174
. |
| - | Rich--The long term fuel trim value will be approaching -20 percent.
Refer to Diagnostic Aids in
DTC P0172
and
DTC P0175
. |
|
Ignition System Checks
|
| • | Check the ignition coil and spark plug boots for cracks, carbon
tracking, and moisture. |
|
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
and
Symptoms - Engine Mechanical
in Engine Mechanical.
|
Additional Checks
|
| • | 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 powertrain control module (PCM) grounds and engine
grounds for clean and tight connections in the correct locations. Refer to
Engine Controls Schematics
. |
| • | Check for DTC PC1215 in the antilock brakes (ABS) idle up
circuit. Refer to
DTC C1215
in Brakes. |
| • | 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. Refer to
Electrical/Accessory Load Idle-Up Signal Diagnosis
. |
| • | 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. |
| • | 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 thorough
the PRNDL pattern. Service the TR switch as necessary. Refer to
DTC P0705
in Automatic Transmission. |
| • | 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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